Citation
Influence of Cover Crops on Nutrient Availability in a Sweet Potato Cropping System in South Florida

Material Information

Title:
Influence of Cover Crops on Nutrient Availability in a Sweet Potato Cropping System in South Florida
Creator:
RAGSDALE, JEANNA
Copyright Date:
2008

Subjects

Subjects / Keywords:
Biomass ( jstor )
Cover crops ( jstor )
Green manures ( jstor )
Nutrients ( jstor )
Planting ( jstor )
Soil science ( jstor )
Soils ( jstor )
Tillage ( jstor )
Velvet ( jstor )
Weeds ( jstor )
Miami metropolitan area ( local )

Record Information

Source Institution:
University of Florida
Holding Location:
University of Florida
Rights Management:
Copyright Jeanna Ragsdale. Permission granted to University of Florida to digitize and display this item for non-profit research and educational purposes. Any reuse of this item in excess of fair use or other copyright exemptions requires permission of the copyright holder.
Embargo Date:
8/31/2006
Resource Identifier:
649814541 ( OCLC )

Downloads

This item has the following downloads:

ragsdale_j ( .pdf )

ragsdale_j_Page_14.txt

ragsdale_j_Page_53.txt

ragsdale_j_Page_09.txt

ragsdale_j_Page_28.txt

ragsdale_j_Page_08.txt

ragsdale_j_Page_70.txt

ragsdale_j_Page_52.txt

ragsdale_j_Page_34.txt

ragsdale_j_Page_51.txt

ragsdale_j_Page_59.txt

ragsdale_j_Page_05.txt

ragsdale_j_Page_41.txt

ragsdale_j_Page_12.txt

ragsdale_j_Page_21.txt

ragsdale_j_Page_23.txt

ragsdale_j_Page_71.txt

ragsdale_j_Page_36.txt

ragsdale_j_Page_61.txt

ragsdale_j_Page_35.txt

ragsdale_j_Page_65.txt

ragsdale_j_Page_17.txt

ragsdale_j_Page_02.txt

ragsdale_j_Page_33.txt

ragsdale_j_Page_54.txt

ragsdale_j_Page_68.txt

ragsdale_j_Page_40.txt

ragsdale_j_Page_31.txt

ragsdale_j_Page_22.txt

ragsdale_j_Page_20.txt

ragsdale_j_Page_69.txt

ragsdale_j_Page_19.txt

ragsdale_j_Page_07.txt

ragsdale_j_Page_58.txt

ragsdale_j_Page_30.txt

ragsdale_j_Page_45.txt

ragsdale_j_Page_37.txt

ragsdale_j_Page_39.txt

ragsdale_j_Page_43.txt

ragsdale_j_Page_60.txt

ragsdale_j_Page_57.txt

ragsdale_j_Page_01.txt

ragsdale_j_Page_63.txt

ragsdale_j_Page_38.txt

ragsdale_j_Page_06.txt

ragsdale_j_pdf.txt

ragsdale_j_Page_15.txt

ragsdale_j_Page_24.txt

ragsdale_j_Page_10.txt

ragsdale_j_Page_48.txt

ragsdale_j_Page_44.txt

ragsdale_j_Page_62.txt

ragsdale_j_Page_72.txt

ragsdale_j_Page_50.txt

ragsdale_j_Page_25.txt

ragsdale_j_Page_66.txt

ragsdale_j_Page_67.txt

ragsdale_j_Page_26.txt

ragsdale_j_Page_32.txt

ragsdale_j_Page_49.txt

ragsdale_j_Page_13.txt

ragsdale_j_Page_18.txt

ragsdale_j_Page_47.txt

ragsdale_j_Page_16.txt

ragsdale_j_Page_46.txt

ragsdale_j_Page_56.txt

ragsdale_j_Page_04.txt

ragsdale_j_Page_55.txt

ragsdale_j_Page_42.txt

ragsdale_j_Page_03.txt

ragsdale_j_Page_11.txt

ragsdale_j_Page_27.txt

ragsdale_j_Page_29.txt

ragsdale_j_Page_64.txt


Full Text












INFLUENCE OF COVER CROPS ON NUTRIENT AVAILABILITY IN A
SWEET POTATO CROPPING SYSTEM IN SOUTH FLORIDA















By

JEANNA RAGSDALE


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OFTHE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2006

































Copyright 2006

by

Jeanna Ragsdale















ACKNOWLEDGMENTS

There is a long list of people I would like to thank for a variety of reasons. The

problem is with where to begin the list and where to end it.

First, I would like to extend thanks to my major advisor, Dr. Yuncong Li, for his

support, guidance, encouragement, and technical advice throughout this study and my

time as a graduate student. I'd also like to thank the other members of my committee,

Thomas Obreza, Ashok Alva, and Zhenli He, for their support, suggestions, and feedback

in this study. I would like to extend a special thanks to M & M Farms, Manelo Hevia,

Teresa Olczyk, and Waldy Klassen for their support and help with organizing my

research. Special thanks are also extended to the many people in the soil and hydrology

lab at the Tropical Research and Education Center (TREC), Newton Campbell, Tina

Dispenza, Guodong Lui, Laura Rosado, Yun Quan, Xing Wang, Qingren Wang, and

Guiqin Yu, for all their help and time. Appreciation is also extended to the whole farm

crew at TREC for all their time, help, and hours in the sun.

Second, I would like to thank the many other academic mentors that in one way or

another inspired my current pursuits: Lawry Gold for instilling a deep sense of self-

awareness in my life's direction and purpose, Dr. Karl Lillquist for inspiring my interest

in soils, Chris Kent for keeping my motivations high, and Kenneth Buhr for reminding

me of the importance of both sustainability and practicality.

Finally, it is without question that I owe my greatest thanks to so many people that

have inspired me, pushed me, and stuck with me throughout my life. This of course









includes my family for their love, support, and belief in my abilities, and a long list of

friends, of which I will only name a few: Glen Erickson for pushing me out of the nest

and teaching me how to fly, MaxZine Weinstein for bringing out my true love for

gardening, David Long for his never ending love, encouragement, and faith in me, and

George Voellmer for providing me with inspiration when it was most needed and never

letting go of my hand.
















TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ................................................................................................. iii

LIST OF TABLES ..................................................... ............ ................ vii

LIST OF FIGURES ......... ............................... ........ ............ ix

ABSTRACT ........ .............. ............. ...... ...................... xi

INTRODUCTION .................................. .. ... .... ........ ...............

LITER A TU R E R EV IEW .................................................................. ......................... 2

C over C rops U sed in F lorida .......................................................................... .... ...2
Cover Crops Tested in Florida .............. ..... ............ ............................ .2
Sorghum-sudan grass (S. bicolor x S. bicolor var. sudanense (Piper)
Stapf.)......................................... ....... ............ ........... 2
Sunn hemp (Crotalariajuncea L. cv. 'Tropic Sun')....................................3
Velvetbean (Mucuna deeringiana (Bort.), Merr.) ..........................................4
Cowpea (Vigna unguiculata L. cv. 'Iron Clay').......................................4
Aeschynomene (Aeschynomene evenia C. Wright) ....................................4
Sesbania (Sesbania exaltata Raf.).......................................... .....................5
German millet (Setaria italica (L.) P. Beav.)......................... .... ........... 5
Cover Crops Used to Improve Weed Suppression and Pest Control........................5
Cover Crops U sed to Im prove Soil Quality...............................................................7
Cover Crops Used to Improve Crop Yield ............................................................. 12
Micronutrient Deficiency of Plants Grown in Calcareous Soils .............................15

OBJECTIVE ................................ ................... ................ ........... 17

M ATERIALS AND M ETHODS ....................................................... .............. 18

Establishment of Field Experiment .................................................. ............... 18
C over C rop M anagem ent............................................. ......................................... 18
Sw eet Potato Crop M anagem ent ........................................ .......................... 19
Sam ple Collection and Chemical Analysis...................................... ............... 20
Statistical A n aly sis............................................................................. ............... 2 1









RESULTS AND DISCUSSION ......................................................... ...............22

C ov er C rop B iom ass........................................................................ .... ........ .... 22
Nutrient Concentrations in Cover Crop ....................................... ...............23
Total Nutrients in Cover Crop Biom ass .......................................... ............... 26
Soil Nutrients Prior to Planting Cover Crops .................................. ............... 30
Soil Nutrients Prior to Cutting Cover Crops................................... ............... 30
Soil Nutrients at 9 Weeks After Incorporation of Cover Crops ..............................39
Soil Nematodes ............................................... ... ............ .. .. .......... 42
Nutrient Concentrations in Sweet Potato Leaves ................................................. 46
Sw eet Potato Yield .................................... ..... .......... .......... ...51

SU M M A R Y ...................................... ................................................... 54

LIST OF REFEREN CES ............................................................ .................... 56

B IO G R A PH IC A L SK E TCH ..................................................................... ..................60















LIST OF TABLES


Table pge

1 Seedling rates for cover crops. ............................................................................ 19

2 Sam ple collection type and dates. ........................................ ........................ 20

3 Cover crop heights (cm) measured at 28, 42, 55, and 68 days after planting
(D A P ). .............................................................................. 22

4 Cover crop and weed biomass yield (Mg/ha) collected 68DAP (07 Nov. 2004).....23

5 Nutrient concentrations in cover crop tissues collected on 7 Nov. 2004. ................24

6 Total nutrient amounts in cover crop biomass. .............................. ......... ...... .28

7 Total N, C and AB-DTPA extractable nutrients (P, K, Mg, Fe, Zn, B, Mn, and
Cu) in soil samples collected on 31 Aug. 2004 prior to planting cover crops. ........32

8 Extractable nutrient concentration in soil samples collected on 07 Nov. 2004
prior to cutting cover crops. ............................................ ............................. 32

9 Extractable nutrient concentration in soil samples collected on 19 Feb. 2005
after incorporation of cover crops. ........................................ ....................... 34

10 Extractable soil nutrient concentrations in weed plots compared between
sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov. 2004,
and after incorporation on 19 Feb. 2005. ..................................... ............... 35

11 Extractable soil nutrient concentrations in sorghum-sudan grass plots compared
between sampling dates: before planting on 31 Aug. 2004, before cutting on 7
Nov. 2004, and after incorporation on 19 Feb. 2005. ............................................ 35

12 Extractable soil nutrient concentrations in sunnhemp plots compared between
sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov. 2004,
and after incorporation on 19 Feb. 2005. ..................................... ............... 36

13 Extractable soil nutrient concentrations in velvetbean plots compared between
sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov. 2004,
and after incorporation on 19 Feb. 2005. ..................................... ............... 36









14 Nematode population (direct count) in soil samples collected prior to planting
cover crops on 31 Aug, before cutting 07 Nov, and after incorporation on 19
F eb 2004 .............................................................................44

15 Sweet potato leaf nutrient concentrations in samples collected on 03 April 2005,
1 week after foliar applications of Fe and Zn.......................................... ....... 47

16 Iron tissue concentration in iron subplots in samples collected on 03 April 2005,
1 week after foliar application of Fe. .............................. ... ........................ 49

17 Zinc tissue concentration in zinc subplots in samples collected on 03 April 2005
1 week after foliar application of Zn. ................................................. ............... 50

18 Sweet potato harvest collected on 30 June 2005.................................................52
















LIST OF FIGURES


Figure pge

1 Cover crop biomass collected on 07 Nov. 2004 prior to incorporation of cover
c ro p s ............................................................................. 2 3

2 Cover crop plant tissue macronutrient concentrations in samples collected on 07
N ov. 2004 prior to cutting cover crops ........................................ .....................25

3 Cover crop plant tissue carbon concentration in samples collected on 07 Nov.
2004 prior to cutting cover crops. ........................................ ........................ 26

4 Cover crop plant tissue micronutrient concentrations in samples collected on 07
Nov. 2004 prior to cutting cover crops. ...................................... ............... 26

5 Cover crop total biom ass m acronutrients.............................................................. 27

6 Cover crop total biom ass carbon ........................................ ......................... 29

7 Cover crop total biomass micronutrients ...................................... ............... 29

8 Total N in soil collected on 31 Aug. 2004 prior to planting cover crops..............31

9 Total C in soil collected on 31 Aug. 2004 prior to planting cover crops.................31

10 AB-DTPA extractable P, K, and Mg in soil collected on 31 Aug. 2004 prior to
planting cover crops ................................................ .......... .....33

11 AB-DTPA extractable Fe, Zn, B, Mn, and Cu in soil collected on 31 Aug. 2004
prior to planting cover crops .............................................................................. 34

12 Total nitrogen in soil collected on 07 Nov. 2004 prior to cutting cover crops ........37

13 Total carbon in soil collected on 07 Nov. 2004 prior to cutting cover crops...........37

14 AB-DTPA extractable macronutrients in soil collected on 07 Nov. 2004 prior to
cutting cover crops ................................. .. ............... .......... .....38

15 AB-DTPA extractable micronutrients in soil collected on 07 Nov. 2004 prior to
cutting cover crops ................................. .. ............... .......... .....38









16 Total N in soil samples collected on 19 Feb. 2005 after incorporation of cover
c ro p s ............................................................................. 3 9

17 Total C in soil samples collected on 19 Feb. 2005 after incorporation of cover
crop s. ............................................................................... 4 0

18 AB-DTPA extractable soil nutrients in samples collected on 19 Feb. 2005 after
incorporation of cover crops .............................................................................. 40

19 AB-DTPA extractable soil nutrients in samples collected on 19 Feb. 2005 after
incorporation of cover crops .............................................................................. 42

20 Soil nem atodes counted in fallow plots......................................... ............... 43

21 Soil nematodes counted in sorghum-sudan plots. .................................................45

22 Soil nematodes counted in sunnhemp plots .................................. ............... 45

23 Soil nem atodes counted in velvet bean plots ................................. ............... 46

24 Sweet potato leaf macronutrient concentrations in samples collected on 03 April
2005 1 week after foliar applications of Fe and Zn...................................... 48

25 Sweet potato leaf carbon concentrations in samples collected on 03 April 2005 1
week after foliar applications of Fe and Zn................................ ................48

26 Sweet potato leaf micronutrient concentrations in samples collected on 03 April
2005 1 week after foliar applications of Fe and Zn...................................... 49

27 Iron in sweet potato leaf tissue collected on 03 April 2005 in Fe subplots 1 week
after foliar application of iron ....................................................... ............... 50

28 Zinc in sweet potato leaf tissue collected on 03 April 2005 in Zn subplots 1
w eek after foliar application of iron................................ ................................. 51

29 Sweet potato harvest in cover crop treatment plots (each plot equals 9.3 m2)
collected on 30 June 2005 ............................................... ............................. 52

30 Sweet potato harvest in micronutrient subplots (each plot equals 9.3 m2)
collected on 30 June 2005 .............................................. .............................. 53















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree Master of Science

INFLUENCE OF COVER CROPS ON NUTRIENT AVAILABILITY IN A SWEET
POTATO CROPPING SYSTEM IN SOUTH FLORIDA

By

Jeanna Ragsdale

August 2006

Chair: Yuncong Li
Major Department: Soil and Water Science

The use of cover crops serves a variety of purposes from taking up excess soil

nutrients to carbon sequestration and controlling soil erosion. Other benefits include

weed and nematode suppression, and use as a green manure for improving soil fertility.

The objective of this research was to improve nutrient availability in a sweet potato

[Ipomea batatas (L.) Lam] cropping system grown on a south Florida calcareous soil.

Sweet potato response to micronutrient fertilizer additions of iron (Fe) and zinc (Zn) were

evaluated for their effect on crop yield and growth. In addition to micronutrient fertilizer

application, the cover crops Sunnhemp (Crotalaria juncea L. cv. Tropic Sun), velvet bean

(Mucuna deeringiana), and sorghum-sudan grass (Sorghum bicolor x S. bicolor var.

Sudanese) were grown and incorporated into the soil prior to planting sweet potato.

Biomass samples were collected for each cover crop. Cover crops were analyzed for

nutrient concentrations, and total biomass nutrient accumulation for N, C, P, K, Mg, Fe,

Zn, B, Mn, and Cu. Sorghum-sudan produced the greatest amount of biomass at 13.01









Mg/ha, followed by sunnhemp at 8.01 Mg/ha and velvet bean at 5.22 Mg/ha. Total

biomass nutrients C, P, K, Mg, Fe, Zn, and B were greatest in sorghum-sudan grass.

Sunnhemp total biomass contained the greatest amount of nitrogen (180.36 kg/ha), and

manganese (368.66 g/ha). Velvet bean plant tissue had the highest concentrations of zinc

and copper. There were no significant differences in soil nutrients among the treatment

plots prior to planting cover crops. After incorporation of cover crops, total nitrogen and

AB-DTPA P increased in all treatment plots, while Mg and Mn decreased. The total

number of nematodes decreased in all treatment plots after incorporation of cover crops.

Sweet potato leaf tissue samples collected from sunnhemp plots contained the greatest

concentration of N, P, and K. Samples collected from sorghum-sudan grass plots

contained the greatest amount concentration of C, and Fe. Foliar applications of Fe and

Zn had no significant effect on Fe or Zn concentrations in sweet potato leaf tissue. The

sweet potato crop was damaged by freezes shortly after planting, which greatly affected

the total harvest. There were no significant differences found between the main treatment

plots or subplots for sweet potato harvest.















INTRODUCTION

The use of cover crops serves a variety of purposes that improve environmental

quality, ranging from taking up excess soil nutrients to carbon sequestration and

controlling soil erosion. Other benefits of cover crops include pest control such as weed

and nematode suppression (Morris and Walker, 2002), and use as a green manure for

improving soil fertility (Blackshaw et al., 2001). Cover crops used as green manures add

organic matter to the soil (Chambliss et al., 2003), which improves soil structure and can

help increase soil water holding capacity (Chambliss et al., 2003). Soil microorganisms

decompose organic matter through a process of mineralization and therefore play an

important role in nutrient cycling. As the organic matter is broken down, nutrients stored

in plant tissues are released to the soil and become available for plant uptake (Chambliss

et al., 2003). As recalcitrant organic matter continues to decompose, it forms humus that

increases the cation exchange capacity (CEC) of the soil. This process results in a greater

ability to hold nutrients in the soil, keeping them available for plant uptake over a period

of time (Chambliss et al., 2003). Many studies commonly evaluated cover crop effects

on crop growth and organic carbon changes in soils, but few studies evaluated

micronutrient availability in association with cover crops. More information is needed to

determine micronutrient availability as a result of cover crop use.















LITERATURE REVIEW

Cover Crops Used in Florida

Common cover crops in Florida include legumes and grasses that are used to

suppress weeds, prevent soil erosion, remove salts, improve soil fertility, protect water

quality, and control pests. Grasses used as cover crops in Florida include pearl millet

(Pennisetum glaucum), sorghum-sudan, bahiagrass (Paspalum notatum), and pangola

(Digitaria eriantha) (Li et al., 1999; Chambliss et al., 2003). Grasses can produce more

biomass and decompose more slowly than legumes. Legumes have the added benefit of

their ability to fix atmospheric N in association with Rhizobia. Legumes used as green

manures generally decompose more rapidly than grasses due to the higher N content in

their biomass. Common Legumes used in Florida include aeschynomene (Aeschynomene

avenia), hairy indigo (Indigofera hirsuta), sesbania (Sesbania exalta), velvet bean, lupine,

and sunhemp (Chambliss et al., 2003).

Cover Crops Tested in Florida

Sorghum-sudan grass (S. bicolor x S. bicolor var. sudanense (Piper) Stapf.)

Sorghum-sudan grass, also known as sorghum-sudan, has been used as a cover crop

throughout Florida for decades, and is still used by some growers during the fallow

summer period in south Florida. Sorghum-sudan grass usually produces 11-16 metric

tons of dry matter per hectare (5 to 7 tons dry mass per acre). Since 0.92% of this

material is N, the amounts of N potentially available to the subsequent crop range from

about 490 to 708 kilograms per hectare (90 to 130 pounds per acre). While meeting some









of the criteria for a good cover crop, Sorghum-sudan falls short in Florida. This plant

grows poorly in many Florida soils, having been developed for the finer textured soils of

the Midwest and Southwest USA. Sorghum-sudan often grows quite tall, requiring

mowing to prepare the crop for green manuring, which adds cost to its management. Its

large fibrous stems have a high C:N ratio, which slows decomposition, and may

immobilize N from the soil during decomposition. Lastly, Sorghum-sudan often attracts

armyworms and corn silk flies, which may be detrimental to subsequent vegetable crops.

However, sorghum-sudan grass suppresses weeds and some parasitic nematodes, and the

seed is inexpensive ( $2.20 to $3.31 per kilogram or $1.00 to 1.50 per pound of seed).

