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Agricultural Sciences and should be
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the Institute and its staff. Current IFAS
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site maintained by the Florida
Cooperative Extension Service.
Copyright 2005, Board of Trustees, University
Bulletin 301 September, 1936.
UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATION
WILMON NEWELL, Director
EFFECTS OF SUMMER COVER CROPS
ON CROP YIELDS AND
ON THE SOIL
I. YIELDS OF CORN AND SWEETPOTATOES FOL-
LOWING SUMMER COVER
II. INFLUENCE OF SUMMER COVER ON NITRATE
AND ORGANIC MATTER CONTENT OF A POOR
GRADE OF NORFOLK SOIL
W. E. STOKES
R. M. BARNETTE
J. B. HESTER
Bulletins will be sent free to Florida residents upon application to
AGRICULTURAL EXPERIMENT STATION
EXECUTIVE STAFF BOARD OF CONTROL
John J. Tigert, M.A., LL.D., President of Geo. H. Baldwin, Chairman, Jacksonville
the University Oliver J. Semmes, Pensacola
Wilmon Newell, D.Sc., Director Harry C. Duncan, Tavares
H. Harold Hume, M.S., Asst. Dir., Research Thomas W. Bryant, Lakeland
Harold Mowry, M.S.A., Asst. Dir., Adm. R. P. Terry, Miami
J. Francis Cooper, M.S.A., Editor J. T. Diamond, Secretary, Tallahassee
Jefferson Thomas, Assistant Editor
Clyde Beale, A B.J., Assistant Editor
Ida Keeling Cresap, Librarian BRANCH STATIONS
Ruby Newhall, Administrative Manager
K. H. Graham, Business Manager NORTH FLORIDA STATION, QUINCY
Rachel McQuarrie, Accountant Gratz, Ph.D., Plant Pathologist in
MAIN STATION, GAINESVILLE R. R Kincaid, Ph.D., Asso. Plant Pathologist
AGRONOMY J. D. Warner, M.S., Agronomist
Jesse Reeves, Farm Superintendent
W. E. Stokes, M.S., Agronomist**
W. A. Leukel, Ph.D., Agronomist CITRUS STATION, LAKE ALFRED
G. E. Ritchey, M.S.A., Associate* A. F. Camp, Ph.D., Horticulturist in Charge
Fred H. Hull, Ph.D., Associate John H. Jefferies, Superintendent
W. A. Carver, Ph.D., Associate Michael Peech, Ph.D., Soil Chemist
John P. Camp, M.S., Assistant W. A. Kuntz, A.M., Assoc. Plant Pathologist
B. R. Fudge, Ph.D., Associate Chemist
ANIMAL HUSBANDRY W. L. Thompson, B.S., Asst. Entomologist
A. L. Shealy, D.V.M., Animal Husbandman** EVERGLADES STATION, BELLE GLADE
R. B. Becker, Ph.D., Dairy Husbandman
L. M. Thurston, Ph.D., Dairy Technician A. Daane, Ph.D., Agronomist in Charge
W. M. Neal, Ph.D., Asso. in An. Nutrition R. N. Lobdell, M.S., Entomologist
D. A. Sanders, D.V.M., Veterinarian F. D. Stevens, B.S., Sugarcane Agronomist
M. W. Emmel, D.V.M., Veterinarian Thomas Bregger, Ph.D., Sugarcane Physiologist
N. R. Mehrhof, M.Agr., Poultry Husbandman G. R. Townsend, Ph.D., Assistant Plant
W. W. Henley, B.S.A., Asst. An. Husb.* Pathologist
W. G. Kirk, Ph.D., Asst. An. Husbandman J. R. Neller, Ph.D., Biochemist
R. M. Crown, B.S.A., Asst. An. Husbandman R. W. Kidder, BS., Assistant Animal
P. T. Dix Arnold, B.S.A., Assistant Dairy Husbandman*
Husbandman Ross E. Robertson, B.S., Assistant Chemist
L. L. Rusoff, M.S., Laboratory Assistant B. S. Clayton, B.S.C.E., Drainage Engineer*
Jeanette Shaw, M.S., Laboratory Technician SUB-TROPICAL STATION, HOMESTEAD
CHEMISTRY AND SOILS H. S. Wolfe, Ph.D., Horticulturist in Charge