Sunn hemp (Crotalaria juncea L. cv. 'Tropic Sun')

Sunn hemp has a number of advantages compared with Sorghum-sudan as a cover

crop. This plant is an annual tropical legume that has a fast-growing, 60 to 80 day

production cycle during which the plant may exceed 2 m in height. Sunn hemp is a short-

day plant that is quite drought tolerant, grows well in both high and low pH soils, and is

also resistant to root-knot nematode. Typically Sunn hemp produces 6 to 8.5 tons of dry

mass per acre. Since 2.85% of this material is N, the amounts of N potentially available

to the subsequent cash crop range from about 13 to 19 kilograms per hectare (340 to 450

pounds per acre). However, Sunn hemp does have several limitations. Seed is rather high-

priced ($3.30 to $8.80 per kilogram or $1.50 to $4.00 per pound) due to the need to

import it, hence limiting seed availability. Seeds require Rhizobium inoculation before

planting. In some fields, Sunn hemp stands may be reduced due to damping-off from the

effects of Pythium or a form of Fusarium. Even with these possible limitations, Sunn

hemp was among the best of the tested cover crops in southern Florida conditions.









Velvetbean (Mucuna deeringiana (Bort.), Merr.)

Velvetbean is also an annual tropical legume that produces a large amount of

biomass, is drought tolerant, suppresses parasitic nematodes, and grows well in both high

and low pH soils. Velvetbean may produce 11-16 metric tons of dry biomass per hectare

(5 to 7 tons per acre of dry biomass) consisting of 2.6% N, which may provide from 570

to 790 kilograms (260 to 360 pounds) of N to the subsequent cash crop. However,

velvetbean's large seed requires a special planter, and volunteer plants may persist into

the next cash crop, requiring weed control. However a small seeded cultivar, 'Georgia

bush', can be seeded with some conventional seeders. Additionally, velvetbean may be

potentially allopathic to some subsequent vegetable crops. In field trials, velvetbean

ranked among the best of the tested cover crops.

Cowpea (Vigna unguiculata L. cv. 'Iron Clay')

Cowpea is a legume that grows well in a variety of soils, is resistant to root-knot

nematodes, and has a short growing season of 40 to 50 days. Cowpea may produce 7 to

11 metric tons per hectare (3 to 5 tons per acre) of biomass consisting of 2% N, which

may provide from 265 to 440 kilograms (120 to 200 pounds) of N to the subsequent cash

crop. However, in Florida conditions, cowpea is not tolerant to flooding and produces a

rather low biomass.

Aeschynomene (Aeschynomene evenia C. Wright)

Aeschynomene grows well on calcareous soils in southern Florida, and is a warm-

season legume forage. It is resistant to root-knot nematode. Aeschynomene's single

apparent disadvantage for Florida conditions is its low biomass production.









Sesbania (Sesbania exaltata Raf.)

Sesbania, like aeschynomene and cowpea, is a warm-season legume forage that is

well adapted to Florida conditions. However, sesbania is susceptible to root-knot

nematode, and does not quickly form a closed canopy, competing rather poorly with

some of Florida's persistent weeds.

German millet (Setaria italica (L.) P. Beav.)

German millet grows well in southern Florida and has been proven to be resistant

to root-knot nematode; however, like some of the other tested crops, German millet

produces low biomass.

Cover Crops Used to Improve Weed Suppression and Pest Control

Using cover crops for pest control can reduce the use of pesticides on subsequent

crops. Cover crops have been proven to control nematodes and suppress weeds, and have

biocidal properties. Nematicidal properties of plant residues may be a result of

ammonification, phytochemicals, or other compounds produced from plant tissues during

the breakdown process (Morris and Walker, 2002).

A study using dried plant tissue from 20 leguminous species showed lower

nematode infestation from each of the legumes (Morris and Walker, 2002). Dried

legume tissue was incorporated into soil at four rates: 1, 2, 2.5, and 5%. The soil was kept

moist enough for seed germination and incubated at 21 and 270 C for 1 week, at which

time 2-week old tomato plants were transplant into each soil treatment. Soil samples

infested with nematodes without dried legume tissue were maintained to serve as

controls. All 20 leguminous species were found to lower the amount of nematode

infestation in the samples, but the range of effect varied widely, with the greatest effect

occurring at the highest incorporation rate.









In a study by Blackshaw et al. (2001), yellow sweet clover was shown to suppress

weeds. Yellow sweet clover as a living biomass suppressed weeds up to 83% in the 1st

year of the study, but had no significant effect in the succeeding 2 years; however, it had

a greater affect on weed biomass than on weed density in all three years. Yellow sweet

clover plant residue left on each plot had fewer weeds than plots without yellow sweet

clover, but amount and species of weeds varied depending on companion plant grown.

This result may have been due to both physical and allelopathic effects (Blackshaw et al.,

2001).

A study conducted in southwestern Nigeria examined the effectiveness of cover

crops at reducing weed seedbanks in maize-cassava systems (Ekeleme et al., 2003).

Total seedbank was evaluated from 1993 to 1995 under three fallow types: bush,

leucaena, and kudzu. In all 3 years, seed population was less in plots with kudzu than

with leucaena. Both kudzu and leucaena were more effective at reducing seed population

than bush fallow. Kudzu, a tropical legume, reaches full ground cover in 1 year. It

creates a closed canopy and therefore reduces the light that reaches the soil, which in turn

reduces weed seed germination.

Methyl bromide is commonly used as a soil fumigant to control fungi and other

pests. The Brassicaceae family has been shown to have biocidal properties. Brassica

species contain glycosidic properties that hydrolyze into cytotoxic compounds, which

affect fungi and other soil pests harmful to crops. Lazzeri et al. (2002) evaluated two

species of Brassica against a conventional green manure (barley), and methyl bromide (a

common soil fumigant) on strawberry performance for 2 years. Plots treated with methyl









bromide had higher yields than all other plots; however, plots that incorporated Brassica

had higher yields than barley or untreated plots.

Cover Crops Used to Improve Soil Quality

In a study by Chander et al. (1997), the use of green manures improved organic

matter, soil microbial activity and productivity of the soil. Crop rotations increased

organic carbon and total nitrogen in the soil. Sesbania aculeate, a green manure, added

the greatest amount of soil organic carbon and total nitrogen when used in the rotation.

Nutrient cycling in the soil is affected by soil biota in the labile fraction of organic

matter, and is therefore affected by soil management practices including crop rotations,

cover crops, green manures, tillage, and fertilization (Chander et al., 1997).

Organic C and N have been used as soil quality indicators but have been

unresponsive during short periods. However, soil microbial biomass and enzyme activity

are more responsive to agricultural management practices and environmental conditions

than total organic C and N and can be used as early indicators of soil quality (Balota et

al., 2003).

Agricultural management practices such as cover crops, crop rotations, and tillage

have an effect on soil microorganisms and therefore soil quality (Chander et al., 1997).

Cover crops and crop rotations help maintain soil organic matter (SOM) and soil

structure, thereby increasing soil porosity and water holding capacity. Conventional

tillage practices can significantly decrease SOM, increase soil erosion, and disturb

microenvironments within the soil. These alterations affect water and oxygen content at

the soil surface (Curci et al., 1997). All these factors influence substrate availability for

microbial activity, which in turn affects soil fertility and quality (Curci et al., 1997).









The most labile carbon and nitrogen pools in soil are represented by soil microbial

biomass C at 1 to 3% of total soil C and as much as 5% of the total soil N is represented

by microbial biomass N (Moore et al., 2000). A study by Moore et al. (2000) found the

highest microbial biomass C in multicropping systems on 4-year rotations and the lowest

in monocropping systems of corn and soybean. Monocropping systems generally contain

less organic matter, microbial biomass, and soil structural stability than agroecosystems

that incorporate crop rotations (Moore et al., 2000). However, while changes in soil

properties are related to crop management practices, it is also important to note that the

extent of these changes are also affected by the soil type (Acosta-Martinez et al., 2004).

Where one study in a loamy sand soil found greater increases in microbial biomass, N-

mineralization, enzyme activities, and organic matter in rotations with corn, soybean, and

oats when compared with continuous corn alone, another study found no difference in

enzyme activities in a fine sandy loam soil under continuous cotton compared with that

under a cotton-peanut rotation (Acosta-Martinez et al., 2004).

Reduced soil productivity, nutrient imbalances, and yield decline can be linked to

loss of soil organic matter (SOM). In a study by Yadvinder-Singh et al. (2004), seven

treatments were analyzed for their effect on soil fertility, yield, and contribution to SOM

in a rice-wheat rotation. The study found that farmyard manure in combination with

green manure (Sesbania cannabina L.) had higher rice yields than other green manure

treatments. Rice yields were significantly higher for wheat straw in combination with

green manure than with green manure alone. Wheat straw and farmyard manure

increased soil organic carbon (SOC) significantly compared with other treatments. The









greatest increases in SOC were found when green manure, wheat straw and farmyard

manure were applied in combination.

Soil physical, biological and chemical properties can be enhanced by additions of

organic matter (Sangakkara et al., 2004). Green manures add organic matter and can have

positive effects on soil quality and nutrient supply. In a study by Sangakkara et al., two

tropical green manures were studied to determine their effect on root and shoot growth of

maize. In 3 years, soil physical properties and available nutrients N, phosphorus (P) and

potassium (K) all increased after continued additions of green manures. The two green

manures used in this study were Crotalariajuncea and Tithonia diversifolia. Shoot

growth was greatest with additions of Crotalaria due to its high nitrogen content, while

Tithonia, which has the ability to mobilize soil P, enhanced development of an extended

root system. Over the three-season study, SOC increased by 8% with additions of

Crotalaria and by 12% with additions of Tithonia compared with untreated soil. Tithonia

increased soil P, while Crotalaria increased soil N.

Phosphorus availability may be enhanced for crops following the use of green

manures (Cavigelli and Thien, 2003), possibly due to a green manure's ability to convert

unavailable forms of P to available forms, thereby enhancing P availability for the

succeeding crop. Organic P in decomposing green manure tissue is potentially labile and

available for the succeeding crop. This decomposition process will also release CO2,

which in soil solution may form H2CO3 causing dissolution of P in minerals.

Decomposition of green manures will also release organic acids, which may further

dissolve soil mineral P (Cavigelli and Thien, 2003).









Some cover crops like white lupine are able to release P due to secretion of organic

anions. White lupine has cluster roots, which secrete citric acid when soils are low in

available P. Other cover crops like fodder rape have extensive fine root systems that are

able to scavenge P at greater distances from the plant (Little et al., 2004). A study on P

bioavailability and green manures incorporated prior to growing sorghum (Cavigelli and

Thien, 2003) found that sorghum P uptake correlated with perennial forage P uptake.

However, they also found that plant type, rather than P uptake, in selected winter cover

crops may have a greater influence on subsequent sorghum crop P uptake.

A study by Franchini et al. (2004) conducted in Brazil tested a variety of cover

crops to determine their ability to transport P down to the roots and into the subsoil

layers. They found that transport of P below 10 to 55cm was best done by black oats,

white lupine and IAPAR-74 pea. The greatest accumulation of P in the aerial parts of the

plant, without P fertilizer applications, was with white lupine. The greatest accumulation

in the roots was by common vetch. In addition to P accumulation by different cover

crops, they may also increase the solubility of native soil P by controlling the rhizosphere

pH, plant exudates, and root phosphatases (Franchini et al., 2004).

A study conducted using three legume green manure sources compared with

inorganic fertilizer NPK for two different varieties of sweet potato found that soil

nitrogen was significantly higher with Mucuna than the other legumes. It also found that

sweet potato tuber yields were statistically similar in plots with Mucuna as they were for

NPK fertilizer treatment plots (Okapara et al., 2004). Mucuna also produced higher

biomass than the other two legumes, Pueraria phaseoloides and Centrosema pubescens

(Okapara et al., 2004).









Avena sativa, Lupinus angustifolius, and oats-lupine mix were evaluated for their

effect on nitrogen loss and availability in an organic cropping system. Using green

manures reduced the amount of N leaching during winter. Annual ryegrass was planted

as a subsequent crop to evaluate each green manure's ability to supply N back to the soil

after incorporation. Green manure treatment plots showed greater N uptake by ryegrass

than non-amended plots. There were no significant differences in biomass production

between treatments; however, lupine had significantly greater N concentration (Fowler et

al., 2004).

A study conducted in Cauca, Columbia evaluated the decomposition and nutrient

release of different green manures (Cobo et al., 2002). The volcanic-ash soils in this area

can be limited by low availability of N and P due to mineral particles in the soil organic

matter. These soils can also be deficient in some micronutrients, notably copper, zinc,

and cobalt. During a 20-week study, Mucuna was shown to release higher amounts of

both N and P compared with that of Tithonia, which released higher amounts of K, Ca,

and Mg (Cobo et al., 2002).

A study by Goyal (1999) found that SOM and mineralized carbon and nitrogen

increased when inorganic fertilizers were used in combination with farmyard manure,

wheat straw, or Sesbania bispinosa green manure. Soil amended with wheat straw and

inorganic fertilizers showed an increase in microbial biomass C from 147 mg/kg soil to

423 mg/kg soil (Goyal et al., 1999). The study concluded that treatments receiving a

combination of organic amendments and inorganic fertilizers resulted in a greater

increase in soil organic C and total N than soils that received only inorganic fertilizers.

Above-ground crop biomass was not incorporated into the soil, but instead was removed,









which may explain why soils amended with wheat straw, adding 3048 kg C/ha/yr, had

such a significant increase in microbial biomass C compared with soils that received only

inorganic fertilizers.

Cover Crops Used to Improve Crop Yield

Cover crops used as green manures help build soil fertility, improve soil structure,

and increase water-holding capacity. In northern Honduras, Mucuna used as a green

manure was shown to increase organic matter, infiltration, and porosity of the soil while

increasing maize yields (Buckles and Triomphe, 1999). It was also shown to reduce

drought stress and suppress weeds, especially broadleaf Mucuna was intercropped with

maize and used as mulch but not incorporated into the soil. Maize yields in rotation with

Mucuna were typically double that of those without it.

A study in Kenya using Tithonia diversifolia in combination with fertilizers

improved maize yields and phosphorus recovery (Nziguheba et al., 2002). Phosphorus is

a limiting nutrient for crop production in western Kenya as a result of small farm holders

with limited purchasing power and the high cost of inorganic fertilizers. Addition of

green manures do not increase total P in the system but can, in some cases, increase the

bioavailability of P already in the soil. Maize yields were greatest with the addition of P

fertilizer, but yield did increase with increasing additions of Tithonia. Maize yields in

combination with fertilizer and Tithonia were higher than with fertilizer or Tithonia

alone. However, maize yields and recovered P in above ground biomass were higher in

treatments where Tithonia was used compared with treatments that received only

fertilizer.

In another study in western Kenya, post-fallow maize yields increased with the

addition of Tithonia diversifolia and Crotalaria grahamiana (Smestad et al., 2002).









Crotalaria produced a greater biomass than Tithonia or weed fallow and also made the

greatest contribution to soil N and P.

In Ghana, yield of plantain was increased by the addition of Leucaena leucocephala

and Flemingia macrophylla (Banful et al., 2000). Higher yields were obtained by the

addition of Flemingia than with Leucaena. The presence of certain species of nematodes

was also found to be lower in Flemingia plots than Leucaena plots.

A study in Uganda evaluated yield and farmer perceptions for green manures in

maize-bean systems (Fischler and Wortmann, 1999). Crotalaria (Crotalaria ochroleuca),

Mucuna (Mucuna pruriens var. utilis), lablab (Dolichos lablab), and Canavalia (Canavalia

ensiformis) were used in short-term fallows as green manures. Farmers reported

improved soil fertility, moisture, and tilth. Weed suppression, erosion control, and higher

yields after the addition of the green manures were also noted, but lablab and Mucuna

were favored due to greater benefits and reduced labor requirements. Maize yields

increased 41% and bean yields 43% with the addition of Crotalaria compared with that of

weed fallow. Maize yield increased 60% with the addition of Mucuna and 50% with the

addition of lablab compared with a continuous maize cropping system.

A study in Bangladesh using Crotalariajuncea and Sesbania aculeata in

combination with urea-N on sugarcane increased cane yield 2 to 57% (Bokhtiar et al.,

2003). No fertilizers were added to the green manure crops. Concentrated

superphosphate, muriate of potash, gypsum, and zinc sulfate were applied to sugarcane

crops. Green manure additions increased total number of tillers and cane stalks. The

highest cane yield was produced by Sesbania in combination with 150 kg N/ha.









A study by Sangakkara et al. (2003) found that maize intercropped with beans

(Phaseolus vulgaris L.) produced maize yields similar to that of monoculture in the first

season and increased by the third season, where when intercropped with crotalaria, yields

decreased below that of monoculture. A study of Sesbania rostrata, Sesbania aculeata,

and Vigna radiata effects on soil properties and crop growth in a rice wheat cropping

system found that soil physical properties improved in green manure plots compared with

that of fallow (Mandal et al., 2003). Soil organic matter increased, bulk density

decreased, soil aggregation improved, and hydraulic conductivity improved especially in

S. rostrata treatment plots followed by S. aculeate and V. radiata. Total soil nitrogen was

also found to be higher in green manure treatment plots compared with fallow (Mandal et

al., 2003).

A study of the effects of green manures on nitrogen availability to organic sweet

corn found that lupine and lupine-mustard mix both significantly increased soil mineral N

by 30 to 45%, where ryegrass decreased soil mineral N by 33 to 43% (Hanly and Greg,

2004). Accumulation ofN by sweet corn was also significantly increased by both lupine

and lupine-mustard mix and significantly decreased by ryegrass treatment. However,

though yield of sweet corn was significantly reduced by ryegrass, lupine and lupine-

mustard mix did not significantly increase yield of sweet corn (Hanly and Greg, 2004).

A study on the effect of intercropping Sesbania cannabina on nitrogen levels for

growth and yield of sugarcane showed that Sesbania improved soil quality. Soil organic

carbon increased from 0.37% to 0.49%. Both Sesbania's ability to fix nitrogen and its

nutrient release upon decomposition of the incorporated biomass improved the soil

fertility in treatment plots. N fixation increased with age of plant with the greatest









increase at 60 days. Yield of sugarcane decreased when intercropped with Sesbania.

This result may have been due to competition for space; however, yield of the ratoon crop

significantly increased (Singh et al., 2003).

Micronutrient Deficiency of Plants Grown in Calcareous Soils

Nutrient availability is highly correlated with soil pH. At high soil pH Fe, Mn, Cu,

Zn, and B may be deficient for plant growth (Brady and Weil, 2002). Calcareous soils

contain high levels of free calcium carbonate (Brady and Weil, 2002), low organic

matter, and pH 7.5-8.5 (Zou et al., 2000). Micronutrient and phosphorus deficiencies are

common in calcareous soils (Misra and Tyler, 2000). Micronutrient availability may also

be affected by soil moisture (Misra and Tyler, 2000), organic matter (Brady and Weil,

2002) and plant exudates (Zou et al., 2000).

Low iron and zinc availability is common in calcareous soils (Misra and Tyler,

2000). Plant available iron is affected by organic compounds in the soil solution (Havlin

et al., 1999), thus soils low in organic matter may contain low plant available Fe. Soil pH

and bicarbonate also affect the availability of Fe, with the greatest deficiencies occurring

between 7.3 an 8.5 (Havlin et al., 1999). Organic matter and high soil pH also affect the

availability and absorption of Zn (Hacisalihoglu and Kochiam, 2003). Other factors such

as mycorrhizal fungi can affect the availability of iron and zinc in the soil solution (Liu et

al., 2000). Organic manure (animal manures and green manure) can supply

micronutrients to plants and may also mobilize soil metal cations by chelation and

completing with organic compounds, making them more available for plant uptake

(Savithri et al., 1999).

In a study by Chander (1997), the use of green manures improved organic matter,

soil microbial activity and productivity of the soil. Crop rotations increased organic






16


carbon and total nitrogen in the soil. Sesbania aculeate, a green manure, added the

greatest amount of soil organic carbon and total nitrogen when used in the rotation.

Nutrient cycling in the soil is affected by soil biota in the labile fraction of organic

matter, and therefore affected by soil management practices including crop rotations,

cover crops, green manures, tillage and fertilization (Chander et al., 1997).















OBJECTIVE

The objectives of this research were to 1) use cover crops to increase nutrient

availability in a calcareous soil, and 2) to improve growth of sweet potato [Ipomea

batatas (L.) Lam] in south Florida. Three cover crops, Sunnhemp, velvet bean, and

sorghum-sudan grass, were grown and incorporated into the soil prior to sweet potato

planting. Each cover crop was evaluated for its effect on nutrient availability in the soil

and average nutrient amount taken up by the cover crop. In addition to cover crop

treatments, sweet potato response to foliar applications of micronutrient fertilizer, iron

(Fe) and zinc (Zn) was evaluated in terms of effect on crop growth and yield. Iron and

zinc were chosen for application because of their limited availability in alkaline soil.















MATERIALS AND METHODS

Establishment of Field Experiment

The field experiment was carried out at a commercial vegetable farm in

Homestead, Florida in 2004. The experiment was set up using a split plot design with

three cover crop treatments and a control (fallow) as the main plots and three subplots for

foliar applications of iron and zinc. There were four replications of each treatment

combination for a total of 16 main plots and 48 subplots. The three cover crop treatments

were sorghum-sudan grass (Sorghum sudanense L.), sunnhemp (Crotalariajuncea L.) and

velvet bean (Mucuna pruriens). There were three subplots within each main plot; one

received foliar applications of chelated zinc, one received foliar applications of chelated

iron, and the third served as a control that received the water carrier without chemicals.