R. W. Ruprecht, Ph.D., Chemist** W. M. Fifield, M.S., Asst. Horticulturist
R. M. Barnette, Ph.D., Chemist Geo. D. Ruehle, Ph.D., Associate Plant
C. E. Bell, Ph.D., Associate Pathologist
R. B. French, Ph.D., Associate W. CENTRAL FLA. STA., BROOKSVILLE
H. W. Winsor, B.S.A., Assistant
W. F. Ward, M.S., Asst. An. Husbandman
ECONOMICS, AGRICULTURAL in charge*
C. V. Noble, Ph.D., Agricultural Economist**
Bruce McKinley, A.B., B.S.A., Associate FIELD STATIONS
Zach Savage, M.S.A., Associate
A. H. Spurlock, M.S.A., Assistant Leesburg
ECONOMICS, HOME M. N. Walker, Ph.D., Plant Pathologist in
Ouida Davis Abbott, Ph.D., Specialist** W. B. Shippy, Ph.D., Asso. Plant Pathologist
C. F. Ahmann, Ph.D., Physiologist K. W. Loucks, M.S., Asst. Plant Pathologist
J. W. Wilson, Ph.D., Associate Entomologist
ENTOMOLOGY Plant City
J. R. Watson, A.M., Entomologist** A. N. Brooks, Ph.D., Plant Pathologist
A. N. Tissot, Ph.D., Associate
H. E. Bratley, M.S.A., Assistant Cocoa
HORTICULTURE A. S. Rhoads, Ph.D., Plant Pathologist
G. H. Blackmon, M.S.A., Horticulturist and A. H. Eddis, Ph. Pathologist
Acting Head of Department A.H. Eddins, Ph.D., Plant Pathologist
A. L. Stahl, Ph.D., Associate Montieelle
F. S. Jamrison, Ph.D., Truck Horticulturist
R. J. Wilmot, M.S.A., Specialist, Fumigation ------, Asst. Entomologist
R. Dickey, B.S.A., Assistant Horticulturist Bradenton
Divid G. Kelbert, Asst. Plant Pathologist
PLANT PATHOLOGY C. C. Goff, M.S., Assistant Entomologist
W. B. Tisdale, Ph.D., Plant Pathologist** Sanford
George F. Weber, Ph.D., Plant Pathologist E. R. Purvis, Ph.D., Assistant Chemist,
R. K. Voorhees, M.S., Assistant Celery Investigations
Stacy O. Hawkins, M.A., Assistant
Erdman West, M.S., Mycologist Lakeland
Lillian E. Arnold, M.S., Assistant Botanist E. S. Ellison, Ph.D., Meteorologist*
SPECTROGRAPHIC LABORATORY B. H. Moore, A.B., Asst. Meteorologist*
L. W. Gaddum, Ph.D., Biochemist In cooperation with U.S.D.A.
L. H. Rogers, M.A'., Spectroscopic Analyst ** Head of Department.
EFFECTS OF SUMMER COVER CROPS
ON CROP YIELDS AND
ON THE SOIL
I. YIELDS OF CORN AND SWEETPOTATOES FOL-
LOWING SUMMER COVER
By W. E. STOKES
The growing and turning under of cover crops or green
manure crops to influence favorably the yield of succeeding
crops has been practiced in most sections of the country for
a long time and with good results. For- the sandy soils of
Florida, the question is often asked if it is better to plant a
leguminous cover crop or to depend upon a volunteer cover
crop. To answer this question and to determine the relative
value of several cover crops to increase succeeding crop yields,
a series of two-year rotations were established using corn and
sweetpotatoes-as indicator crops and "Florida pusley" or "Mexi-
can clover",1 Richardia scabra St. Hil. (a volunteer crop), Brab-
ham cowpeas, beggarweed, Georgia speckled velvet beans and
Crotalaria striata D. C. as summer cover crops. The rotation
with the volunteer cover crop was similar to the practice of
land "resting", except that when land is "rested" no cultiva-
tion is given after the final summer plowing of the planted
crop, while the land in this rotation was given a spring plow-
ing and allowed to grow up to "pusley".