Cover Crop Management

The field site was disked several times between 23 and 30 Aug. 2004 in preparation

for cover crop planting. Soil samples were taken on 30 Aug. 2004 for each main plot

prior to cover crop planting. Cover crops were planted on 31 Aug. 2004. (Seeding rates

are shown in Table 1.) No fertilizer or irrigation was applied at any time to any of the

treatment plots during the cover crop growing season. Soil, plant tissue, and biomass

samples were taken on 7 Nov. 2004 prior to cutting cover crops. Cover crops were

mowed and tilled on 9 Nov. 2004 using a flail mower and a rototiller to minimize

carrying cover crops into adjacent treatment plots. The site was tilled a second time on









24 Nov. 2004. A third set of soil samples was collected 9 weeks after cover crop

incorporation on 19 Feb. 2005 before sweet potato flowering.

Table 1. Seedling rates for cover crops.
Cover Crop Seeding Rate

lb/ac kg/ha

Sorghum sudan 25 28

Sunnhemp* 91 100

Velvet Bean 80 88


*Inoculant used: Cowpea/Peanut/Lespedeza.

Sweet Potato Crop Management

The field was disked and prepared for sweet potato planting on 15 Dec. 2004.

Sweet potatoes were planted on 20 Dec. 2004 with 1.52 m row spacing using slip cuttings

of approximately 30 cm. Soil samples were collected on 19 Feb. 2005 after cover crop

incorporation. Plant tissue samples using newly mature leaves were collected on 26 Mar.

2005 before flowering just prior to foliar applications of micronutrient fertilizers, and

again on 03 Apr. 2005 at flowering, 1 week after application ofFe (Becker Underwood,

EDTA, 10% Fe) and Zn (Ciba, EDTA, 14% Zn). The sweet potatoes were harvested on

30 June 2005.

The grower fertilized the sweet potato crop with a 7-10-15 fertilizer. It also

contained several micronutrients, 6.05% S, 1.1% Ca, 3% Mg, 0.45% Fe, and 0.06% B.

Other additions made to the sweet potato crop by the grower were 16 ounces of Pencap

M and 1 Quart of Moniter after planting, 6 ounces perthrium and 1 quart of indo-sulfin

two weeks after planting, and 5 ounces of capture 4 weeks after planting. Occasionally

the grower may add snail bait, Oracal, to the sweet potato crop.










The table of contents and lists of tables and figures are treated by Word as if they

were single objects. If you update a table of contents or list, you will discover that the

entire table of contents and all lists will be updated, not just one entry. If minor changes

are required to a table of contents or list after it has been created, then remember that

these changes will need to be made every time the table of contents or list is recreated.

Sample Collection and Chemical Analysis

Soil samples were collected prior to cover crop planting, prior to cutting cover

crops, and 9 weeks after incorporation (Table. 2). Samples were collected from 0-15 cm

depth, air-dried and sieved (2 mm). Samples were analyzed for total N and C using a

CNS Analyzer (Vario MAX CNS, Germany), and for AB-DTPA extractable P, K, Mg,

Fe, Zn, B, Mn, and Cu using an atomic absorption spectrometer (AAS, AA-6300,

Shimadzu, Japan).

Biomass samples were collected for cover crops prior to cutting (Table. 2). Whole

plants within a 103-cm2 sampling area were collected from each treatment plot and

fallow plot. Wet and dry weights were recorded and used to determine the biomass of

each treatment in Mg/ha. Nematode samples were collected prior to planting cover

crops, prior to cutting cover crops, and 9 weeks after incorporation of cover crops.

Table 2. Sample collection type and dates.

Sample Type Collection dates
8/31/2004 11/07/2004 2/19/2005 3/26/2005 4/3/2005 6/30/2005
Soil
Plant Tissue
Biomass
Nematode
Harvest









Cover crop tissue samples were collected for chemical analysis prior to cutting

(Table 2). Whole plants within sampling area were collected. Tissue samples for sweet

potato were collected prior to flowering, at flowering prior to foliar applications of

micronutrient fertilizers, and again 1 week after foliar applications of micronutrient

fertilizers. A final plant tissue sample was collected prior to sweet potato harvest. Newly

mature leaves were collected for sweet potato tissue samples. Samples were washed with

detergent (Liquinox), diluted HC1, and rinsed with DDI water. Then samples were dried

at 70 C and ground. Samples were analyzed for N and C using a CNS Analyzer. The

concentrations of P, K, Mg, Fe, Zn, B, Mn, and Cu, were measured by dry ash and

determined using the AAS.

Statistical Analysis

All data were analyzed with SAS statistical software (version 8.1, SAS Inst. Inc.,

Cary, NC). Duncan's multiple range test was used to separate the means between

treatment plots and sampling dates.















RESULTS AND DISCUSSION

Cover Crop Biomass

Cover crop heights were measured 28, 42, 55, and 68 days after planting (DAP)

(Table 3). Sorghum-sudan produced the greatest amount ofbiomass at 13.01 Mg/ha,

followed by sunnhemp at 8.01 Mg/ha and velvet bean at 5.22 Mg/ha (Table 4, Fig. 1).

The control plot (fallow with weeds) produced the least amount of biomass, 3.10 Mg/ha,

of any treatments. However, both sunnhemp and velvet bean did not reach their expected

biomass production. The short days during the time of year they were planted caused

both cover crops to flower early (Gardner et al., 1985), which slowed their growth. In

addition, velvet bean is typically grown as a summer cover crop and did not grow well

due to the short days during the fall. It also needs a longer time to establish and build

biomass than the other cover crops used in this study. Other studies in south Florida have

shown sunnhemp and velvet bean to produce as much as 12.1 to 19.7 Mt/ha and 7.8 to

9.95 Mt/ha, respectively (Wang et al., 2003; 2005).

Table 3. Cover crop heights (cm) measured at 28, 42, 55, and 68 days after planting (DAP).
Cover crop/weed 28DAP 42DAP 55DAP 68DAP


Sorghum sudan 25-31 91-97 173-183 173-183

Sunnhemp 25-31 61-71 102-112 112-122

Velvet Bean 25-31 25-36 31-41 36-41


DAP-Days after planting.










Table 4. Cover crop and weed biomass yield (Mg/ha) collected 68DAP (07 Nov. 2004).
Cover crop biomass Dry weight

------Mg/ha------

Fallow with weeds 3.10*'

Sorghum-sudan 13.01a

Sunnhemp 8.01b

Velvet bean 5.22


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).


Cover Crop Biomass

14 a
12
10

S6
4



Weeds Sorghum- Sunnhemp Velvet Bean
sudan





Figure 1. Cover crop biomass collected on 07 Nov. 2004 prior to incorporation of cover
crops. Means with the same letter are not significantly different by Duncan's
multiple range test (P 0.05).

Nutrient Concentrations in Cover Crop

Plant tissue samples were taken for each treatment plot 68 DAP. Sunnhemp

samples contained a significantly higher concentration ofN, 17.47 g/kg, than sorghum-

sudan grass but not significantly different from velvet bean or fallow with weed plots

(Table 5, Fig. 2). Velvet bean and weeds both had a significantly higher concentration of










N, 14.26 g/kg and 13.16 g/kg respectively, than sorghum-sudan grass, but not

significantly different from each other (Table 5, Fig. 2). Both sunnhemp and velvet bean

are legumes that fix N, which attributes the greater concentrations ofN in sunnhemp

compared with the other treatments; however, though velvet bean is also a legume it

contained only slightly higher concentrations of N than fallow with weed plots (Table 5,

Fig. 2).

Table 5. Nutrient concentrations in cover crop tissues collected on 7 Nov. 2004.


Cover crop/weeds N C P K Mg

----------------------------------- g/kg----------------------

Fallow with weeds 13.16ba4 405.18 6.84 25.29 2.74

Sorghum-Sudan 7.44b 420.65 a 3.39b 17.97b 1.79

Sunnhemp 17.47" 414.70 a 4.04 b 16.62b 2.62 a

Velvet Bean 14.26ba 401.83 b 6.68 15.85 b 2.22 b


Fallow with weeds

Sorghum-sudan

Sunnhemp

Velvet bean


Fe Zn B

---------------------------------------- mg/kg

62.82a 52.39b 101.02a

27.63 b 41.03 b 74.62 b

41.02b 30.73 c 97.02a

40.50 b 86.36 a 92.63 a


Mn Cu

--------------------------

30.04 b 11.31 b

14.29 6.39

46.27 7.56

27.47 b 26.87a


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).

Carbon concentrations were greatest in sorghum-sudan grass and sunnhemp

samples (Table 5, Fig. 3). These cover crops also produced the greatest amount of










biomass compared with other treatment plots and therefore would have accumulated

more carbon in their biomass through photosynthesis (Brady and Weil, 2002). Sunnhemp

samples contained significantly higher concentrations of manganese (Mn) compared with

other treatments (Table 5, Fig. 4), which could contribute significant quantities of Mn to

a subsequent crop.


Macronutrients

30

25

20 ---- Weeds
S1 ESorghum-sudan
l ESunnhemp
10 a a Velvet Bean
a a
5 b acab


N P K Mg





Figure 2. Cover crop plant tissue macronutrient concentrations in samples collected on 07
Nov. 2004 prior to cutting cover crops. Means with the same letter are not
significantly different by Duncan's multiple range test (P 0.05).

Velvet bean samples had significantly higher concentrations of Zn and Cu, 86.36

mg/kg and 26.87 mg/kg respectively, compared with other treatments (Table 5, fig. 4).

Velvet bean tissue samples also contained significantly greater concentrations ofP than

either sorghum-sudan grass or sunnhemp samples. This result suggests that given a large

production of biomass under different growing conditions with longer day length, velvet

bean could contribute significant quantities ofZn, Cu, and P to a subsequent crop.











Carbon


425
420
415
410
S405
400
395
390


b


Weeds Sorghum- Sunnhemp Velvet Bean
sudan


Figure 3. Cover crop plant tissue carbon concentration in samples collected on 07 Nov.
2004 prior to cutting cover crops. Means with the same letter are not
significantly different by Duncan's multiple range test (P 0.05).


Micronutrients

120


a60
100

80 Weeds

60 Sorghum-sudan
E bb Sunnhemp
40 -b Velvet Bean

20


Fe Zn B Mn Cu


Figure 4. Cover crop plant tissue micronutrient concentrations in samples collected on 07
Nov. 2004 prior to cutting cover crops. Means with the same letter for same
nutrient are not significantly different by Duncan's multiple range test (P
0.05).

Total Nutrients in Cover Crop Biomass

Sunnhemp contained significantly more N in its total biomass than any other

treatment (Table. 6, Fig. 5). Since sunnhemp is a legume, it is able to fix its own N and









therefore contained more N in its biomass than the greater biomass produced by

sorghum-sudan grass. Sorghum-sudan and velvet bean contained significantly more N in

their biomass than weed biomass, but were not significantly different from each other

(Table. 6, Fig. 5). Sunnhemp contained significantly more Mn in its total biomass

compared with the other treatments. It also had a significantly higher concentration of

Mn in its plant tissue than other treatments (Table 5, Fig. 4). The total amount of Mn in

each treatment's biomass did not correspond with the amount of biomass produced by

each treatment (Table 4, Fig. 1). Manganese is a key nutrient used for N transformation,

metabolism, and assimilation (Brady and Weil, 2002). This may be why sunnhemp

contained significantly more Mn in its biomass, since sunnhemp is a legume and fixes N.


Total Biomass Macronutrients

250

200 -
U Weeds
S150 b
c I Sorghum-sudan
.r 100 Ub Sunnhemp
U Velvet bean
50 c a

0
N P K Mg




Figure 5. Cover crop total biomass macronutrients. Means with the same letter for same
nutrient are not significantly different by Duncan's multiple range test (P
0.05).









Table 6. Total nutrient amounts in cover crop biomass.


Cover crop/weeds N C P K Mg

---------------------------------kg/ha---------------------- -----

Fallow with weeds 40.520 1255.300 20.97b 78.26b 8.51b

Sorghum-Sudan 96.10b 5468.50 a 44.17a 232.98a 23.16"

Sunnhemp 180.36" 3325.30 b 32.60 b" 134.14b 21.07 a

Velvet Bean 74.40b 2032.90 36.03 ba 87.60 b 11.43 b


Fallow with weeds

Sorghum-sudan

Sunnhemp

Velvet Bean


Fe Zn B Mn Cu

--------------------------------g/ha---------------------------

190.30b 160.340 315.50b 94.810 34.68

357.68a 526.11 975.92a 180.31 b 84.87ba

330.69a 246.04b 778.50a 368.66a 61.010b

207.25 b 467.85ba 478.10 b 135.29 b 138.19a


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).

Sorghum-sudan grass contained more C, K, Mg, Fe, and B in its total biomass than

other treatments, followed by sunnhemp, velvet bean, and weeds (Table 5, Fig. 6). The

total amount of these nutrients in each treatment's biomass corresponds with the amount

of biomass produced by each treatment, where sorghum-sudan produced the greatest

amount of biomass, followed by sunnhemp, velvet bean and weeds (Table 4, Fig. 1).

Sorghum-sudan grass contained significantly more P in its biomass than the weed plots,

but not significantly more than sunnhemp or velvet bean plots (Table 5, Fig.6). Sorghum-











sudan grass total biomass contained greater amounts of all nutrients except for N, Mn,

and Cu.


Total Biomass Carbon


6000

5000

4000

3000

2000

1000

0


c


Weeds Sorghum-sudan Sunnhemp Velvet bean


Figure 6. Cover crop total biomass carbon. Means with the same letter are not
significantly different by Duncan's multiple range test (P 0.05).


Total Biomass Micronutrients

1200

1000
a
800 l Weeds
0 Sorghum-sudan
600 a
ba b *Sunnhemp
400 aa b a EVelvet bean
b b b
200 -

0
Fe Zn B Mn Cu


Figure 7. Cover crop total biomass micronutrients. Means with the same letter are not
significantly different by Duncan's multiple range test (P 0.05).


c









The total amount of Zn and Cu in each treatment's biomass did not correspond with

the amount of biomass produced by each treatment (Table 6; Fig. 7). Sorghum-sudan

grass contained more Zn in its total biomass than other treatments, but not significantly

more than velvet bean (Table 6; Fig. 7). Velvet bean also contained significantly higher

zinc and copper concentrations in its plant tissue than any of the other treatments except

for sorghum-sudan grass. This result suggests that velvet bean, which contained

significantly more Zn and Cu with less biomass than sunnhemp or weeds, may be able to

access additional pools of soil Zn and Cu. Thus, under other growing conditions when the

days are longer, velvet bean could contribute large amounts of Zn and Cu to a succeeding

crop.

Soil Nutrients Prior to Planting Cover Crops

Concentrations of soil nutrients within each of the treatment plots were uniform

prior to planting cover crops. There were no significant differences between treatment

plots prior to planting cover crops for either total N or total C (Table 7; Figs. 8 and 9).

There were also no significant differences found between treatment plots for AB-DTPA

extractable P, K, Mg, Fe, Zn, B, Mn, or Cu (Table 7; Figs. 10 and 11).

Soil Nutrients Prior to Cutting Cover Crops

There were no significant differences in total P, or extractable P, Zn, Mn, and Cu

between treatment plots prior to cutting cover crops (Table 8; Figs. 12, 14, and 15). There

were also no significant differences in these nutrients in samples collected prior to

planting cover crops and prior to cutting cover crops, except for manganese, which was

greater in soil samples prior to planting than before cutting cover crops. Sunnhemp,

velvet bean and weed plots had significantly more carbon removed than sorghum-sudan

plots (Table. 8, Fig. 13); however, there were no significant differences found between











samples prior to planting or prior to cutting cover crops for any of the treatment plots


(Tables 10, 11, 12, and 13).


Total Nitrogen


1.8
1.6
1.4
1.2
I 1.0
m 0.8
0.6
0.4
0.2
0.0


Weeds Sorghum- Sunnhemp Velvet Bean
sudan


Figure 8. Total N in soil collected on 31 Aug. 2004 prior to planting cover crops. Means
with the same letter are not significantly different by Duncan's multiple range
test (P 0.05).


Total Carbon


70
60
50
I 40
o 30
20
10
0


Weeds Sorghum- Sunnhemp Velvet Bean
sudan


Figure 9. Total C in soil collected on 31 Aug. 2004 prior to planting cover crops. Means
with the same letter are not significantly different by Duncan's multiple range
test (P 0.05).










Table 7. Total N, C and AB-DTPA extractable nutrients (P, K, Mg, Fe, Zn, B, Mn, and
Cu) in soil samples collected on 31 Aug. 2004 prior to planting cover crops.
Cover crop/weeds N C P K Mg Fe Zn B Mn Cu

---- g/kg ----- ----------------------------- mg/kg ------------------------

Fallow with

weeds 1.48a 54.70a 33.58a 148.63a 42.55a 32.59a 10.35a 0.16a 6.35a 55.80a

Sorghum-Sudan 1.54a 57.43a 37.82a 147.57 43.63 34.11 10.82 0.17a 8.49 57.28

Sunnhemp 1.38a 53.47a 31.67a 138.33a 41.55a 31.77a 10.50a 0.17a 7.10a 54.94a

VelvetBean 1.41a 54.40a 39.35 a 141.73 a 42.09 a 32.78 a 10.70 a 0.17a 6.19a 58.07 a


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).

Table 8. Extractable nutrient concentration in soil samples collected on 07 Nov. 2004
prior to cutting cover crops.
Cover crop/weeds N C P K Mg Fe Zn B Mn Cu

---- g/kg ----- ---------------------------------- mg/kg ----------- -------------

Fallow with

weeds 1.20 53.64 ba 39.11a 54.34a 36.49 30.99b 10.68a 0.08 3.74a 57.68a

Sorghum-sudan 1.35a 55.18a 39.12a 38.19 b 33.96a 32.45a 10.50 0.07 ba 3.40a 60.13a

Sunnhemp 1.33 52.94ba 40.78a 49.27 ba 36.38 32.00 ba 10.69a 0.06b 3.55a 59.83a

Velvet bean 1.26 a 51.44b 33.90 a 52.73 a 36.96 a 31.23 ba 10.63 a 0.08 a 3.33 a 59.62 a


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).

There was a greater amount of potassium removed from sorghum-sudan grass and

sunnhemp plots than velvet bean or weed plots (Table 8; Fig. 14), in part because both

cover crops produced more biomass than velvet bean or weeds (Table 4; Fig. 1) and as a

consequence contained more potassium in their total biomass (Table 6; Fig. 5). There was

a greater amount of iron removed from sunnhemp and velvet bean plots than from

sorghum-sudan plots (Table. 8). This result may be attributable to legumes requiring iron










for the enzyme nitrogenase, which is necessary for nitrogen fixation (Brady and Weil,

2002). However, there were no significant differences found between samples prior to

planting cover crops and prior to cutting cover crops for any cover crop treatment plots

(Table 10; 11, 12, and 13).


AB-DTPA Soil nutrients


C1 a


* Weeds
* Sorghum-sudan
* Sunnhemp
E Velvet Bean


Figure 10. AB-DTPA extractable P, K, and Mg in soil collected on 31 Aug. 2004 prior to
planting cover crops. Means with the same letter for same nutrient are not
significantly different by Duncan's multiple range test (P 0.05).

Sorghum-sudan grass and sunnhemp plots contained less boron than velvet bean or

weed plots (Table 8). Both sorghum-sudan grass and sunnhemp produced more biomass

(Table 4) and therefore contained more boron in their total biomass than velvet bean or

weeds (Table 6), and in turn took up more boron from the soil.


160
140
120
100
80
60
40
20
0


aa a a a a


I











AB-DTPA Soil Nutrients


Sa


aa aa
a ,a


aaaa


* Weeds
* Sorghum-sudan
1Sunnhemp
E Velvet Bean


Fe Zn


B Mn Cu
B Mn Cu


Figure 11. AB-DTPA extractable Fe, Zn, B, Mn, and Cu in soil collected on 31 Aug.
2004 prior to planting cover crops. Means with the same letter for same
nutrient are not significantly different by Duncan's multiple range test (P
0.05).

Table 9. Extractable nutrient concentration in soil samples collected on 19 Feb. 2005
after incorporation of cover crops.


Cover crop/weeds


C P K Mg


Fe Zn Mn Cu


----- g/kg -----


------------------------------------ mg/kg------------------------------


weeds 2.39a 57.65 a 56.73 a 198.69a 17.29b

Sorghum-sudan 2.20a 59.11 a 58.79a 154.39 ba 17.06

Sunnhemp 2.30a 55.16a 59.75 a 150.22ba 17.34b

Velvet bean 2.39 a 53.60a 53.58 a 129.43 b 17.53 a


34.27 ba 30.03a 3.32a 49.52a

35.21 a 29.85a 3.55a 51.13a

34.29 ba 29.61 a 4.00a 50.39 a

30.68b 28.91 a 3.92a 47.69a


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).


60

50

40

S30
E
20


10 +


F ,llli il!i









Table 10. Extractable soil nutrient concentrations in weed plots compared between
sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov.
2004, and after incorporation on 19 Feb. 2005.
Sampling dates N C P K Mg Fe Zn Mn Cu

----- g/kg ----- ------------------------------- mg/kg ------------------------------


1.48ba 54.70a 33.58b 148.63a

1.20b 53.64a 39.11b 54.34b


2.39a 57.65 a 57.73a 198.69a 17.29c


32.59ba 10.35b 6.35a 55.80ba


36.49b 30.99b 10.68b 3.74b


57.68a


34.27a 30.03a 3.92b 49.52b


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).

Table 11. Extractable soil nutrient concentrations in sorghum-sudan grass plots compared
between sampling dates: before planting on 31 Aug. 2004, before cutting on 7
Nov. 2004, and after incorporation on 19 Feb. 2005.
Sampling dates N C P K Mg Fe Zn Mn Cu

----- g/kg ----- ------------------------------ mg/kg ----------------------------

31 Aug 2004 1.54a 57.43" 37.82b 147.57" 43.63" 34.11" 10.82b 8.49a 57.28"

07 Nov 2004 1.35" 55.18a 39.12b 38.19b 33.96b 32.45" 10.50b 4.00b 60.13"

19 Feb 2005 2.21" 59.11 a 58.78a 154.39" 17.06c 35.21" 29.85" 3.40b 51.13b


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).


31 Aug 2004

07 Nov 2004

19 Feb 2005










Table 12. Extractable soil nutrient concentrations in sunnhemp plots compared between
sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov.
2004, and after incorporation on 19 Feb. 2005.
Sampling dates N C P K Mg Fe Zn Mn Cu

----- g/kg ----- ------.---------------------------.. mg/kg ------------------------------


1.37b 53.47a 31.67b 138.33" 41.55"

1.33b 52.94 a 40.78b 49.29b 36.38b

2.30" 55.16a 58.75a 150.22" 17.34c


31.77" 10.50b 7.10" 54.94ba

32.00a 10.69b 3.55b 59.83"

34.30" 29.61" 3.87b 50.40b


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).

Table 13. Extractable soil nutrient concentrations in velvetbean plots compared between
sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov.
2004, and after incorporation on 19 Feb. 2005.
Sampling dates N C P K Mg Fe Zn Mn Cu


----- g/kg -----


1.41b 54.40a 39.35a

1.26b 51.44a 33.90a

2.39a 53.59a 53.58a


------------------------------------- mg/kg--------------------------------


141.73a 42.09a 32.78a 10.70b 6.19a 58.07a

52.73b 36.96b 31.23a 10.63b 3.33b 60.13a

129.43a 17.53' 30.68a 28.90a 3.32b 47.69b


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).


31 Aug 2004

07 Nov 2004

19 Feb 2005


31 Aug 2004

07 Nov 2004

19 Feb 2005











Total Nitrogen

1.6
1.4 a




S0.6m-
0.4
0.2
0.0
Weeds Sorghum- Sunnhemp Velvet Bean
sudan






Figure 12. Total nitrogen in soil collected on 07 Nov. 2004 prior to cutting cover crops.
Means with the same letter are not significantly different by Duncan's
multiple range test (P 0.05).


Total Carbon

60 ba a ba b

50

40

--. 3o

20 -

10

0
Weeds Sorghum- Sunnhemp Velvet Bean
sudan






Figure 13. Total carbon in soil collected on 07 Nov. 2004 prior to cutting cover crops.
Means with the same letter are not significantly different by Duncan's
multiple range test (P 0.05).













AB-DTPA Soil Macronutrients

60
a

50

a
40
a a a

Sa1 Weeds
30 Sorghum-sudan
30
ME 1Sunnhemp


20


10


0
P K Mg



Figure 14. AB-DTPA extractable macronutrients in soil collected on 07 Nov. 2004 prior
to cutting cover crops. Means with the same letter for same nutrient are not

significantly different by Duncan's multiple range test (P 0.05).


AB-DTPA Soil Micronutrients


70


60


50


40 Weeds
U Sorghum-sudan
b a ba ba Sunnhemp
30 E Velvet Bean


20





0

Fe Zn B Mn Cu



Figure 15. AB-DTPA extractable micronutrients in soil collected on 07 Nov. 2004 prior
to cutting cover crops. Means with the same letter for same nutrient are not
significantly different by Duncan's multiple range test (P 0.05).









Soil Nutrients at 9 Weeks After Incorporation of Cover Crops

There were no significant differences found between treatment plots for total

nitrogen, total carbon, or AB-DTPA phosphorus, zinc, manganese, or copper after

incorporation of cover crops (Table 9; Fig. 16, and 17). There were significant increases

for soil total nitrogen, AB-DTPA phosphorus and zinc after incorporation of cover crops

compared with prior to planting cover crops; however, there were no significant

differences for these nutrients between treatment plots (Tables 10, 11, 12, and 13). This

result may be in part due to fertilizer added by the grower to the field site during previous

plantings and to the subsequent sweet potato crop.


Soil Total Nitrogen


Weeds Sorghum- Sunnhemp Velvet Bean
sudan


Figure 16. Total N in soil samples collected on 19 Feb. 2005 after incorporation of cover
crops. Means with the same letter are not significantly different by Duncan's
multiple range test (P 0.05).







40



Soil Total Carbon

70

60 a a

50

cn 40
S30
20

10

0
Weeds Sorghum- Sunnhemp Velvet Bean
sudan


Figure 17. Total C in soil samples collected on 19 Feb. 2005 after incorporation of cover
crops. Means with the same letter are not significantly different by Duncan's
multiple range test (P 0.05).


AB-DTPA soil Nutrients

250


200

ba ba
150 U Weeds
N Sorghum-sudan
1E lSunnhemp
100 N Velvet Bean

aaa
50
bcba

0
P K Mg


Figure 18. AB-DTPA extractable soil nutrients in samples collected on 19 Feb. 2005
after incorporation of cover crops. Means with the same letter for same
nutrient are not significantly different by Duncan's multiple range test (P
0.05).









There was no significant difference in extractable K prior to planting or after

incorporation of cover crops for any of the treatment plots except that velvet bean plots

contained significantly less potassium than fallow with weed plots (Table 9). All

treatment plots contained significantly less K prior to cutting cover crops compared with

either pre-planting or after incorporation of cover crops (Tables 10, 11, 12, and 13). This

result may be because K is readily leached from the soil. Potassium also has a higher

leaching capacity in soils with less negatively charged cation exchange sites (Brady and

Weil, 2002). However, K leaching is reduced in soils where Ca2+ and Mg2+ are present,

such as in limed soils, suggesting that in a higher pH calcareous soil K leaching would be

reduced (Brady and Weil, 2002). The likely explanation for the significant reduction in

plant available K prior to incorporation of the cover crops is that plants tend to take up

soluble K in greater amounts than needed for plant growth (Brady and Weil, 2002)

suggesting that this reduction in K was simply due to plant uptake.

In all treatment plots, extractable Mg was significantly lower after incorporation of

cover crops compared with prior to cutting or prior to planting the cover crops (Tables

10, 11, 12, and 13). After incorporation of cover crops, velvet bean plots contained

significantly more extractable Mg than all other treatment plots, and sunnhemp and

fallow with weed plots contained significantly more Mg than sorghum-sudan grass plots

(Table 9).

After incorporation of cover crops, Sorghum-sudan grass plots contained

significantly more extractable Fe than velvet bean plots, but not significantly different

that sunnhemp or fallow plots (Table 9; Fig. 19). There was no significant difference in

extractable Fe concentration before planting, after planting, or after incorporation of










cover crops in velvet bean, sorghum-sudan, or sunnhemp plots. However, before

planting and after incorporation in fallow with weed plots, iron was significantly higher

than before cutting cover crops (Table 10). Both Mg and Mn were greater in all treatment

plots prior to planting cover crops compared with before cutting or after incorporation of

cover crops (Tables 10, 11, 12, and 13; Figs. 18 and 19). Plant available Cu was

significantly lower for all treatments after incorporation of cover crops compared with

before planting cover crops (Tables 10, 11, 12, and 13).


AB-DTPA Soil Micronutrients

60

a
50

40
ba a ba E Weeds
Cn b
0 a aia a m Sorghum-sudan
Cn 3o
E 30 Sunnhemp
N Velvet Bean
20

10
aaaa
0
Fe Zn Mn Cu


Figure 19. AB-DTPA extractable soil nutrients in samples collected on 19 Feb. 2005
after incorporation of cover crops. Means with the same letter for same
nutrient are not significantly different by Duncan's multiple range test (P
0.05).

Soil Nematodes

There were minimal counts of Aphelenchus, Belonolaimus, Pratylenchus,

Quinisulcius, and Scutellonema found in all treatment plots. The highest nematode

counts were for Helicotylenchus, Meloidogyne (root-knot nematode), and

Paratrichodorus in all treatment plots. The total number of nematodes, for all species











counted, decreased after incorporation of cover crops. In addition, velvet bean decreased

the number of Helicotylenchus and Meloidogyne during its growing period with a

significant decrease in the number of Meloidogyne after incorporation of velvet bean.

Fallow plots had a significant decrease in the number of Meloidogyne and

Paratrichodorus after incorporation. Sunnhemp plots also had a significant decrease in

the number of Paratrichodorus present in the soil after incorporation. Sorghum-sudan

showed significant decreases in Helicotylenchus, Meloidogyne, Quinisulcius, and

Paratrichodorus species present in the soil after incorporation (Table 14; Figs. 20, 21, 22,

and 23).


Fallow
40
a
35

30

25

20 -

15 ba

10 -a

5 -
a a b aa aaa a
0aa a aa a Bb a aa a


.o. o .


Spre-planting
* pre-cutting
" after incorporation


Figure 20. Soil nematodes counted in fallow plots. Samples collected prior to planting
cover crops on 31 Aug. 2004, before cutting 07 Nov. 2004, and after
incorporation on 19 Feb. 2005. Means with the same letter for same nematode
are not significantly different by Duncan's multiple range test (P 0.05).



















Table 14. Nematode population (direct count) in soil samples collected prior to planting cover crops on 31 Aug, before cutting 07

Nov, and after incorporation on 19 Feb. 2004.
Fallow with weeds Aphelenchus Belonolaimus Criconemella Helicotylenchus Meloidogyne Pratylenchus Quinisulcius Scutellonema Paratrichodorus

pre-planting 0Oa 1.5 a 0.75 a 8.25 a 14.25ba 0 a 1 a 1.25 a 23 a

pre-cutting Oa 3.5 a 1.25 a 10.25 a 37 a 0 a 0.5 a 0.5 a 14.25 a

after incorporation 0.5a 0 a 0a 0 a 0.5b 1.5 a 0a 0 a 0b


Sorghum

pre-planting

pre-cutting

after incorporation




Sunnhemp

pre-planting

pre-cutting

after incorporation




Velvet

pre-planting

pre-cutting

after incorporation


Aphelenchus Belonolaimus

0 0.25a

0a 1.75 a

0.5a 0 a


Aphelenchus

0a

0a

0a




Aphelenchus

0a

0a

0.5 a


Belonolaimus

2.5

3.75 a

0a




Belonolaimus

0 b

6.75 a

0 b


Criconemella

1.75a

0.25 a

0a




Criconemella

0.75 a

0a

0a




Criconemella

la

0a

0a


Helicotylenchus

20.5 a

29 a

Ob




Helicotylenchus

16 a

36.75 a

0a




Helicotylenchus

34.25 a

25.25 a

0a


IMeloidogyne

27

36 a

0.5 b




Meloidogyne

18.5 a

5.5 a

0.5 a




Meloidogyne

16.25 a

6.25 b

1b


Pratylenchus

2.25 a

1.25 a

0.5 a




Pratylenchus

1 a

0.75 a

0a




Pratylenchus

0.25 a

0.25 a

la


Quinisulcius

2.25 a

Ob

0Ob




Quinisulcius

0.5 a

3.5 a

0 a




Quinisulcius

3.25 a

0.5 a

0a


Scutellonema

3 a

0.5 a

0a




Scutellonema

2.25a

1.25a

0a




Scutellonema

1.25 a

0.5 a

0a


Paratrichodorus

12.75 a

9ba

Ob



Paratrichodorus

8.75 a

5.75ba

0b




Paratrichodorus

9.5 a

7.75 a

0a


tMeans with the same letter for same nematode are not significantly different by Duncan's multiple range test (P 0.05).













Sorghum-sudan
40

35

30

25
0 -a pre-planting
0 pre-cutting
15 E after incorporation

10

5 a
a a
a aa a a a8 h a a I a aa b

'_, M ,C A


2^^e^


Figure 21. Soil nematodes counted in sorghum-sudan plots. Samples collected prior to
planting cover crops on 31 Aug. 2004, before cutting 07 Nov. 2004, and after
incorporation on 19 Feb. 2005. Means with the same letter for same nematode
are not significantly different by Duncan's multiple range test (P 0.05).


Sunnhemp
40
a
35

30

25
0 pre-planting
+j 2 0 ------------ a------------
20 I pre-cutting
S15 I ___ after incorporation

10

5
Sa a









Figure 22. Soil nematodes counted in sunnhemp plots. Samples collected prior to planting
cover crops on 31 Aug. 2004, before cutting 07 Nov. 2004, and after
incorporation on 19 Feb. 2005. Means with the same letter for same nematode
are not significantly different by Duncan's multiple range test (P 0.05).












Velvet Bean
40

35 a

30

25

20
a
15

10

5 a-------- [a ---------d ^
a a a
aa b b a a a aa a
o0
4 *\Z)~ ,cfi ( ~c

,c`r~ c 9'&~ o# O
(5'e .(` 'z.


* pre-planting
E pre-cutting
* after incorporation


Figure 23. Soil nematodes counted in velvet bean plots. Samples collected prior to
planting cover crops on 31 Aug. 2004, before cutting 07 Nov. 2004, and after
incorporation on 19 Feb. 2005. Means with the same letter for same nematode
are not significantly different by Duncan's multiple range test (P 0.05).

Nutrient Concentrations in Sweet Potato Leaves

Sweet potato leaf tissue collected from sunnhemp plots contained greater


concentrations of N, P, K, and Cu, but contained the lowest concentrations of Fe and Mg


compared with tissue samples collected from other treatment plots (Table 15; Figs. 24


and 26). Tissue samples collected from sorghum-sudan grass plots contained


significantly more C than velvet bean plots, but not significantly more than sunnhemp or


fallow with weed plots (Table 15; Fig. 25).


There was no significant difference in Fe concentration of sweet potato leaf tissue


in Fe subplots within each cover crop treatment plot after foliar application of Fe;


however, sunnhemp plots did have a lower concentration of Fe than other treatment plots


(Table 16; Fig. 27). There was also no significant difference in Zn concentration of









sweet potato leaf tissue in Zn subplots within each cover crop treatment plot after foliar

application of Zn (Table 17; Fig. 28).

Table 15. Sweet potato leaf nutrient concentrations in samples collected on 03 April
2005, 1 week after foliar applications of Fe and Zn.
Cover crop/weeds N C P K Mg


Fallow with weeds

Sorghum-Sudan

Sunnhemp

Velvet Bean


Fallow with weeds

Sorghum-sudan

Sunnhemp

Velvet Bean


---------------------------------- g/kg ----------------------------------

28.85b 421.91ba 2.01ba 27.29b 3.76a

29.01b 425.78a 1.98b 28.02b 3.56a

34.12" 422.49 ba 2.12" 30.14a 3.10b

30.84 b 419.99b 2.08 ba 28.61 ba 3.60a


------------------------- mg/kg ------------------- ----

97.01 29.97a 1483.69a 282.30b

97.09a 27.35a 1379.64a 295.76b

63.57b 30.94a 1302.15a 335.45a

85.73 ba 29.39 1448.42 a 302.48 b


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).







48



40

35 a

30 b


25 Weeds
0 nESorghum-sudan
C-b 20
lU Sunnhemp
15 E Velvet Bean

10

5
ba b a ba


N P K Mg

Figure 24. Sweet potato leaf macronutrient concentrations in samples collected on 03
April 2005 1 week after foliar applications of Fe and Zn. Means with the same
letter for same nutrient are not significantly different by Duncan's multiple
range test (P 0.05).


Carbon

427
426
425
424
423
S422
421 b
420
419
418
417
Weeds Sorghum- Sunnhemp Velvet Bean
sudan





Figure 25. Sweet potato leaf carbon concentrations in samples collected on 03 April 2005
1 week after foliar applications of Fe and Zn. Means with the same letter are
not significantly different by Duncan's multiple range test (P 0.05).











1600
a
a
1400

1200

1000
U Weeds
U Sorghum-sudan
800
E Sunnhemp
U Velvet Bean
600

400

200

0
Fe Zn Mn Cu

Figure 26. Sweet potato leaf micronutrient concentrations in samples collected on 03
April 2005 1 week after foliar applications of Fe and Zn. Means with the same
letter for same nutrient are not significantly different by Duncan's multiple
range test (P 0.05).

Table 16. Iron tissue concentration in iron subplots in samples collected on 03 April
2005, 1 week after foliar application of Fe.


Iron sub-plots




Fallow with

weeds

Sorghum-sudan

Sunnhemp

Velvet Bea n


Fe (mg/kg)


95.29a

89.94 a


66.30 a

88.64 a


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).







50


Table 17. Zinc tissue concentration in zinc subplots in samples collected on 03 April
2005 1 week after foliar application of Zn.
Zinc sub-plots Zn (mg/kg)


Fallow with


weeds


Sorghum-sudan

Sunnhemp

Velvet Bea n


40.64 a

32.06 a

32.62a

32.13 a


tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).


Iron (Fe subplots)


80 -

60


20 -

0


a


Weeds Sorghum-sudan Sunnhemp


Velvet Bean


Figure 27. Iron in sweet potato leaf tissue collected on 03 April 2005 in Fe subplots 1
week after foliar application of iron. Means with the same letter are not
significantly different by Duncan's multiple range test (P 0.05).







51



Zinc (Zn subplots)

45
a
40
35
30 -
25
E 20
15
10
5
0
Weeds Sorghum-sudan Sunnhemp Velvet Bean



Figure 28. Zinc in sweet potato leaf tissue collected on 03 April 2005 in Zn subplots 1
week after foliar application of iron. Means with the same letter are not
significantly different by Duncan's multiple range test (P 0.05).

Sweet Potato Yield

The boniato tropical sweet potato can be grown year round in south Florida and

takes 120 to 180 days to reach maturity Shortly after planting there were some freezes

in the study area that damaged the young slips. Large areas of the young plants did not

recover from these winter freezes. The sweet potato yield was higher in sunnhemp and

sorghum-sudan treatment plots (Table 18; Fig. 29), as well as Fe subplots (Table 18; Fig.

30); however, there were no significant differences among cover crop treatment plots or

among micronutrient subplots. This result may be due to the time of year the study was

conducted and the damage to the crop as a result of the winter freezes.







52


Table 18. Sweet potato harvest collected on 30 June 2005.
Treatments kg/plot*

Fallow with weeds plots 3.12 a

Sorghum-sudan plots 6.10 a

Sunnhemp plots 6.04 a

Velvet Bean plots 4.25 a



Iron sub-plots 5.61 a

Zinc sub-plots 4.59 a

Control sub-plots 4.43 a


*Each plot equals 9.3 sq. m.
tMeans in a column followed by the same letter are not significantly different by
Duncan's multiple range test (P 0.05).



10
9
8
7
a a
6
a
v 4 a


2
1
0 r
Weeds Sorghum-sudan Sunnhemp Velvet Bean




Figure 29. Sweet potato harvest in cover crop treatment plots (each plot equals 9.3 m2)
collected on 30 June 2005. Means with the same letter are not significantly
different by Duncan's multiple range test (P 0.05).







53



6

5 -

4



2-

1 -
0





Iron Zinc Control






Figure 30. Sweet potato harvest in micronutrient subplots (each plot equals 9.3 m2)
collected on 30 June 2005. Means with the same letter are not significantly
different by Duncan's multiple range test (P 0.05).















SUMMARY

Sorghum-sudan grass and sunnhemp produced more biomass than velvet beans or

weeds, at 13.01 Mg/ha and 8.01 Mg/ha respectively. Neither sunnhemp nor velvet bean

reached their expected biomass production. This result was likely due, in part, to the

short days during the time of year they were planted, which caused them to flower early

and slowed their growth. Sunnhemp contained significantly more N in its total biomass

than other tested cover crops. Sunnhemp also contained significantly more Mn in its total

biomass, which is a key nutrient for N transformation, metabolism, and assimilation.

Soil samples taken prior to planting cover crops showed no significant difference in

total or AB-DTPA extractable nutrients. There were some differences found in measured

soil nutrients prior to cutting cover crops, but in general these samples contained less of

each nutrient than samples taken prior to planting cover crops. However, after

incorporation of cover crops some nutrients did increase, though not significantly

between treatment plots except for Mg, which was significantly greater in velvet bean

plots than other cover crop plots.

Fe and Mg concentrations were lowest in sweet potato leaf tissue samples collected

from sunnhemp plots; however, tissue samples collected from sunnhemp plots contained

significantly greater concentrations ofN, P, K, and Cu. Fe concentrations were not

significantly different in Fe subplots for any of the cover crop main treatment plots. Zn

concentrations were also not significantly different in Zn subplots in any of the cover

crop main treatment plots.