LOCATION OF EXPERIMENT AND DESCRIPTION OF SOIL
A two-acre field on the Florida Experiment Station farm
was used for rotation tests. This field is typical of large areas
of sandy lands of low fertility in Alachua, Gilchrist, Levy, La-
fayette, Taylor, Columbia and other counties in Florida.
1The colloquial terms "Florida pusley" or "Mexican clover" are used
in Florida to designate Riehardia scabra. St. Hil. Neither of these terms
has any standing nor has been used in available botanical literature and
both are misleading in that Richardia is not a clover and neither is it a
Portulaca which may be called purslane or pusley.
4 Florida Agricultural Experiment Station
The soil is a Norfolk medium fine sand (deep phase). The
surface soil to a depth of about six inches is gray in color and
contains a small amount of organic matter. The surface soil
grades off into a light yellow, somewhat finer textured subsoil,
almost devoid of organic matter. The light yellow subsoil ex-
tends to a depth of eight to 10 feet where clay is encountered.
The field had been used for cultivated crops for a number
of years previous to the experiment but fertilizer and cultural
treatment had been uniform. The low fertility of the virgin
soil had been depleted largely during this period. The native
tree growth on an adjoining tract of the same soil type is long-
PLAN OF EXPERIMENT
In 1923 the experimental field grew a volunteer non-legumin-
ous crop of "Mexican clover" or "Florida pusley" which was
turned into the soil. In 1924 the field was divided into four
equal parts, hereafter referred to as Sections A, B, C and D.
Each section was subsequently divided into 11 plots 19 by 104
feet. Sections A and B were used for the sweetpotato-cover
crop rotations and Sections C and D were used for the corn-
cover crop rotations. A diagram showing the arrangement of
the plots is given in Figure 1.
Beginning in 1924, Section A was planted to cover crops
and B to sweetpotatoes; each year through 1928 these crops
were alternated on the two sections. Also in 1924 Section C was
grown to cover crops and Section D to corn; each year through
1928 these crops were alternated on the two sections in a similar
manner. Cover crops used included cowpeas, beggarweed, vel-
vet beans and Crotalaria striate in duplicate plots and "Florida
pusley" in triplicate plots. The cover crops were incorporated
in the soil each January before the indicator crops were planted.
Yearly cropping record of plots in the four sections:
Year A B C D
1924 Cover crop Sweetpotatoes Cover crop Corn
1925 Sweetpotatoes Cover crop Corn Cover crop
1926 Cover crop Sweetpotatoes Cover crop Corn
1927 Sweetpotatoes Cover crop Corn Cover crop
1928 Cover crop Sweetpotatoes Cover crop Corn
Thus, there were two distinct two-year rotations involved;
namely, sweetpotatoes following various summer cover crops
grown and turned under, and corn following various summer
Effects of Summer Cover Crops on Crop Yields 5
cover crops grown and turned under. All crops were grown
each year. This experiment offered ample opportunity for both
crop and soil studies.
Plot Corn-cover crop rotations
11 "Florida pusley" (volunteer crop) Corn
10 Cowpeas "
9 Beggarweed "
8 Velvet beans
7 I Crotalaria striata ._ _____"
6 "Florida pusley" a )
5 Cowpeas "
4 Beggarweed "
3 Velvet beans "
2 i Crotalaria striata "
1 "Florida pusley" V
Sweetpotato--cover crop rotations
11 "Florida pusley" (volunteer crop) c oSweetpotatoes
10 Cowpeas "__
9 Beggarweed t"
8 Velvet beans __
6 "Floridapuley" ___ __
7 Crotalaria striata ____
6 "Florida pusley" U U
5 Cowpeas "__
4 I Beggarweed "
3 Velvet beans
2 Crotalaria striata _______
1 "Florida pusley" _____
<--- 104.35' ---
<- 208.7' ->
Fig. 1.-Diagram of plot arrangement used in cover crop and rotation
experiments begun in 1924.
6 Florida Agricultural Experiment Station
CULTURAL AND FERTILIZER PRACTICES
Preparation of the seedbed and cultivation of the crops were
such as to preclude, as far as possible, mixing of soil of the
The customary system of cultivation used with sweetpotatoes
and corn in Florida was followed. The sweetpotatoes were
planted and grown on low beds. The corn was planted slightly
below the level and given shallow, level cultivation.
The cover crops, except the volunteer growth, were planted
in the spring and none were fertilized. All plots were prepared
by thoroughly disking each spring, whether they were to grow
volunteer or planted cover. The beggarweed and Crotalaria
striata were planted broadcast and the cowpeas and velvet beans
were planted thickly in 21/2-foot rows. Every care was taken
to insure a perfect stand and good -growth of the crops. The
beggarweed was inoculated, but even so its yields usually were
light, since this crop is poorly adapted to deep Norfolk sand.
None of the other leguminous crops required artificial inocu-
Two 64 square foot areas were harvested from each cover
crop plot each fall when the crop had reached maximum growth.
Green weights of the top growth of cover crops from these
areas were recorded. Dry weight determinations were made
on sub-samples of the plant materials and yields were computed
in pounds of dry matter (8% moisture) per acre.
Cuban Flint corn and Porto Rican sweetpotato draws were
used in the experiment. These crops were planted each year
in March and June, respectively. Both the corn and sweet-
potatoes were fertilized with 600 pounds per acre of a 4-8-4
(NH--P2Os-K20) fertilizer made from nitrate of soda, super-
phosphate and muriate of potash. One-half of the nitrogen
of this fertilizer was withheld for side-dressing. The corn
was side-dressed with nitrate of soda 45 to 50 days after plant-
ing, and the sweetpotatoes after vine growth started.
It may be noted from Figure 1 that each year, except the
first, duplicate plots of corn and sweetpotatoes followed each
of the planted cover crops, and triplicate plots of corn and
sweetpotatoes followed the volunteer cover crops. The first
year corn and sweetpotatoes followed a volunteer cover crop
consisting mainly of "Florida pusley" on all plots. The entire
plot (1/30 acre) of sweetpotatoes was harvested and weighed
upon reaching maturity. The yield of potatoes was calculated
Effects of Summer Cover Crops on Crop Yields 7
in bushels per acre on the basis of 60 pounds of freshly dug
sweetpotatoes per bushel. The three inside rows of the corn
plots were harvested upon maturity. This gave an area of
1/40 acre. The corn was husked and weighed. The number
of bushels of shelled corn per acre was calculated from the
weight of the husked corn, allowing 70 pounds per bushel.
Approximate planting dates, period of maximum growth, and
dates of maturity of the different cover crops and of corn and
sweetpotatoes are given in Table 1.
TABLE 1.-APPROXIMATE- DATES OF PLANTING, PERIOD OF MAXIMUM
GROWTH, AND DATES OF MATURITY OF CORN AND SWEETPOTATOES AND
DIFFERENT COVER CROPS.
Crop dates of Period of maximum Approximate date
planting rate growth of maturity
Corn March June, July, August Early part of
Sweetpotatoes June July, August, October-November
Cowpeas April May, June, July Last part of August
Beggarweed April June, July, August September to mid-
dle of October
Velvet beans April May, June, July, October
Crotalaria striata April May, June, July, October-November
YIELD AND NITROGEN CONTENT OF SUMMER
The yearly and average yields of summer cover crops for the
years 1924 to 1927, inclusive, expressed in pounds of air-dry
material (8% moisture) per acre for the rotations are given
in Table 2. These yields were calculated from the average yield
of duplicate plots in each section. Yields of plots bordering
roadways were not used. In each instance such plots were
growing "Florida pusley", and as these plots were duplicated
within the experimental area no great objection may be raised
TABLE 2.-YEARLY AND AVERAGE YIELDS OF SUMMER COVER CROPS IN POUNDS AIR-DRYI MATERIAL PER ACRE IN THE
SWEETPOTATO AND CORN ROTATIONS.