55


There were no significant differences in sweet potato yield between any of the main

treatment plots or subplots. The sweet potato crop had been damaged by winter freezes

and did not produce as expected.

Sunnhemp and Sorghum-sudan grass both grew well during the shorter fall days.

Velvet bean may be a good choice as a cover crop if it has a longer time to get

established. It would be recommended to run the study for a second year and during a

time of year when the days are longer to see what the effect on the sweet potato yield.
















LIST OF REFERENCES


Acosta-Martinez, V., D.R. Upchurch, A.M. Schubert, D. Porter, and T. Wheeler. 2004.
Early impacts of cotton and peanut cropping systems on selected soil chemical,
physical, microbiological and biochemical properties. Biol. Fertil. Soils, 40:44-54.

Balota, E. L., A. Colozzi-Filho, D. S. Andrade, and R. P. Dick. 2003. Microbial biomass
in soils under different tillage and crop rotation systems. Biol. Fertil. Soils, 38:15-
20.

Banful, B., A. Dzietror, I. Ofori, and O. B. Hemeng. 2000. Yield of plantain alley
cropped with Leucaena leucocephala and Flemingia macrophylla in Kumasi,
Ghana. Agroforestry Systems, 49:189-199.

Blackshaw, R., J. R. Moyer, R. C. Doram, and A. L. Boswell. 2001. Yellow sweet
clover, green manure, and its residues effectively suppress weeds during fallow.
Weed Sci., 49:406-413.

Bokhtiar, S. M., M. A. Gafur, and A.B.M.M. Rahman. 2003. Effects of Crotalaria and
Sesbania aculeata green manures and N fertilizer and the productivity of sugar
cane. J. Agric. Sci., 140:305-309.

Brady, N. C., and R. R. Weil. 2002. The Nature and Properties of Soils. Thirteenth
Edition, Prentice Hall, Upper Saddle River, NJ.

Buckles, D., and B. Triomphe. 1999. Adoption ofmucuna in the farming systems of
northern Honduras. Agroforestry Systems, 47:67-91.

Cavigelli, M. A. and S. J. Thien. 2003. Phosphorus bioavailability following
incorporation of green manure crops. Soil Sci. Soc. Am. J.. 67:1186-1194.

Chambliss, C. G., R. M. Muchovej, and J. J. Mullahey. 2003. Cover crops, UF, IFAS,
SS-AGR-66.

Chander, K., S. Goyal, M.C. Mundra, and K.K. Kapoor. 1997. Organic matter,
microbial biomass and enzyme activity of soils under different crop rotations in the
tropics. Bio. Fertil. Soils, 24:306-310.

Cobo, J.G., E. Barrios, D.C.L. Kass, and R.J. Thomas. 2002. Decomposition and
nutrient release by green manures in a tropical agroecosystem. Plant Soil,
240:331-342.









Curci, M., M.D.R. Pizzigallo, C. Crecchio, R. Mininni, and P. Ruggiero. 1997. Effects
of conventional tillage on biochemical properties of soils. Bio. Fertil. Soils, 25:1-6.

Ekeleme, F., I. O. Akobundu, A.O. Ishichei, and D. Chikoye. 2003. Cover Crops
Reduce weed seedbanks in maize-cassava systems in southwestern Nigeria. Weed
Sci., 51:774-780.

Fischler, M., and C. S. Wortmann. 1999. Green manures for maize-bean systems in
eastern Uganda: Agronomic performance and farmers' perceptions. Agroforestry
Systems, 47:123-138.

Fowler, C.J.E., L.M. Condron, and R.D. McLenaghen. 2004. Effects of green manures
on nitrogen loss and availability in an organic cropping system. New Zealand J.
Agric. Res., 47:95-100.

Franchini, J. C., M. A. Pavan, and M. Miyazawa. 2004. Redistribution of phosphorus in
soil through cover crop roots. Brazilian Archives Bio. Tech., 47(3):381-386.

Gardner, F. P., R. B. Pearce, R. L. Mitchel. 1985. Physiology of Crop Plants. Iowa
State Univ. Press, Ames, IA.

Goyal, S., K. C., M.C. Mundra, and K.K. Kapoor. 1999. Influence of inorganic
fertilizers and organic amendments on soil organic matter and microbial biomass
properties under tropical conditions. Bio. Fertil. Soils, 29:196-200.

Hacisalihoglu, G. and L. V. Kochiam. 2003. How do some plants tolerate low levels of
soil zinc? Mechanisms of zinc efficiency in crop plants. New Phyt., 159:341-350.

Hanly, J.A. and P.E.H. Gregg. 2004. Green-manure impacts on nitrogen availability to
organic sweetcorn (zea mays). New Zealand J. Crop Hort. Sci.. 32:295-307.

Havlin, J. L., J. D. Beaton, S. L. Tisdale, and Werner L. Nelson. 1999. Soil Fertility and
Fertilizers: An Introduction to Nutrient Management. Sixth Edition, Prentice-Hall,
Inc, Upper Saddle River, NJ

Lazzeri, L., G. Baruzzi, L. Malaguti, and L. Antoniacci. 2003. Replacing methyl
bromide in annual strawberry production with glucosinolate-containing green
manure crops. Pest Manage. Sci., 59:983-990.

Li, Y.C., H.H. Bryan, R. Rao, N. Heckert, and T. Olczyk. 1999. Summer cover crops
for tomato production in south Florida, P.18-21. Proc. Conference Florida Tomato
Institute, Citrus &Vegetable Magazine, Tampa, Fl.

Little, S.A., P.J. Hocking, and R.S.B. Greene. 2004. A preliminary study of the role of
cover crops in improving soil fertility and yield for potato production. Commun.
Soil Plant Anal., 35:471-494.









Lui, A., C. Hamel, R.I. Hamilton, B.L. Ma and D.L. Smith. 2000. Acquisition of Cu,
Zn, Mn and Fe by mycorrhizal maize (Zea mays L.) grown on soil at different P
and micronutrient levels. Mycorrhiza, 9:331-336.

Mandal, U. K., G. Singh, U.S. Victor, K.L. Sharma. 2003. Green manuring: its effect on
soil properties and crop growth under rice-wheat cropping system. European
J.Agron., 19:225-237.

Misra, A., and G. Tyler. 2000. Effect of wet and dry cycles in calcareous soil on mineral
uptake of two grasses, Agrostis stolonifera L. and Festuca ovina L. Plant Soil,
24:297-303.

Moore, J.M., S. Klose, and M.A. Tabatabai. 2000. Soil Microbial biomass carbon and
Nitrogen as affected by cropping systems. Bio. Fertil. Soils, 31: 200-210.

Morris, J. B., and J. T Walker. 2002. Non-traditional legumes as potential soil
amendments for nematode control. J. Nematol., 34(4):358-361.

Nziguheba, G., R. Merckx, C. A. Palm, and P. Mutuo. 2002. Combining Tithonia
diversifolia and fertilizers for maize production in a phosphorus deficient soil in
Kenya. Agroforestry Systems, 55:165-174.

Okpara, D.A., J.C. Njoku and J.E. Asiegbu. 2004. Response of two sweet potato
varieties to four green manure sources and inorganic fertilizer in a humid tropical
ultisol. Bio. Agric. Hortic., 22:81-90.

Sangakkara, U.R., M. Liedgens, A. Soldati, and P. Stamp. 2004. Root and shoot growth
of maize (zea mays) as affected by incorporation of Crotalaria juncea and Tithonia
dicversifolia as green manures. J. Agron. Crop Sci., 190:339-346.

Sangakkara, U.R., W. Richner, M.K. Schneider, P. Stamp. 2003. Impact of
intercropping beans (Phaseolus vulgaris L.) and sunnhemp (Crotalariajuncea L.)
on growth, yields and nitrogen uptake of maize (Zea mays L.) grown in the humid
tropics during the minor rainy season. Maydica., 48:233-238.

Savithri, P., R. Perumal, and R. Nagarajan. 1999. Soil and crop management
technologies for enhancing rice production under micronutrient constraints. Nutr.
Cycling Agroecosystems, 53:81-92.

Singh, D., R.L. Yadav, J.P. Singh and R. Pandey. 2003. Effect of intercropped prickly
sesban (Sesbania cannabina) green-manure and nitrogen levels on growth and yield
of sugarcane. Indian J. Agric. Sci., 73:664-667.

Smestad, B. Thor, H. Tiessen, and R. J. Buresh. 2002. Short fallows of Tithonia
diversifolia and Crotalaria grahamiana for soil fertility improvement in western
Kenya. Agroforestry Systems, 55:151-194.









Wang, Q., Y. Li and W. Klassen. 2005a. Influence of summer cover crops on
conservation of soil water and nutrients in a subtropical area. J. Soil Water
Conserv., 60:58-63.

Wang, Q., Y. C. Li, and W. Klassen. 2005b. Determination of cation exchange capacity
on low to highly calcareous soils. Commun. Soil Sci. Plant Anal., 36 (11&12):
1479-1498.

Wang, Q., Y.C. Li, and W. Klassen. In press a. Summer cover crops and soil
amendments to improve growth and nutrient uptake of okra (Abelmoschus
esculentus L.). HortTechnology.

Wang, Q., Y. C. Li, and W. Klassen. In press b. Microbial biomass of carbon and
nitrogen in tomato field with cover crops. J. Plant Nutr.

Wang, Q., Y.C. Li, W. Klassen. 2003a. Effects of soil amendments at a heavy loading
rate associated with cover crops as green manures on the leaching of nutrients and
heavy metals from a calcareous soil. J. Environ. Sci. Health, B38(6): 865-881.

Wang, Q., W. Klassen, H.H. Bryan, and Y. Li. 2003b. Influence of summer cover crops
in growth and yield of a subsequent tomato crop in south Florida. Proc. Fl. State
Hortic. Soc., 116:140-143.

Wang, Q., W. Klassen, Y.C. Li, M. Codallo, A. Abdul-Baki. 2005c. Influence of cover
crops and irrigation rates on tomato yields and quality in a subtropical region.
HortScience. 40(7):2125-2131.

Yadvinder-Singh, B., J.K. Ladha, C.S. Khind, R.K. Gupta, O.P. Meelu, and E. Pasuquin.
2004. Long-term effects of organic inputs on yield and soil fertility in rice-wheat
rotation. Soil Sci. Soc. Am. J., 68:845-853.

Zou, Y., F. Zhang, X. Li, and Y. Cao. 2000. Studies on the improvement in iron
nutrition of peanut by intercropping with maize on a calcareous soil. Plant Soil,
220:13-25.















BIOGRAPHICAL SKETCH

I'm originally from Washington State. I earned a BFA in painting and sculpture

from Pacific Lutheran University in 2000 and a BA in geography from Central

Washington University in 2002. This change in academic direction awakened my interest

in the environment, sustainable agriculture and soil science, which led me to pursue a MS

in soil science at the University of Florida.




Full Text

PAGE 1

INFLUENCE OF COVER CROPS ON NUTRIENT AVAILABILITY IN A SWEET POTATO CROPPING SYS TEM IN SOUTH FLORIDA By JEANNA RAGSDALE A THESIS PRESENTED TO THE GRADUATE SCHOOL OFTHE UNIVERSITY OF FLORI DA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSTITY OF FLORIDA 2006

PAGE 2

Copyright 2006 by Jeanna Ragsdale

PAGE 3

iii ACKNOWLEDGMENTS There is a long list of people I would like to thank for a variety of reasons. The problem is with where to begin the list and where to end it. First, I would like to extend thanks to my major advisor, Dr. Yuncong Li, for his support, guidance, encouragement, and technical advice throughout this study and my time as a graduate student. Id also like to thank the other members of my committee, Thomas Obreza, Ashok Alva, and Zhenli He, for their support, suggestions, and feedback in this study. I would like to extend a special thanks to M & M Farms, Manelo Hevia, Teresa Olczyk, and Waldy Klassen for thei r support and help with organizing my research. Special thanks are also extended to the many pe ople in the soil and hydrology lab at the Tropical Research and Educati on Center (TREC), Newton Campbell, Tina Dispenza, Guodong Lui, Laura Rosado, Y un Quan, Xing Wang, Qingren Wang, and Guiqin Yu, for all their help and time. Appr eciation is also extende d to the whole farm crew at TREC for all their tim e, help, and hours in the sun. Second, I would like to thank the many other academic mentors that in one way or another inspired my current pursuits: Lawry Gold for instilling a deep sense of selfawareness in my lifes direction and purpose, Dr Karl Lillquist for inspiring my interest in soils, Chris Kent for keeping my motiv ations high, and Kenneth Buhr for reminding me of the importance of both sustainability and practicality. Finally, it is without question that I owe my great est thanks to so many people that have inspired me, pushed me, and stuck with me throughout my life. This of course

PAGE 4

iv includes my family for their love, support, a nd belief in my abilitie s, and a long list of friends, of which I will only name a few: Gl en Erickson for pushing me out of the nest and teaching me how to fly, MaxZine Weinst ein for bringing out my true love for gardening, David Long for his never ending l ove, encouragement, and faith in me, and George Voellmer for providing me with inspiration when it was most needed and never letting go of my hand.

PAGE 5

v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES............................................................................................................vii LIST OF FIGURES...........................................................................................................ix ABSTRACT.......................................................................................................................xi INTRODUCTION...............................................................................................................1 LITERATURE REVIEW....................................................................................................2 Cover Crops Used in Florida........................................................................................2 Cover Crops Tested in Florida..............................................................................2 Sorghum-sudan grass (S. bicolor S. bicolor var. sudanense (Piper) Stapf.)........................................................................................................2 Sunn hemp (Crotalaria juncea L. cv. Tropic Sun).......................................3 Velvetbean (Mucuna deeringiana (Bort.), Merr.)..........................................4 Cowpea (Vigna unguiculata L. cv. Iron Clay).............................................4 Aeschynomene (Aeschynomene evenia C. Wright )......................................4 Sesbania (Sesbania exaltata Raf.)...................................................................5 German millet (Setaria italica (L.) P. Beav.)..................................................5 Cover Crops Used to Improve Weed Suppression and Pest Control............................5 Cover Crops Used to Improve Soil Quality..................................................................7 Cover Crops Used to Improve Crop Yield.................................................................12 Micronutrient Deficiency of Plan ts Grown in Calcareous Soils................................15 OBJECTIVE......................................................................................................................17 MATERIALS AND METHODS.......................................................................................18 Establishment of Field Experiment............................................................................18 Cover Crop Management............................................................................................18 Sweet Potato Crop Management................................................................................19 Sample Collection and Chemical Analysis.................................................................20 Statistical Analysis......................................................................................................21

PAGE 6

vi RESULTS AND DISCUSSION........................................................................................22 Cover Crop Biomass...................................................................................................22 Nutrient Concentrations in Cover Crop......................................................................23 Total Nutrients in Cover Crop Biomass.....................................................................26 Soil Nutrients Prior to Planting Cover Crops.............................................................30 Soil Nutrients Prior to Cutting Cover Crops...............................................................30 Soil Nutrients at 9 Weeks After Incorporation of Cover Crops.................................39 Soil Nematodes...........................................................................................................42 Nutrient Concentrations in Sweet Potato Leaves.......................................................46 Sweet Potato Yield.....................................................................................................51 SUMMARY.......................................................................................................................54 LIST OF REFERENCES...................................................................................................56 BIOGRAPHICAL SKETCH.............................................................................................60

PAGE 7

vii LIST OF TABLES Table page 1 Seedling rates for cover crops..................................................................................19 2 Sample collection type and dates.............................................................................20 3 Cover crop heights (cm) measured at 28, 42, 55, and 68 da ys after planting (DAP).......................................................................................................................22 4 Cover crop and weed biomass yield (M g/ha) collected 68DAP (07 Nov. 2004).....23 5 Nutrient concentrations in cover crop tissues collected on 7 Nov. 2004.................24 6 Total nutrient amounts in cover crop biomass.........................................................28 7 Total N, C and AB-DTPA extractable nut rients (P, K, Mg, Fe, Zn, B, Mn, and Cu) in soil samples collected on 31 A ug. 2004 prior to planting cover crops.........32 8 Extractable nutrient con centration in soil sample s collected on 07 Nov. 2004 prior to cutting cover crops......................................................................................32 9 Extractable nutrient con centration in soil sample s collected on 19 Feb. 2005 after incorporation of cover crops............................................................................34 10 Extractable soil nutrien t concentrations in weed plots compared between sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov. 2004, and after incorporation on 19 Feb. 2005..................................................................35 11 Extractable soil nutrient c oncentrations in sorghum-sudan grass plots compared between sampling dates: before plan ting on 31 Aug. 2004, before cutting on 7 Nov. 2004, and after incorporation on 19 Feb. 2005...............................................35 12 Extractable soil nutrient concentrations in sunnhemp plots compared between sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov. 2004, and after incorporation on 19 Feb. 2005..................................................................36 13 Extractable soil nutrient c oncentrations in velvetbean plots compared between sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov. 2004, and after incorporation on 19 Feb. 2005..................................................................36

PAGE 8

viii 14 Nematode population (direct count) in so il samples collected prior to planting cover crops on 31 Aug, before cutting 07 Nov, and after incorporation on 19 Feb. 2004..................................................................................................................44 15 Sweet potato leaf nutrient concentrati ons in samples collected on 03 April 2005, 1 week after foliar applic ations of Fe and Zn...........................................................47 16 Iron tissue concentration in iron subplo ts in samples collected on 03 April 2005, 1 week after foliar application of Fe........................................................................49 17 Zinc tissue concentration in zinc subp lots in samples collected on 03 April 2005 1 week after foliar application of Zn........................................................................50 18 Sweet potato harvest collected on 30 June 2005......................................................52

PAGE 9

ix LIST OF FIGURES Figure page 1 Cover crop biomass collected on 07 Nov. 2004 prior to incorporation of cover crops.........................................................................................................................23 2 Cover crop plant tissue macronutrient con centrations in samples collected on 07 Nov. 2004 prior to cutting cover crops....................................................................25 3 Cover crop plant tissue carbon concentr ation in samples collected on 07 Nov. 2004 prior to cutting cover crops.............................................................................26 4 Cover crop plant tissue micronutrient con centrations in samples collected on 07 Nov. 2004 prior to cutting cover crops....................................................................26 5 Cover crop total biomass macronutrients.................................................................27 6 Cover crop total biomass carbon..............................................................................29 7 Cover crop total biomass micronutrients.................................................................29 8 Total N in soil collected on 31 A ug. 2004 prior to planting cover crops.................31 9 Total C in soil collected on 31 A ug. 2004 prior to planting cover crops.................31 10 AB-DTPA extractable P, K, and Mg in soil collected on 31 Aug. 2004 prior to planting cover crops.................................................................................................33 11 AB-DTPA extractable Fe, Zn, B, Mn, and Cu in soil collected on 31 Aug. 2004 prior to planting cover crops....................................................................................34 12 Total nitrogen in soil collected on 07 Nov. 2004 prior to cutting cover crops........37 13 Total carbon in soil collected on 07 Nov. 2004 prior to cutting cover crops...........37 14 AB-DTPA extractable macr onutrients in soil collect ed on 07 Nov. 2004 prior to cutting cover crops...................................................................................................38 15 AB-DTPA extractable micronutrients in soil collected on 07 Nov. 2004 prior to cutting cover crops...................................................................................................38

PAGE 10

x 16 Total N in soil samples collected on 19 Feb. 2005 after incorporation of cover crops.........................................................................................................................39 17 Total C in soil samples collected on 19 Feb. 2005 after incorporation of cover crops.........................................................................................................................40 18 AB-DTPA extractable soil nutrients in samples collected on 19 Feb. 2005 after incorporation of cover crops....................................................................................40 19 AB-DTPA extractable soil nutrients in samples collected on 19 Feb. 2005 after incorporation of cover crops....................................................................................42 20 Soil nematodes counted in fallow plots....................................................................43 21 Soil nematodes counted in sorghum-sudan plots.....................................................45 22 Soil nematodes counted in sunnhemp plots.............................................................45 23 Soil nematodes counted in velvet bean plots...........................................................46 24 Sweet potato leaf macronutrient concentr ations in samples collected on 03 April 2005 1 week after foliar app lications of Fe and Zn..................................................48 25 Sweet potato leaf carbon concentrations in samples collected on 03 April 2005 1 week after foliar applica tions of Fe and Zn..............................................................48 26 Sweet potato leaf micronutrient concentrations in samples collected on 03 April 2005 1 week after foliar app lications of Fe and Zn..................................................49 27 Iron in sweet potato leaf tissue collec ted on 03 April 2005 in Fe subplots 1 week after foliar application of iron..................................................................................50 28 Zinc in sweet potato leaf tissue co llected on 03 April 2005 in Zn subplots 1 week after foliar application of iron.........................................................................51 29 Sweet potato harvest in cover crop tr eatment plots (each plot equals 9.3 m2) collected on 30 June 2005........................................................................................52 30 Sweet potato harvest in micronutrien t subplots (each plot equals 9.3 m2) collected on 30 June 2005........................................................................................53

PAGE 11

xi Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree Master of Science INFLUENCE OF COVER CROPS ON NUTRIENT AVAILABILITY IN A SWEET POTATO CROPPING SYSTEM IN SOUTH FLORIDA By Jeanna Ragsdale August 2006 Chair: Yuncong Li Major Department: Soil and Water Science The use of cover crops serves a variet y of purposes from taking up excess soil nutrients to carbon sequestration and contro lling soil erosion. Other benefits include weed and nematode suppression, and use as a green manure for improving soil fertility. The objective of this research was to impr ove nutrient availability in a sweet potato [Ipomea batatas (L.) Lam] cropping system grown on a south Florid a calcareous soil. Sweet potato response to micronutri ent fertilizer additions of ir on (Fe) and zinc (Zn) were evaluated for their effect on crop yield and grow th. In addition to mi cronutrient fertilizer application, the cover crops S unnhemp (Crotalaria juncea L. cv. Tropic Sun), velvet bean (Mucuna deeringiana), and sorghum-sudan gr ass (Sorghum bicolor S. bicolor var. Sudanese) were grown and incorporated into the soil prior to planting sweet potato. Biomass samples were collected for each c over crop. Cover crops were analyzed for nutrient concentrations, and total biomass nut rient accumulation for N, C, P, K, Mg, Fe, Zn, B, Mn, and Cu. Sorghum-sudan produced the greatest amount of biomass at 13.01

PAGE 12

xii Mg/ha, followed by sunnhemp at 8.01 Mg/ha and velvet bean at 5.22 Mg/ha. Total biomass nutrients C, P, K, Mg, Fe, Zn, and B were greatest in sorghum-sudan grass. Sunnhemp total biomass contained the greate st amount of nitrogen (180.36 kg/ha), and manganese (368.66 g/ha). Velvet bean plant tissu e had the highest concentrations of zinc and copper. There were no signi ficant differences in soil nutrients among the treatment plots prior to planting cover crops. After inco rporation of cover crops total nitrogen and AB-DTPA P increased in all treatment plot s, while Mg and Mn decreased. The total number of nematodes decreased in all treatment plots after incorporation of cover crops. Sweet potato leaf tissue sample s collected from sunnhemp plots contained the greatest concentration of N, P, and K. Sample s collected from sorghum-sudan grass plots contained the greatest amount c oncentration of C, and Fe. Fo liar applications of Fe and Zn had no significant effect on Fe or Zn con centrations in sweet pot ato leaf tissue. The sweet potato crop was damaged by freezes shortly after planting, which greatly affected the total harvest. There were no significant differences f ound between the main treatment plots or subplots for sweet potato harvest.