Sections A and B (sweetpotatoes) Sections C and D (corn) Sections A, B, C and D (both rotations)
Summer Cover I I Ave. Ave. Ave.
Crop 1924 1925 1926 1927 1928 1924 1924 1925 1926 1927 1928 1924 1924 1925 1926 1927 1928 1924 *
Se.A Sec.BSec.A Sc.B Sec.A to Se. C Sec.D SeC Se.D See. C to See. Sec. Sec. Sec. Sec. to
1927 1927 A+C B-4D A+C B+D A+JC 1927
incl. incl. inel. 9
lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. Ibs. Ibs. lbs. lbs. lbs. lbs. lbs, lbs. Ilbs.
"Florida pusley" ..... 1,450 851 1,443 1,1502 a 1,191 851 1,021 1,0212 s a 1,320 851 1,232 1,085
Crotalaria strata .... 5,173 3,492 8,678 4,084 9,528 5,357 3,587 2,722 6,296 2,82 8,508 3,747 4,380 ) 3,107 7,487 8,233 9,018 4,552
Velvet beans ............. 2,164 1,463 1,450 429 4,254 1,377 1,497 1,450 1,361 769 2,382 1,269 1,830 1,457 1,406 599 3,318 1,325
Beggarweed ............ 1,157 987 395 429 1.150 742 579 1,041 340 259 1,320 555 868 1,014 368 344 1,235 648
Cowpeas .................. 2,722 2,096 939 1,361 1,960 1,779 2,552 2,212 511 1,191 2,042 1,616 2,637 2,154 725 1,276 2,001 1,698
1 8% moisture.
2 2-year average.
No record of "Florida pusley" was kept.
Effects of Summer Cover Crops on Crop Yields 9
to excluding yield records of these border plots. Table 2 also
gives the average yields of summer cover crops for both sets
of rotations for the years 1924 to 1927, inclusive.
The average yield of the different cover crops, together with
percentages of nitrogen in the cover crops and the calculated
amounts of nitrogen per acre (top growth), are given in
TABLE 3.-AVERAGE ANNUAL YIELD IN POUNDS OF AIR-DRY MATERIAL PER
ACRE AND PERCENTAGE AND TOTAL NITROGEN IN TOP GROWTH OF COVER
Summer cover crop Yield Percent top growth
l bs. nitrogen .bs.
"Florida pusley" ................. 1,085 1.081 11.73
Crotalaria strict ................ 4,552 2.177 99.09
Velvet beans ......................... 1,323 1.998 26.43
Beggarweed ........................ 648 1.207 7.82
Cowpeas ...........................(. 1,698 1.662 28.22
Crotalaria striata, with its larger yields and high percentage
of nitrogen, returned to the soil an average of 99.09 pounds of
nitrogen per acre. This was approximately four times as much
as the velvet beans or the cowpeas, nine times as much as the
native cover crop of "Florida pusley" and 13 times as much
as the crop of beggarweed.
SWEETPOTATO YIELDS FOLLOWING SUMMER
Table 4 gives the yearly and average yields of sweetpotatoes
obtained during the period of the rotation and the increases
due to planted cover crops. The yearly yields are averages
from duplicate plots.
The yield of sweetpotatoes obtained from the plots of Sec-
tion B (in 1924) before the various summer cover crops were
planted indicate that Section B had a fairly uniform yielding
capacity. The maximum variation in the average yields of
the plots was 5.4 bushels per acre.