PAGE 13

1 INTRODUCTION The use of cover crops serves a variety of purposes that improve environmental quality, ranging from taking up excess soil nutrients to carbon sequestration and controlling soil erosion. Other benefits of cover crops include pest control such as weed and nematode suppression (Morris and Walker, 2002), and use as a green manure for improving soil fertility (Blackshaw et al., 2001). Cover c rops used as green manures add organic matter to the soil (Chambliss et al., 2003), which improves soil structure and can help increase soil water holding capacity (Chambliss et al., 2003). Soil microorganisms decompose organic matter through a process of mineralization and therefore play an important role in nutrient cycling. As the organic matter is broken down, nutrients stored in plant tissues are released to the soil and become available for plant uptake (Chambliss et al., 2003). As recalcitrant organi c matter continues to decompose, it forms humus that increases the cation exchange capacity (CEC) of the soil. This process results in a greater ability to hold nutrients in the soil, keeping them available for plant uptake over a period of time (Chambliss et al., 2003). Many studies commonly evaluated cover crop effects on crop growth and organic carbon changes in soils, but few studies evaluated micronutrient availability in association with cover crops. More information is needed to determine micronutr ient availability as a result of cover crop use.

PAGE 14

2 LITERATURE REVIEW Cover Crops Used in Florida Common cover crops in Florida include legumes and grasses that are used to suppress weeds, prevent soil erosion, remove salts, improve soil fertility, protect water quality, and control pests. Grasses used as cover crops in Florida include pearl millet (Pennisetum glaucum), sorghum sudan, bahiagrass (Paspalum notatum), and pangola (Digitaria eriantha) (Li et al., 1999; Chambliss et al., 2003). Grasses can prod uce more biomass and decompose more slowly than legumes. Legumes have the added benefit of their ability to fix atmospheric N in association with Rhizobia. Legumes used as green manures generally decompose more rapidly than grasses due to the higher N co ntent in their biomass. Common Legumes used in Florida include aeschynomene (Aeschynomene avenia), hairy indigo (Indigofera hirsuta), sesbania (Sesbania exalta), velvet bean, lupine, and sunhemp (Chambliss et al., 2003). Cover Crops Tested in Florida Sor ghum sudan grass (S. bicolor S. bicolor var. sudanense (Piper) Stapf .) Sorghum sudan grass, also known as sorghum sudan, has been used as a cover crop throughout Florida for decades, and is still used by some growers during the fallow summer period in so uth Florida. Sorghum sudan grass usually produces 11 16 metric tons of dry matter per hectare (5 to 7 tons dry mass per acre). Since 0.92% of this material is N, the amounts of N potentially available to the subsequent crop range from about 490 to 708 kilo grams per hectare (90 to 130 pounds per acre) While meeting some

PAGE 15

3 of the criteria for a good cover crop, Sorghum sudan falls short in Florida. This plant grows poorly in many Florida soils, having been developed for the finer textured soils of the Midwest and Southwest USA. Sorghum sudan often grows quite tall, requiring mowing to prepare the crop for green manuring, which adds cost to its management. Its large fibrous stems have a high C:N ratio, which slows decomposition, and may immobilize N from the soi l during decomposition. Lastly, Sorghum sudan often attracts armyworms and corn silk flies, which may be detrimental to subsequent vegetable crops. However, sorghum sudan grass suppresses weeds and some parasitic nematodes, and the seed is inexpensive ( $2 .20 to $3.31 per kilogram or $1.00 to 1.50 per pound of seed). Sunn hemp (Crotalaria juncea L. cv. Tropic Sun) Sunn hemp has a number of advantages compared with Sorghum sudan as a cover crop. This plant is an annual tropical legume that has a fast growi ng, 60 to 80 day production cycle during which the plant may exceed 2 m in height. Sunn hemp is a short day plant that is quite drought tolerant, grows well in both high and low pH soils, and is also resistant to root knot nematode. Typically Sunn hemp pro duces 6 to 8.5 tons of dry mass per acre. Since 2.85% of this material is N, the amounts of N potentially available to the subsequent cash crop range from about 13 to 19 kilograms per hectare (340 to 450 pounds per acre). However, Sunn hemp does have sever al limitations. Seed is rather high priced ($3.30 to $8.80 per kilogram or $1.50 to $4.00 per pound) due to the need to import it, hence limiting seed availability. Seeds require Rhizobium inoculation before planting. In some fields, Sunn hemp stands may b e reduced due to damping off from the effects of Pythium or a form of Fusarium Even with these possible limitations, Sunn hemp was among the best of the tested cover crops in southern Florida conditions.

PAGE 16

4 Velvetbean (Mucuna deeringiana (Bort.), Merr.) Velv etbean is also an annual tropical legume that produces a large amount of biomass, is drought tolerant, suppresses parasitic nematodes, and grows well in both high and low pH soils. Velvetbean may produce 11 16 metric tons of dry biomass per hectare (5 to 7 tons per acre of dry biomass) consisting of 2.6% N, which may provide from 570 to 790 kilograms (260 to 360 pounds) of N to the subsequent cash crop. However, velvetbean's large seed requires a special planter, and volunteer plants may persist into the ne xt cash crop, requiring weed control. However a small seeded cultivar, Georgia bush, can be seeded with some conventional seeders. Additionally, velvetbean may be potentially allopathic to some subsequent vegetable crops. In field trials, velvetbean rank ed among the best of the tested cover crops. Cowpea (Vigna unguiculata L. cv. Iron Clay) Cowpea is a legume that grows well in a variety of soils, is resistant to root knot nematodes, and has a short growing season of 40 to 50 days. Cowpea may produce 7 to 11 metric tons per hectare (3 to 5 tons per acre) of biomass consisting of 2% N, which may provide from 265 to 440 kilograms (120 to 200 pounds) of N to the subsequent cash crop. However, in Florida conditions, cowpea is not tolerant to flooding and pro duces a rather low biomass. Aeschynomene (Aeschynomene evenia C. Wright ) Aeschynomene grows well on calcareous soils in southern Florida, and is a warm season legume forage. It is resistant to root knot nematode. Aeschynomenes single apparent disadvantage for Florida conditions is its low biomass production.

PAGE 17

5 Sesbania (Sesbania exaltata Raf. ) Sesbania like aeschynomene and cowpea, is a warm season legume forage that is well adapted to Florida conditions. However, sesbania is susceptible to root knot nemato de, and does not quickly form a closed canopy, competing rather poorly with some of Florida's persistent weeds. German millet (Setaria italica (L.) P. Beav. ) German millet grows well in southern Florida and has been proven to be resistant to root knot nema tode; however, like some of the other tested crops, German millet produces low biomass. Cover Crops Used to Improve Weed Suppression and Pest Control Using cover crops for pest control can reduce the use of pesticides on subsequent crops. Cover crops have been proven to control nematodes and suppress weeds, and have biocidal properties. Nematicidal properties of plant residues may be a result of ammonification, phytochemicals, or other compounds produced from plant tissues during the breakdown process (Mo rris and Walker, 2002). A study using dried plant tissue from 20 leguminous species showed lower nematode infestation from each of the legumes (Morris and Walker, 2002). Dried legume tissue was incorporated into soil at four rates: 1, 2, 2.5, and 5%. Th e soil was kept moist enough for seed germination and incubated at 21 and 27 C for 1 week, at which time 2 week old tomato plants were transplant into each soil treatment. Soil samples infested with nematodes without dried legume tissue were maintained t o serve as controls. All 20 leguminous species were found to lower the amount of nematode infestation in the samples, but the range of effect varied widely, with the greatest effect occurring at the highest incorporation rate.

PAGE 18

6 In a study by Blackshaw et al. (2001), yellow sweet clover was shown to suppress weeds. Yellow sweet clover as a living biomass suppressed weeds up to 83% in the 1st year of the study, but had no significant effect in the succeeding 2 years; however, it had a greater affect on weed biomass than on weed density in all three years. Yellow sweet clover plant residue left on each plot had fewer weeds than plots without yellow sweet clover, but amount and species of weeds varied depending on companion plant grown. This result may have been due to both physical and allelopathic effects (Blackshaw et al., 2001). A study conducted in southwestern Nigeria examined the effectiveness of cover crops at reducing weed seedbanks in maize cassava systems (Ekeleme et al., 2003). Total seedbank wa s evaluated from 1993 to 1995 under three fallow types: bush, leucaena, and kudzu. In all 3 years, seed population was less in plots with kudzu than with leucaena. Both kudzu and leucaena were more effective at reducing seed population than bush fallow. Kudzu, a tropical legume, reaches full ground cover in 1 year. It creates a closed canopy and therefore reduces the light that reaches the soil, which in turn reduces weed seed germination. Methyl bromide is commonly used as a soil fumigant to control fu ngi and other pests. The Brassicaceae family has been shown to have biocidal properties. Brassica species contain glycosidic properties that hydrolyze into cytotoxic compounds, which affect fungi and other soil pests harmful to crops. Lazzeri et al. ( 2002) evaluated two species of Brassica against a conventional green manure (barley), and methyl bromide (a common soil fumigant) on strawberry performance for 2 years. Plots treated with methyl

PAGE 19

7 bromide had higher yields than all other plots; however, plo ts that incorporated Brassica had higher yields than barley or untreated plots. Cover Crops Used to Improve Soil Quality In a study by Chander et al. (1997), the use of green manures improved organic matter, soil microbial activity and productivity of the soil. Crop rotations increased organic carbon and total nitrogen in the soil. Sesbania aculeate, a green manure, added the greatest amount of soil organic carbon and total nitrogen when used in the rotation. Nutrient cycling in the soil is affected by soil biota in the labile fraction of organic matter, and is therefore affected by soil management practices including crop rotations, cover crops, green manures, tillage, and fertilization (Chander et al., 1997). Organic C and N have been used as soil quality indicators but have been unresponsive during short periods. However, soil microbial biomass and enzyme activity are more responsive to agricultural management practices and environmental conditions than total organic C and N and can be used as early indic ators of soil quality (Balota et al., 2003). Agricultural management practices such as cover crops, crop rotations, and tillage have an effect on soil microorganisms and therefore soil quality (Chander et al., 1997). Cover crops and crop rotations help mai ntain soil organic matter (SOM) and soil structure, thereby increasing soil porosity and water holding capacity. Conventional tillage practices can significantly decrease SOM, increase soil erosion, and disturb microenvironments within the soil. These alte rations affect water and oxygen content at the soil surface (Curci et al., 1997). All these factors influence substrate availability for microbial activity, which in turn affects soil fertility and quality (Curci et al., 1997).

PAGE 20

8 The most labile carbon and n itrogen pools in soil are represented by soil microbial biomass C at 1 to 3% of total soil C and as much as 5% of the total soil N is represented by microbial biomass N (Moore et al., 2000). A study by Moore et al. (2000) found the highest microbial biomas s C in multicropping systems on 4 year rotations and the lowest in monocropping systems of corn and soybean. Monocropping systems generally contain less organic matter, microbial biomass, and soil structural stability than agroecosystems that incorporate c rop rotations (Moore et al., 2000). However, while changes in soil properties are related to crop management practices, it is also important to note that the extent of these changes are also affected by the soil type (Acosta Martinez et al., 2004). Where one study in a loamy sand soil found greater increases in microbial biomass, N mineralization, enzyme activities, and organic matter in rotations with corn, soybean, and oats when compared with continuous corn alone, another study found no difference in en zyme activities in a fine sandy loam soil under continuous cotton compared with that under a cotton peanut rotation (Acosta Martinez et al., 2004). Reduced soil productivity, nutrient imbalances, and yield decline can be linked to loss of soil organic matt er (SOM). In a study by Yadvinder Singh et al. (2004), seven treatments were analyzed for their effect on soil fertility, yield, and contribution to SOM in a rice wheat rotation. The study found that farmyard manure in combination with green manure (Sesb ania cannabina L.) had higher rice yields than other green manure treatments. Rice yields were significantly higher for wheat straw in combination with green manure than with green manure alone. Wheat straw and farmyard manure increased soil organic carb on (SOC) significantly compared with other treatments. The

PAGE 21

9 greatest increases in SOC were found when green manure, wheat straw and farmyard manure were applied in combination. Soil physical, biological and chemical properties can be enhanced by additions o f organic matter (Sangakkara et al., 2004). Green manures add organic matter and can have positive effects on soil quality and nutrient supply. In a study by Sangakkara et al., two tropical green manures were studied to determine their effect on root and shoot growth of maize. In 3 years, soil physical properties and available nutrients N, phosphorus (P) and potassium (K) all increased after continued additions of green manures. The two green manures used in this study were Crotalaria juncea and Tithonia diversifolia. Shoot growth was greatest with additions of Crotalaria due to its high nitrogen content, while Tithonia, which has the ability to mobilize soil P, enhanced development of an extended root system. Over the three season study, SOC increased by 8% with additions of Crotalaria and by 12% with additions of Tithonia compared with untreated soil. Tithonia increased soil P, while Crotalaria increased soil N. Phosphorus availability may be enhanced for crops following the use of green manures (Ca vigelli and Thien, 2003), possibly due to a green manures ability to convert unavailable forms of P to available forms, thereby enhancing P availability for the succeeding crop. Organic P in decomposing green manure tissue is potentially labile and avail able for the succeeding crop. This decomposition process will also release CO2, which in soil solution may form H2CO3 causing dissolution of P in minerals. Decomposition of green manures will also release organic acids, which may further dissolve soil mi neral P (Cavigelli and Thien, 2003).

PAGE 22

10 Some cover crops like white lupine are able to release P due to secretion of organic anions. White lupine has cluster roots, which secrete citric acid when soils are low in available P. Other cover crops like fodder r ape have extensive fine root systems that are able to scavenge P at greater distances from the plant (Little et al., 2004). A study on P bioavailability and green manures incorporated prior to growing sorghum (Cavigelli and Thien, 2003) found that sorghum P uptake correlated with perennial forage P uptake. However, they also found that plant type, rather than P uptake, in selected winter cover crops may have a greater influence on subsequent sorghum crop P uptake. A study by Franchini et al. (2004) conduc ted in Brazil tested a variety of cover crops to determine their ability to transport P down to the roots and into the subsoil layers. They found that transport of P below 10 to 55cm was best done by black oats, white lupine and IAPAR 74 pea. The greates t accumulation of P in the aerial parts of the plant, without P fertilizer applications, was with white lupine. The greatest accumulation in the roots was by common vetch. In addition to P accumulation by different cover crops, they may also increase the solubility of native soil P by controlling the rhizosphere pH, plant exudates, and root phosphatases (Franchini et al., 2004). A study conducted using three legume green manure sources compared with inorganic fertilizer NPK for two different varieties of sweet potato found that soil nitrogen was significantly higher with Mucuna than the other legumes. It also found that sweet potato tuber yields were statistically similar in plots with Mucuna as they were for NPK fertilizer treatment plots (Okapara et al. 2004). Mucuna also produced higher biomass than the other two legumes, Pueraria phaseoloides and Centrosema pubescens (Okapara et al., 2004).

PAGE 23

11 Avena sativa, Lupinus angustifolius, and oats lupine mix were evaluated for their effect on nitrogen loss and a vailability in an organic cropping system. Using green manures reduced the amount of N leaching during winter. Annual ryegrass was planted as a subsequent crop to evaluate each green manures ability to supply N back to the soil after incorporation. Gre en manure treatment plots showed greater N uptake by ryegrass than non amended plots. There were no significant differences in biomass production between treatments; however, lupine had significantly greater N concentration (Fowler et al., 2004). A study conducted in Cauca, Columbia evaluated the decomposition and nutrient release of different green manures (Cobo et al., 2002). The volcanic ash soils in this area can be limited by low availability of N and P due to mineral particles in the soil organic ma tter. These soils can also be deficient in some micronutrients, notably copper, zinc, and cobalt. During a 20 week study, Mucuna was shown to release higher amounts of both N and P compared with that of Tithonia, which released higher amounts of K, Ca, and Mg (Cobo et al., 2002). A study by Goyal (1999) found that SOM and mineralized carbon and nitrogen increased when inorganic fertilizers were used in combination with farmyard manure, wheat straw, or Sesbania bispinosa green manure. Soil amended with wheat straw and inorganic fertilizers showed an increase in microbial biomass C from 147 mg/kg soil to 423 mg/kg soil (Goyal et al., 1999). The study concluded that treatments receiving a combination of organic amendments and inorganic fertilizers resulted in a greater increase in soil organic C and total N than soils that received only inorganic fertilizers. Above ground crop biomass was not incorporated into the soil, but instead was removed,

PAGE 24

12 which may explain why soils amended with wheat straw, adding 3048 kg C/ha/yr, had such a significant increase in microbial biomass C compared with soils that received only inorganic fertilizers. Cover Crops Used to Improve Crop Yield Cover crops used as green manures help build soil fertility, improve soil structure, and increase water holding capacity. In northern Honduras, Mucuna used as a green manure was shown to increase organic matter, infiltration, and porosity of the soil while increasing maize yields (Buckles and Triomphe, 1999). It was also shown to reduce drough t stress and suppress weeds, especially broadleaf. Mucuna was intercropped with maize and used as mulch but not incorporated into the soil. Maize yields in rotation with Mucuna were typically double that of those without it. A study in Kenya using Tithonia diversifolia in combination with fertilizers improved maize yields and phosphorus recovery (Nziguheba et al., 2002). Phosphorus is a limiting nutrient for crop production in western Kenya as a result of small farm holders with limited purchasing power an d the high cost of inorganic fertilizers. Addition of green manures do not increase total P in the system but can, in some cases, increase the bioavailability of P already in the soil. Maize yields were greatest with the addition of P fertilizer, but yield did increase with increasing additions of Tithonia. Maize yields in combination with fertilizer and Tithonia were higher than with fertilizer or Tithonia alone. However, maize yields and recovered P in above ground biomass were higher in treatments where Tithonia was used compared with treatments that received only fertilizer. In another study in western Kenya, post fallow maize yields increased with the addition of Tithonia diversifolia and Crotalaria grahamiana (Smestad et al., 2002).

PAGE 25

13 Crotalaria produced a greater biomass than Tithonia or weed fallow and also made the greatest contribution to soil N and P. In Ghana, yield of plantain was increased by the addition of Leucaena leucocephala and Flemingia macrophylla (Banful et al., 2000). Higher yields were obtained by the addition of Flemingia than with Leucaena. The presence of certain species of nematodes was also found to be lower in Flemingia plots than Leucaena plots. A study in Uganda evaluated yield and farmer perceptions for green manures in maize b ean systems (Fischler and Wortmann, 1999). Crotalaria (Crotalaria ochroleuca), Mucuna (Mucuna pruriens var. utilis), lablab (Dolichos lablab), and Canavalia (Canavalia ensiformis) were used in short term fallows as green manures. Farmers reported improved soil fertility, moisture, and tilth. Weed suppression, erosion control, and higher yields after the addition of the green manures were also noted, but lablab and Mucuna were favored due to greater benefits and reduced labor requirements. Maize yields incre ased 41% and bean yields 43% with the addition of Crotalaria compared with that of weed fallow. Maize yield increased 60% with the addition of Mucuna and 50% with the addition of lablab compared with a continuous maize cropping system. A study in Banglades h using Crotalaria juncea and Sesbania aculeata in combination with urea N on sugarcane increased cane yield 2 to 57% (Bokhtiar et al., 2003). No fertilizers were added to the green manure crops. Concentrated superphosphate, muriate of potash, gypsum, and zinc sulfate were applied to sugarcane crops. Green manure additions increased total number of tillers and cane stalks. The highest cane yield was produced by Sesbania in combination with 150 kg N/ha.

PAGE 26

14 A study by Sangakkara et al. (2003) found that maize in tercropped with beans (Phaseolus vulgaris L.) produced maize yields similar to that of monoculture in the first season and increased by the third season, where when intercropped with crotalaria, yields decreased below that of monoculture. A study of Sesb ania rostrata, Sesbania aculeata, and Vigna radiata effects on soil properties and crop growth in a rice wheat cropping system found that soil physical properties improved in green manure plots compared with that of fallow (Mandal et al., 2003). Soil orga nic matter increased, bulk density decreased, soil aggregation improved, and hydraulic conductivity improved especially in S. rostrata treatment plots followed by S. aculeate and V. radiata. Total soil nitrogen was also found to be higher in green manure treatment plots compared with fallow (Mandal et al., 2003). A study of the effects of green manures on nitrogen availability to organic sweet corn found that lupine and lupine mustard mix both significantly increased soil mineral N by 30 to 45%, where ryeg rass decreased soil mineral N by 33 to 43% (Hanly and Greg, 2004). Accumulation of N by sweet corn was also significantly increased by both lupine and lupine mustard mix and significantly decreased by ryegrass treatment. However, though yield of sweet co rn was significantly reduced by ryegrass, lupine and lupine mustard mix did not significantly increase yield of sweet corn (Hanly and Greg, 2004). A study on the effect of intercropping Sesbania cannabina on nitrogen levels for growth and yield of sugarcan e showed that Sesbania improved soil quality. Soil organic carbon increased from 0.37% to 0.49%. Both Sesbanias ability to fix nitrogen and its nutrient release upon decomposition of the incorporated biomass improved the soil fertility in treatment plot s. N fixation increased with age of plant with the greatest

PAGE 27

15 increase at 60 days. Yield of sugarcane decreased when intercropped with Sesbania. This result may have been due to competition for space; however, yield of the ratoon crop significantly increa sed (Singh et al., 2003). Micronutrient Deficiency of Plants Grown in Calcareous Soils Nutrient availability is highly correlated with soil pH. At high soil pH Fe, Mn, Cu, Zn, and B may be deficient for plant growth (Brady and Weil, 2002). Calcareous so ils contain high levels of free calcium carbonate (Brady and Weil, 2002), low organic matter, and pH 7.5 8.5 (Zou et al., 2000). Micronutrient and phosphorus deficiencies are common in calcareous soils (Misra and Tyler, 2000). Micronutrient availability may also be affected by soil moisture (Misra and Tyler, 2000), organic matter (Brady and Weil, 2002) and plant exudates (Zou et al., 2000). Low iron and zinc availability is common in calcareous soils (Misra and Tyler, 2000). Plant available iron is affe cted by organic compounds in the soil solution (Havlin et al., 1999), thus soils low in organic matter may contain low plant available Fe. Soil pH and bicarbonate also affect the availability of Fe, with the greatest deficiencies occurring between 7.3 an 8.5 (Havlin et al., 1999). Organic matter and high soil pH also affect the availability and absorption of Zn (Hacisalihoglu and Kochiam, 2003). Other factors such as mycorrhizal fungi can affect the availability of iron and zinc in the soil solution (Liu et al., 2000). Organic manure (animal manures and green manure) can supply micronutrients to plants and may also mobilize soil metal cations by chelation and complexing with organic compounds, making them more available for plant uptake (Savithri et al., 1999). In a study by Chander (1997), the use of green manures improved organic matter, soil microbial activity and productivity of the soil. Crop rotations increased organic

PAGE 28

16 carbon and total nitrogen in the soil. Sesbania aculeate, a green manure, add ed the greatest amount of soil organic carbon and total nitrogen when used in the rotation. Nutrient cycling in the soil is affected by soil biota in the labile fraction of organic matter, and therefore affected by soil management practices including crop rotations, cover crops, green manures, tillage and fertilization (Chander et al., 1997).

PAGE 29

17 OBJECTIVE The objectives of this research were to 1) use cover crops to increase nutrient availability in a calcareous soil, and 2) to improve growth of sweet potat o [Ipomea batatas (L.) Lam] in south Florida. Three cover crops, Sunnhemp, velvet bean, and sorghum sudan grass, were grown and incorporated into the soil prior to sweet potato planting. Each cover crop was evaluated for its effect on nutrient availabili ty in the soil and average nutrient amount taken up by the cover crop. In addition to cover crop treatments, sweet potato response to foliar applications of micronutrient fertilizer, iron (Fe) and zinc (Zn) was evaluated in terms of effect on crop growth and yield. Iron and zinc were chosen for application because of their limited availability in alkaline soil.

PAGE 30

18 MATERIALS AND METHOD S Establishment of Field Experiment The field experiment was carried out at a commercial vegetable farm in Homestead, Flori da in 2004. The experiment was set up using a split plot design with three cover crop treatments and a control (fallow) as the main plots and three subplots for foliar applications of iron and zinc. There were four replications of each treatment combinat ion for a total of 16 main plots and 48 subplots. The three cover crop treatments were sorghum sudan grass (Sorghum sudanense L.), sunnhemp (Crotalaria juncea L.) and velvet bean (Mucuna pruriens). There were three subplots within each main plot; one rece ived foliar applications of chelated zinc, one received foliar applications of chelated iron, and the third served as a control that received the water carrier without chemicals. Cover Crop Management The field site was disked several times between 23 and 30 Aug. 2004 in preparation for cover crop planting. Soil samples were taken on 30 Aug. 2004 for each main plot prior to cover crop planting. Cover crops were planted on 31 Aug. 2004. (Seeding rates are shown in Table 1.) No fertilizer or irrigation was applied at any time to any of the treatment plots during the cover crop growing season. Soil, plant tissue, and biomass samples were taken on 7 Nov. 2004 prior to cutting cover crops. Cover crops were mowed and tilled on 9 Nov. 2004 using a flail mower an d a rototiller to minimize carrying cover crops into adjacent treatment plots. The site was tilled a second time on

PAGE 31

19 24 Nov. 2004. A third set of soil samples was collected 9 weeks after cover crop incorporation on 19 Feb. 2005 before sweet potato floweri ng. Table 1. Seedling rates for cover crops. *Inoculant used: Cowpea/Peanut/Lespedeza. Sweet Potato Crop Management The field was disked and prepared for sweet potato planting on 15 Dec. 2004. Sweet potatoes were planted on 20 Dec. 2004 with 1.52 m row s pacing using slip cuttings of approximately 30 cm. Soil samples were collected on 19 Feb. 2005 after cover crop incorporation. Plant tissue samples using newly mature leaves were collected on 26 Mar. 2005 before flowering just prior to foliar applications of micronutrient fertilizers, and again on 03 Apr. 2005 at flowering, 1 week after application of Fe (Becker Underwood, EDTA, 10% Fe) and Zn (Ciba, EDTA, 14% Zn). The sweet potatoes were harvested on 30 June 2005. The grower fertilized the sweet potato c rop with a 7 10 15 fertilizer. It also contained several micronutrients, 6.05% S, 1.1% Ca, 3% Mg, 0.45% Fe, and 0.06% B. Other additions made to the sweet potato crop by the grower were 16 ounces of Pencap M and 1 Quart of Moniter after planting, 6 ounce s perthrium and 1 quart of indo sulfin two weeks after planting, and 5 ounces of capture 4 weeks after planting. Occasionally the grower may add snail bait, Oracal, to the sweet potato crop.

PAGE 32

20 The table of contents and lists of tables and figures are treate d by Word as if they were single objects. If you update a table of contents or list, you will discover that the entire table of contents and all lists will be updated, not just one entry. If minor changes are required to a table of contents or list after it has been created, then remember that these changes will need to be made every time the table of contents or list is recreated. Sample Collection and Chemical Analysis Soil samples were collected prior to cover crop planting, prior to cutting cover crop s, and 9 weeks after incorporation (Table. 2). Samples were collected from 0 15 cm depth, air dried and sieved (2 mm). Samples were analyzed for total N and C using a CNS Analyzer (Vario MAX CNS, Germany), and for AB DTPA extractable P, K, Mg, Fe, Zn, B, Mn, and Cu using an atomic absorption spectrometer (AAS, AA 6300, Shimadzu, Japan). Biomass samples were collected for cover crops prior to cutting (Table. 2). Whole plants within a 103 cm2 sampling area were collected from each treatment plot and fallow p lot. Wet and dry weights were recorded and used to determine the biomass of each treatment in Mg/ha. Nematode samples were collected prior to planting cover crops, prior to cutting cover crops, and 9 weeks after incorporation of cover crops. Table 2. Sam ple collection type and dates.

PAGE 33

21 Cover crop tissue samples were collected for chemical analysis prior to cutting (Table 2). Whole plants within sampling area were collected. Tissue samples for sweet potato were collected prior to flowering, at flowe ring prior to foliar applications of micronutrient fertilizers, and again 1 week after foliar applications of micronutrient fertilizers. A final plant tissue sample was collected prior to sweet potato harvest. Newly mature leaves were collected for sweet potato tissue samples. Samples were washed with detergent (Liquinox), diluted HCl, and rinsed with DDI water. Then samples were dried at 70 C and ground. Samples were analyzed for N and C using a CNS Analyzer. The concentrations of P, K, Mg, Fe, Zn, B, Mn, and Cu, were measured by dry ash and determined using the AAS. Statistical Analysis All data were analyzed with SAS statistical software (version 8.1, SAS Inst. Inc., Cary, NC). Duncans multiple range test was used to separate the means between trea tment plots and sampling dates.

PAGE 34

22 RESULTS AND DISCUSSI ON Cover Crop Biomass Cover crop heights were measured 28, 42, 55, and 68 days after planting (DAP) (Table 3). Sorghum sudan produced the greatest amount of biomass at 13.01 Mg/ha, followed by sunnhemp at 8.01 Mg/ha and velvet bean at 5.22 Mg/ha (Table 4, Fig. 1). The control plot (fallow with weeds) produced the least amount of biomass, 3.10 Mg/ha, of any treatments. However, both sunnhemp and velvet bean did not reach their expected biomass productio n. The short days during the time of year they were planted caused both cover crops to flower early (Gardner et al., 1985), which slowed their growth. In addition, velvet bean is typically grown as a summer cover crop and did not grow well due to the sho rt days during the fall. It also needs a longer time to establish and build biomass than the other cover crops used in this study. Other studies in south Florida have shown sunnhemp and velvet bean to produce as much as 12.1 to 19.7 Mt/ha and 7.8 to 9.95 Mt/ha, respectively (Wang et al., 2003; 2005). Table 3. Cover crop heights (cm) measured at 28, 42, 55, and 68 days after planting (DAP). DAP Days after planting.

PAGE 35

23 Table 4. Cover crop and weed biomass yield (Mg/ha) collected 68DAP (07 Nov. 2004). Mean s in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05). Figure 1. Cover crop biomass collected on 07 Nov. 2004 prior to incorporation of cover crops. Means with the same letter are not signifi cantly different by Duncans multiple range test ( P 0.05). Nutrient Concentrations in Cover Crop Plant tissue samples were taken for each treatment plot 68 DAP. Sunnhemp samples contained a significantly higher concentration of N, 17.47 g/kg, than sorgh um sudan grass but not significantly different from velvet bean or fallow with weed plots (Table 5, Fig. 2). Velvet bean and weeds both had a significantly higher concentration of

PAGE 36

24 N, 14.26 g/kg and 13.16 g/kg respectively, than sorghum sudan grass, but no t significantly different from each other (Table 5, Fig. 2). Both sunnhemp and velvet bean are legumes that fix N, which attributes the greater concentrations of N in sunnhemp compared with the other treatments; however, though velvet bean is also a legum e it contained only slightly higher concentrations of N than fallow with weed plots (Table 5, Fig. 2). Table 5. Nutrient concentrations in cover crop tissues collected on 7 Nov. 2004. Means in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05). Carbon concentrations were greatest in sorghum sudan grass and sunnhemp samples (Table 5, Fig. 3). These cover crops also produced the greatest amount of

PAGE 37

25 biomass compared with other treatment plots and therefore would have accumulated more carbon in their biomass through photosynthesis (Brady and Weil, 2002). Sunnhemp samples contained significantly higher concentrations of manganese (Mn) compared with other treatments (Table 5, Fig. 4), which could cont ribute significant quantities of Mn to a subsequent crop. Figure 2. Cover crop plant tissue macronutrient concentrations in samples collected on 07 Nov. 2004 prior to cutting cover crops. Means with the same letter are not significantly different by Dunc ans multiple range test ( P 0.05). Velvet bean samples had significantly higher concentrations of Zn and Cu, 86.36 mg/kg and 26.87 mg/kg respectively, compared with other treatments (Table 5, fig. 4). Velvet bean tissue samples also contained significantly greater concentrations of P than either sorghum sudan grass or sunnhemp samples. This result suggests that given a large production of biomass under different growing conditions with longer day length, velvet bean could contribute significant quantities of Zn, Cu, and P to a subsequent crop.

PAGE 38

26 Figure 3. Cover crop plant tissue carbon concentration in samples collected on 07 Nov. 2004 prior to cutting cover crops. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05). Figure 4. Cover crop plant tissue micronutrient concentrations in samples collected on 07 Nov. 2004 prior to cutting cover crops. Means with the same letter for same nutrient are not significantly different by Duncans multiple range test ( P 0.0 5). Total Nutrients in Cover Crop Biomass Sunnhemp contained significantly more N in its total biomass than any other treatment (Table. 6, Fig. 5). Since sunnhemp is a legume, it is able to fix its own N and

PAGE 39

27 therefore contained more N in its biomass than t he greater biomass produced by sorghum sudan grass. Sorghum sudan and velvet bean contained significantly more N in their biomass than weed biomass, but were not significantly different from each other (Table. 6, Fig. 5). Sunnhemp contained significantly m ore Mn in its total biomass compared with the other treatments. It also had a significantly higher concentration of Mn in its plant tissue than other treatments (Table 5, Fig. 4). The total amount of Mn in each treatments biomass did not correspond with the amount of biomass produced by each treatment (Table 4, Fig. 1). Manganese is a key nutrient used for N transformation, metabolism, and assimilation (Brady and Weil, 2002). This may be why sunnhemp contained significantly more Mn in its biomass, since sunnhemp is a legume and fixes N. Figure 5. Cover crop total biomass macronutrients. Means with the same letter for same nutrient are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 40

28 Table 6. Total nutrient amounts in cover crop bio mass. Means in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05). Sorghum sudan grass contained more C, K, Mg, Fe, and B in its total biomass than other treatments, followed by sunnhemp, velve t bean, and weeds (Table 5, Fig. 6). The total amount of these nutrients in each treatments biomass corresponds with the amount of biomass produced by each treatment, where sorghum sudan produced the greatest amount of biomass, followed by sunnhemp, velve t bean and weeds (Table 4, Fig. 1). Sorghum sudan grass contained significantly more P in its biomass than the weed plots, but not significantly more than sunnhemp or velvet bean plots (Table 5, Fig.6). Sorghum

PAGE 41

29 sudan grass total biomass contained greater a mounts of all nutrients except for N, Mn, and Cu. Figure 6. Cover crop total biomass carbon. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05). Figure 7. Cover crop total biomass micronutrients. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 42

30 The total amount of Zn and Cu in each treatments biomass did not correspond with the amount of biomass produced by each treatment (Table 6; Fig. 7). Sorghum su dan grass contained more Zn in its total biomass than other treatments, but not significantly more than velvet bean (Table 6; Fig. 7). Velvet bean also contained significantly higher zinc and copper concentrations in its plant tissue than any of the other treatments except for sorghum sudan grass. This result suggests that velvet bean, which contained significantly more Zn and Cu with less biomass than sunnhemp or weeds, may be able to access additional pools of soil Zn and Cu. Thus, under other growing co nditions when the days are longer, velvet bean could contribute large amounts of Zn and Cu to a succeeding crop. Soil Nutrients Prior to Planting Cover Crops Concentrations of soil nutrients within each of the treatment plots were uniform prior to plantin g cover crops. There were no significant differences between treatment plots prior to planting cover crops for either total N or total C (Table 7; Figs. 8 and 9). There were also no significant differences found between treatment plots for AB DTPA extrac table P, K, Mg, Fe, Zn, B, Mn, or Cu (Table 7; Figs. 10 and 11). Soil Nutrients Prior to Cutting Cover Crops There were no significant differences in total P, or extractable P, Zn, Mn, and Cu between treatment plots prior to cutting cover crops (Table 8; F igs. 12, 14, and 15). There were also no significant differences in these nutrients in samples collected prior to planting cover crops and prior to cutting cover crops, except for manganese, which was greater in soil samples prior to planting than before c utting cover crops. Sunnhemp, velvet bean and weed plots had significantly more carbon removed than sorghum sudan plots (Table. 8, Fig. 13); however, there were no significant differences found between

PAGE 43

31 samples prior to planting or prior to cutting cover cr ops for any of the treatment plots (Tables 10, 11, 12, and 13). Figure 8. Total N in soil collected on 31 Aug. 2004 prior to planting cover crops. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05). Figu re 9. Total C in soil collected on 31 Aug. 2004 prior to planting cover crops. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 44

32 Table 7. Total N, C and AB DTPA extractable nutrients (P, K, Mg, Fe, Zn, B, Mn, and Cu) in soil samples collected on 31 Aug. 2004 prior to planting cover crops. Means in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05). Table 8. Extractable nutrient concentration in soil samples collected on 07 Nov. 2004 prior to cutting cover crops. Means in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05). There was a greater amount of potassium removed from sorghum s udan grass and sunnhemp plots than velvet bean or weed plots (Table 8; Fig. 14), in part because both cover crops produced more biomass than velvet bean or weeds (Table 4; Fig. 1) and as a consequence contained more potassium in their total biomass (Table 6; Fig. 5). There was a greater amount of iron removed from sunnhemp and velvet bean plots than from sorghum sudan plots (Table. 8). This result may be attributable to legumes requiring iron

PAGE 45

33 for the enzyme nitrogenase, which is necessary for nitrogen fixa tion (Brady and Weil, 2002). However, there were no significant differences found between samples prior to planting cover crops and prior to cutting cover crops for any cover crop treatment plots (Table 10; 11, 12, and 13). Figure 10. AB DTPA extractabl e P, K, and Mg in soil collected on 31 Aug. 2004 prior to planting cover crops. Means with the same letter for same nutrient are not significantly different by Duncans multiple range test ( P 0.05). Sorghum sudan grass and sunnhemp plots contained less bor on than velvet bean or weed plots (Table 8). Both sorghum sudan grass and sunnhemp produced more biomass (Table 4) and therefore contained more boron in their total biomass than velvet bean or weeds (Table 6), and in turn took up more boron from the soil.

PAGE 46

34 Figure 11. AB DTPA extractable Fe, Zn, B, Mn, and Cu in soil collected on 31 Aug. 2004 prior to planting cover crops. Means with the same letter for same nutrient are not significantly different by Duncans multiple range test ( P 0.05). Table 9. Extra ctable nutrient concentration in soil samples collected on 19 Feb. 2005 after incorporation of cover crops. Means in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 47

35 Table 10. Extractable s oil nutrient concentrations in weed plots compared between sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov. 2004, and after incorporation on 19 Feb. 2005. Means in a column followed by the same letter are not significantly diffe rent by Duncans multiple range test ( P 0.05). Table 11. Extractable soil nutrient concentrations in sorghum sudan grass plots compared between sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov. 2004, and after incorporation on 19 F eb. 2005. Means in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 48

36 Table 12. Extractable soil nutrient concentrations in sunnhemp plots compared between sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov. 2004, and after incorporation on 19 Feb. 2005. Means in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05). Table 13. Extractable soil nutrient concentra tions in velvetbean plots compared between sampling dates: before planting on 31 Aug. 2004, before cutting on 7 Nov. 2004, and after incorporation on 19 Feb. 2005. Means in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 49

37 Figure 12. Total nitrogen in soil collected on 07 Nov. 2004 prior to cutting cover crops. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05). Figure 13. Total carbon in so il collected on 07 Nov. 2004 prior to cutting cover crops. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 50

38 Figure 14. AB DTPA extractable macronutrients in soil collected on 07 Nov. 2004 prior to cutt ing cover crops. Means with the same letter for same nutrient are not significantly different by Duncans multiple range test ( P 0.05). Figure 15. AB DTPA extractable micronutrients in soil collected on 07 Nov. 2004 prior to cutting cover crops. Means wi th the same letter for same nutrient are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 51

39 Soil Nutrients at 9 Weeks After Incorporation of Cover Crops There were no significant differences found between treatment plots for total nitro gen, total carbon, or AB DTPA phosphorus, zinc, manganese, or copper after incorporation of cover crops (Table 9; Fig. 16, and 17). There were significant increases for soil total nitrogen, AB DTPA phosphorus and zinc after incorporation of cover crops com pared with prior to planting cover crops; however, there were no significant differences for these nutrients between treatment plots (Tables 10, 11, 12, and 13). This result may be in part due to fertilizer added by the grower to the field site during pre vious plantings and to the subsequent sweet potato crop. Figure 16. Total N in soil samples collected on 19 Feb. 2005 after incorporation of cover crops. Means with the same letter are not significantly different by Duncans multiple range test ( P 0. 05).

PAGE 52

40 Figure 17. Total C in soil samples collected on 19 Feb. 2005 after incorporation of cover crops. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05). Figure 18. AB DTPA extractable soil nutrients in samples collected on 19 Feb. 2005 after incorporation of cover crops. Means with the same letter for same nutrient are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 53

41 There was no significant difference in extractable K prior to planting or after incorporation of cover crops for any of the treatment plots except that velvet bean plots contained significantly less potassium than fallow with weed plots (Table 9). All treatment plots contained significantly less K prior to cutting c over crops compared with either pre planting or after incorporation of cover crops (Tables 10, 11, 12, and 13). This result may be because K is readily leached from the soil. Potassium also has a higher leaching capacity in soils with less negatively charg ed cation exchange sites (Brady and Weil, 2002). However, K leaching is reduced in soils where Ca2+ and Mg2+ are present, such as in limed soils, suggesting that in a higher pH calcareous soil K leaching would be reduced (Brady and Weil, 2002). The likely explanation for the significant reduction in plant available K prior to incorporation of the cover crops is that plants tend to take up soluble K in greater amounts than needed for plant growth (Brady and Weil, 2002) suggesting that this reduction in K was simply due to plant uptake. In all treatment plots, extractable Mg was significantly lower after incorporation of cover crops compared with prior to cutting or prior to planting the cover crops (Tables 10, 11, 12, and 13). After incorporation of cover c rops, velvet bean plots contained significantly more extractable Mg than all other treatment plots, and sunnhemp and fallow with weed plots contained significantly more Mg than sorghum sudan grass plots (Table 9). After incorporation of cover crops, Sorg hum sudan grass plots contained significantly more extractable Fe than velvet bean plots, but not significantly different that sunnhemp or fallow plots (Table 9; Fig. 19). There was no significant difference in extractable Fe concentration before planting after planting, or after incorporation of

PAGE 54

42 cover crops in velvet bean, sorghum sudan, or sunnhemp plots. However, before planting and after incorporation in fallow with weed plots, iron was significantly higher than before cutting cover crops (Table 10). Both Mg and Mn were greater in all treatment plots prior to planting cover crops compared with before cutting or after incorporation of cover crops (Tables 10, 11, 12, and 13; Figs. 18 and 19). Plant available Cu was significantly lower for all treatments after incorporation of cover crops compared with before planting cover crops (Tables 10, 11, 12, and 13). Figure 19. AB DTPA extractable soil nutrients in samples collected on 19 Feb. 2005 after incorporation of cover crops. Means with the same letter for same nutrient are not significantly different by Duncans multiple range test ( P 0.05). Soil Nematodes There were minimal counts of Aphelenchus, Belonolaimus, Pratylenchus, Quinisulcius, and Scutellonema found in all treatment plots. The highest nema tode counts were for Helicotylenchus, Meloidogyne (root knot nematode), and Paratrichodorus in all treatment plots. The total number of nematodes, for all species

PAGE 55

43 counted, decreased after incorporation of cover crops. In addition, velvet bean decreased t he number of Helicotylenchus and Meloidogyne during its growing period with a significant decrease in the number of Meloidogyne after incorporation of velvet bean. Fallow plots had a significant decrease in the number of Meloidogyne and Paratrichodorus af ter incorporation. Sunnhemp plots also had a significant decrease in the number of Paratrichodorus present in the soil after incorporation. Sorghum sudan showed significant decreases in Helicotylenchus, Meloidogyne, Quinisulcius, and Paratrichodorus spec ies present in the soil after incorporation (Table 14; Figs. 20, 21, 22, and 23). Figure 20. Soil nematodes counted in fallow plots. Samples collected prior to planting cover crops on 31 Aug. 2004, before cutting 07 Nov. 2004, and after incorporation on 19 Feb. 2005. Means with the same letter for same nematode are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 56

44 Table 14. Nematode population (direct count) in soil samples collected prior to planting cover crops on 31 Aug, before cu tting 07 Nov, and after incorporation on 19 Feb. 2004. Means with the same letter for same nematode are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 57

45 45 Figure 21. Soil nematodes counted in sorghum sudan plots. Samples collecte d prior to planting cover crops on 31 Aug. 2004, before cutting 07 Nov. 2004, and after incorporation on 19 Feb. 2005. Means with the same letter for same nematode are not significantly different by Duncans multiple range test ( P 0.05). Figure 22. Soil nematodes counted in sunnhemp plots. Samples collected prior to planting cover crops on 31 Aug. 2004, before cutting 07 Nov. 2004, and after incorporation on 19 Feb. 2005. Means with the same letter for same nematode are not significantly different by Dun cans multiple range test ( P 0.05).

PAGE 58

46 46 Figure 23. Soil nematodes counted in velvet bean plots. Samples collected prior to planting cover crops on 31 Aug. 2004, before cutting 07 Nov. 2004, and after incorporation on 19 Feb. 2005. Means with the same letter for same nematode are not significantly different by Duncans multiple range test ( P 0.05). Nutrient Concentrations in Sweet Potato Leaves Sweet potato leaf tissue collected from sunnhemp plots contained greater concentrations of N, P, K, and Cu, but cont ained the lowest concentrations of Fe and Mg compared with tissue samples collected from other treatment plots (Table 15; Figs. 24 and 26). Tissue samples collected from sorghum sudan grass plots contained significantly more C than velvet bean plots, but not significantly more than sunnhemp or fallow with weed plots (Table 15; Fig. 25). There was no significant difference in Fe concentration of sweet potato leaf tissue in Fe subplots within each cover crop treatment plot after foliar application of Fe; h owever, sunnhemp plots did have a lower concentration of Fe than other treatment plots (Table 16; Fig. 27). There was also no significant difference in Zn concentration of

PAGE 59

47 47 sweet potato leaf tissue in Zn subplots within each cover crop treatment plot after foliar application of Zn (Table 17; Fig. 28). Table 15. Sweet potato leaf nutrient concentrations in samples collected on 03 April 2005, 1 week after foliar applications of Fe and Zn. Means in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 60

48 48 Figure 24. Sweet potato leaf macronutrient concentrations in samples collected on 03 April 2005 1 week after foliar applications of Fe and Zn. Means with the same letter for same nutrient are not sig nificantly different by Duncans multiple range test ( P 0.05). Figure 25. Sweet potato leaf carbon concentrations in samples collected on 03 April 2005 1 week after foliar applications of Fe and Zn. Means with the same letter are not significantly differ ent by Duncans multiple range test ( P 0.05).

PAGE 61

49 49 Figure 26. Sweet potato leaf micronutrient concentrations in samples collected on 03 April 2005 1 week after foliar applications of Fe and Zn. Means with the same letter for same nutrient are not significant ly different by Duncans multiple range test ( P 0.05). Table 16. Iron tissue concentration in iron subplots in samples collected on 03 April 2005, 1 week after foliar application of Fe. Means in a column followed by the same letter are not significant ly different by Duncans multiple range test ( P 0.05).

PAGE 62

50 50 Table 17. Zinc tissue concentration in zinc subplots in samples collected on 03 April 2005 1 week after foliar application of Zn. Means in a column followed by the same letter are not significant ly different by Duncans multiple range test ( P 0.05). Figure 27. Iron in sweet potato leaf tissue collected on 03 April 2005 in Fe subplots 1 week after foliar application of iron. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 63

51 5 1 Figure 28. Zinc in sweet potato leaf tissue collected on 03 April 2005 in Zn subplots 1 week after foliar application of iron. Means with the same letter are not significantly different by Duncans multiple range test ( P 0. 05). Sweet Potato Yield The boniato tropical sweet potato can be grown year round in south Florida and takes 120 to 180 days to reach maturity Shortly after planting there were some freezes in the study area that damaged the young slips. Large areas of the young plants did not recover from these winter freezes. The sweet potato yield was higher in sunnhemp and sorghum sudan treatment plots (Table 18; Fig. 29), as well as Fe subplots (Table 18; Fig. 30); however, there were no significant differences amo ng cover crop treatment plots or among micronutrient subplots. This result may be due to the time of year the study was conducted and the damage to the crop as a result of the winter freezes.

PAGE 64

52 52 Table 18. Sweet potato harvest collected on 30 June 2005. *E ach plot equals 9.3 sq. m. Means in a column followed by the same letter are not significantly different by Duncans multiple range test ( P 0.05). Figure 29. Sweet potato harvest in cover crop treatment plots (each plot equals 9.3 m 2 ) collected on 30 June 2005. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 65

53 53 Figure 30. Sweet potato harvest in micronutrient subplots (each plot equals 9.3 m 2 ) collected on 30 June 2005. Means with the same letter are not significantly different by Duncans multiple range test ( P 0.05).

PAGE 66

54 SUMMARY Sorghum sudan grass and sunnhemp produced more biomass than velvet beans or weeds, at 13.01 Mg/ha and 8.01 Mg/ha respectively. Neither sunnhemp nor velvet bean reached thei r expected biomass production. This result was likely due, in part, to the short days during the time of year they were planted, which caused them to flower early and slowed their growth. Sunnhemp contained significantly more N in its total biomass than other tested cover crops. Sunnhemp also contained significantly more Mn in its total biomass, which is a key nutrient for N transformation, metabolism, and assimilation. Soil samples taken prior to planting cover crops showed no significant difference in total or AB DTPA extractable nutrients. There were some differences found in measured soil nutrients prior to cutting cover crops, but in general these samples contained less of each nutrient than samples taken prior to planting cover crops. However, aft er incorporation of cover crops some nutrients did increase, though not significantly between treatment plots except for Mg, which was significantly greater in velvet bean plots than other cover crop plots. Fe and Mg concentrations were lowest in sweet pot ato leaf tissue samples collected from sunnhemp plots; however, tissue samples collected from sunnhemp plots contained significantly greater concentrations of N, P, K, and Cu. Fe concentrations were not significantly different in Fe subplots for any of th e cover crop main treatment plots. Zn concentrations were also not significantly different in Zn subplots in any of the cover crop main treatment plots.

PAGE 67

55 55 There were no significant differences in sweet potato yield between any of the main treatment plots or subplots. The sweet potato crop had been damaged by winter freezes and did not produce as expected. Sunnhemp and Sorghum sudan grass both grew well during the shorter fall days. Velvet bean may be a good choice as a cover crop if it has a longer time t o get established. It would be recommended to run the study for a second year and during a time of year when the days are longer to see what the effect on the sweet potato yield.

PAGE 68

56 LIST OF REFERENCES Acosta Martinez, V., D.R. Upchurch, A.M. Schubert, D. Porter, and T. Wheeler. 2004. Early impacts of cotton and peanut cropping systems on selected soil chemical, physical, microbiological and biochemical properties. Biol. Fertil. Soils, 40:44 54. Balota, E. L., A. Colozzi Filho, D. S. Andrade, and R. P. D ick. 2003. Microbial biomass in soils under different tillage and crop rotation systems. Biol. Fertil. Soils, 38:15 20. Banful, B., A. Dzietror, I. Ofori, and O. B. Hemeng. 2000. Yield of plantain alley cropped with Leucaena leucocephala and Flemingia macrophylla in Kumasi, Ghana. Agroforestry Systems, 49:189 199. Blackshaw, R., J. R. Moyer, R. C. Doram, and A. L. Boswell. 2001. Yellow sweet clover, green manure, and its residues effectively suppress weeds during fallow. Weed Sci., 49:406 413. Bok htiar, S. M., M. A. Gafur, and A.B.M.M. Rahman. 2003. Effects of Crotalaria and Sesbania aculeata green manures and N fertilizer and the productivity of sugar cane. J. Agric. Sci., 140:305 309. Brady, N. C., and R. R. Weil. 2002. The Nature and Prope rties of Soils. Thirteenth Edition, Prentice Hall, Upper Saddle River, NJ. Buckles, D., and B. Triomphe. 1999. Adoption of mucuna in the farming systems of northern Honduras. Agroforestry Systems, 47:67 91. Cavigelli, M. A. and S. J. Thien. 2003. Phos phorus bioavailability following incorporation of green manure crops. Soil Sci. Soc. Am. J.. 67:1186 1194. Chambliss, C. G., R. M. Muchovej, and J. J. Mullahey. 2003. Cover crops, UF, IFAS, SS AGR 66. Chander, K., S. Goyal, M.C. Mundra, and K.K. Kapoo r. 1997. Organic matter, microbial biomass and enzyme activity of soils under different crop rotations in the tropics. Bio. Fertil. Soils, 24:306 310. Cobo, J.G., E. Barrios, D.C.L. Kass, and R.J. Thomas. 2002. Decomposition and nutrient release by green manures in a tropical agroecosystem. Plant Soil, 240:331 342.

PAGE 69

57 57 Curci, M., M.D.R. Pizzigallo, C. Crecchio, R. Mininni, and P. Ruggiero. 1997. Effects of conventional tillage on biochemical properties of soils. Bio. Fertil. Soils, 25:1 6. Ekeleme, F., I. O. Akobundu, A.O. Ishichei, and D. Chikoye. 2003. Cover Crops Reduce weed seedbanks in maize cassava systems in southwestern Nigeria. Weed Sci., 51:774 780. Fischler, M., and C. S. Wortmann. 1999. Green manures for maize bean systems in easter n Uganda: Agronomic performance and farmers perceptions. Agroforestry Systems, 47:123 138. Fowler, C.J.E., L.M. Condron, and R.D. McLenaghen. 2004. Effects of green manures on nitrogen loss and availability in an organic cropping system. New Zealand J Agric. Res., 47:95 100. Franchini, J. C., M. A. Pavan, and M. Miyazawa. 2004. Redistribution of phosphorus in soil through cover crop roots. Brazilian Archives Bio. Tech., 47(3):381 386. Gardner, F. P., R. B. Pearce, R. L. Mitchel. 1985. Physiology of Crop Plants. Iowa State Univ. Press, Ames, IA. Goyal, S., K. C., M.C. Mundra, and K.K. Kapoor. 1999. Influence of inorganic fertilizers and organic amendments on soil organic matter and microbial biomass properties under tropical conditions. Bio. F ertil. Soils, 29:196 200. Hacisalihoglu, G. and L. V. Kochiam. 2003. How do some plants tolerate low levels of soil zinc? Mechanisms of zinc efficiency in crop plants. New Phyt., 159:341 350. Hanly, J.A. and P.E.H. Gregg. 2004. Green manure impacts on nitrogen availability to organic sweetcorn (zea mays). New Zealand J. Crop Hort. Sci.. 32:295 307. Havlin, J. L., J. D. Beaton, S. L. Tisdale, and Werner L. Nelson. 1999. Soil Fertility and Fertilizers: An Introduction to Nutrient Management. Sixth E dition, Prentice Hall, Inc, Upper Saddle River, NJ Lazzeri, L., G. Baruzzi, L. Malaguti, and L. Antoniacci. 2003. Replacing methyl bromide in annual strawberry production with glucosinolate containing green manure crops. Pest Manage. Sci., 59:983 990. Li, Y.C., H.H. Bryan, R. Rao, N. Heckert, and T. Olczyk. 1999. Summer cover crops for tomato production in south Florida, P.18 21. Proc. Conference Florida Tomato Institute, Citrus &Vegetable Magazine, Tampa, Fl. Little, S.A., P.J. Hocking, and R.S.B. Greene. 2004. A preliminary study of the role of cover crops in improving soil fertility and yield for potato production. Commun. Soil Plant Anal., 35:471 494.

PAGE 70

58 58 Lui, A., C. Hamel, R.I. Hamilton, B.L. Ma and D.L. Smith. 2000. Acquisition of Cu, Zn, Mn and Fe by mycorrhizal maize (Zea mays L.) grown on soil at different P and micronutrient levels. Mycorrhiza, 9:331 336. Mandal, U. K., G. Singh, U.S. Victor, K.L. Sharma. 2003. Green manuring: its effect on soil properties and crop growth under rice wh eat cropping system. European J.Agron., 19:225 237. Misra, A., and G. Tyler. 2000. Effect of wet and dry cycles in calcareous soil on mineral uptake of two grasses, Agrostis stolonifera L. and Festuca ovina L. Plant Soil, 24:297 303. Moore, J.M., S. K lose, and M.A. Tabatabai. 2000. Soil Microbial biomass carbon and Nitrogen as affected by cropping systems. Bio. Fertil. Soils, 31: 200 210. Morris, J. B., and J. T Walker. 2002. Non traditional legumes as potential soil amendments for nematode control. J. Nematol., 34(4):358 361. Nziguheba, G., R. Merckx, C. A. Palm, and P. Mutuo. 2002. Combining Tithonia diversifolia and fertilizers for maize production in a phosphorus deficient soil in Kenya. Agroforestry Systems, 55:165 174. Okpara, D.A., J.C. Nj oku and J.E. Asiegbu. 2004. Response of two sweet potato varieties to four green manure sources and inorganic fertilizer in a humid tropical ultisol. Bio. Agric. Hortic., 22:81 90. Sangakkara, U.R., M. Liedgens, A. Soldati, and P. Stamp. 2004. Root an d shoot growth of maize (zea mays) as affected by incorporation of Crotalaria juncea and Tithonia dicversifolia as green manures. J. Agron. Crop Sci., 190:339 346. Sangakkara, U.R., W. Richner, M.K. Schneider, P. Stamp. 2003. Impact of intercropping be ans (Phaseolus vulgaris L.) and sunnhemp (Crotalaria juncea L.) on growth, yields and nitrogen uptake of maize (Zea mays L.) grown in the humid tropics during the minor rainy season. Maydica., 48:233 238. Savithri, P., R. Perumal, and R. Nagarajan. 1999 Soil and crop management technologies for enhancing rice production under micronutrient constraints. Nutr. Cycling Agroecosystems, 53:81 92. Singh, D., R.L. Yadav, J.P. Singh and R. Pandey. 2003. Effect of intercropped prickly sesban (Sesbania cannab ina) green manure and nitrogen levels on growth and yield of sugarcane. Indian J. Agric. Sci., 73:664 667. Smestad, B. Thor, H. Tiessen, and R. J. Buresh. 2002. Short fallows of Tithonia diversifolia and Crotalaria grahamiana for soil fertility improve ment in western Kenya. Agroforestry Systems, 55:151 194.

PAGE 71

59 59 Wang, Q., Y. Li and W. Klassen. 2005a. Influence of summer cover crops on conservation of soil water and nutrients in a subtropical area. J. Soil Water Conserv., 60:58 63. Wang, Q., Y. C. Li, and W. Klassen. 2005b. Determination of cation exchange capacity on low to highly calcareous soils. Commun. Soil Sci. Plant Anal., 36 (11&12): 1479 1498. Wang, Q., Y.C. Li, and W. Klassen. In press a. Summer cover crops and soil amendments to improve gro wth and nutrient uptake of okra (Abelmoschus esculentus L.). HortTechnology. Wang, Q., Y. C. Li, and W. Klassen. In press b. Microbial biomass of carbon and nitrogen in tomato field with cover crops. J. Plant Nutr. Wang, Q., Y.C. Li, W. Klassen. 2003a Effects of soil amendments at a heavy loading rate associated with cover crops as green manures on the leaching of nutrients and heavy metals from a calcareous soil. J. Environ. Sci. Health, B38(6): 865 881. Wang, Q., W. Klassen, H.H. Bryan, and Y. Li. 2003b. Influence of summer cover crops in growth and yield of a subsequent tomato crop in south Florida. Proc. Fl. State Hortic. Soc., 116:140 143. Wang, Q., W. Klassen, Y.C. Li, M. Codallo, A. Abdul-Baki. 2005c. Influence of cover crops and irrigation rates on tomato yields and quality in a subtropical region. HortScience. 40(7):2125-2131. Yadvinder Singh, B., J.K. Ladha, C.S. Khind, R.K. Gupta, O.P. Meelu, and E. Pasuquin. 2004. Long term effects of organic inputs on yield and soil fertility in ric e wheat rotation. Soil Sci. Soc. Am. J., 68:845 853. Zou, Y., F. Zhang, X. Li, and Y. Cao. 2000. Studies on the improvement in iron nutrition of peanut by intercropping with maize on a calcareous soil. Plant Soil, 220:13 25.

PAGE 72

60 BIOGRAPHICAL SKETCH Im originally from Washington State. I earned a BFA in painting and sculpture from Pacific Lutheran University in 2000 and a BA in geography from Central Washington University in 2002. This change in academic direction awakened my interest in the environment sustainable agriculture and soil science, which led me to pursue a MS in soil science at the University of Florida.