TABLE 4.-YIELDS OF SWEETPOTATOES IN BUSHELS PER ACRE FOLLOWING THE INCORPORATION OF SUMMER COVER CROPS.
i A Increase
Section B1 I Section A2 Average due to
Summer cover Average Average Sections planted 2
crop 1924 1926 1928 1926 and 1925 1927 1925 and B and As cover
__1928 1927 crop
bu. bu. bu. bu. bu. bu. bu. bu. bu.
"Florida pusley" .... 58.8 19.9 6.8 13.3 47.3 9.3 28.3 20.8
Crotalaria striata 57.7 39.8 16.3 28.0 78.0 21.9 49.9 38.9 18.1
Velvet beans ............ 5.3 34.3 4.8 19.5 54.5 14.1 34.3 26.9 6.1
Beggarweed ............ 53.4 27.2 5.3 16.2 55.0 6.1 30.5 23.4 2.6
Cowpeas ............. 5 55.3 33.8 6.0 19.9 61.0 10.8 35.9 27.9 7.1
11924 sweetpotatoes following a volunteer cover crop of "Florida pusley" common to all plots; 1926 sweetpotatoes '
following different summer cover crops; 1928 sweetpotatoes following different summer cover crops for second time.
2 1925 sweetpotatoes following different cover crops; 1927 sweetpotatoes following different cover crops for second time.
3 1924 yields not included in this average.
Effects of Summer Cover Crops on Crop Yields 11
The yield of sweetpotatoes in 1925 following the plowing
under of the summer cover crops planted on Section A in 1924
shows a distinct increase over the yield of sweetpotatoes fol-
lowing the incorporation of a volunteer non-leguminous cover
crop of "Florida pusley" of 1924. Plowing under a crop of
Crotalaria striata gave an increase in yield of 30.7 bushels per
acre over the plots where the volunteer cover crop was turned
under. Velvet beans gave an increase of 7.2 bushels; beggar-
weed 7.7 bushels, and cowpeas 13.7 bushels per acre over that
of the volunteer crop. These increase figures are not actually
given in the table but they can be readily calculated therefrom.
The average yield of sweetpotatoes on Section B for 1926
was distinctly lower than that for 1924. However, in 1926
the yield of sweetpotatoes was considerably higher following
all planted cover crops of 1925 on Section B than it was fol-
lowing the volunteer cover crop. Crotalaria striata increased
the yield 19.9 bushels per acre over the volunteer cover crop;
velvet beans increased the yield 14.4 bushels; beggarweed 7.3
bushels and cowpeas 13.9 bushels per acre.
In 1927 and 1928, despite unfavorable weather conditions
and other possible detrimental factors, the potatoes following
the incorporation of Crotalaria striata gave significantly higher
yields than those following the incorporation of any other cover
crop. The yield of potatoes on the crotalaria plots exceeded
that of those on the "Florida pusley" plots by 12.6 bushels for
1927 and 9.7 bushels for 1928. During 1927 the velvet beans
and cowpeas increased the yields of sweetpotatoes only a few
bushels over the volunteer cover crop, while in 1928 no increases
were obtained from the incorporation of any cover crop except
CORN YIELDS FOLLOWING SUMMER COVER CROPS
Table 5 gives the corn yields on Sections D and C of the
experimental field. In 1924 Section D was planted to corn fol-
lowing a uniform cover crop of "Florida pusley". Average
yields obtained on the duplicate plots indicate a relatively uni-
form yield over the section. The maximum difference in aver-
age yields of plots was 2.3 bushels per acre.
The corn yields of 1925 on Section C and of 1926 on Section
D are much greater following the plowing under of the planted
cover crops of the previous seasons than following the volun-
teer cover crop. In 1925 Crotalaria striata increased the yield
12 Florida Agricultural Experiment Station
of corn 5.7 bushels; velvet beans 7.0 bushels; beggarweed 3.3
bushels and cowpeas 6.3 bushels over the yield of corn fol-
lowing the plowing under of "Florida pusley". In 1926 the
corn yields on Section D were increased by the planted legumin-
ous cover crops over the volunteer non-leguminous cover crop
as follows: after Crotalaria striata 10.7 bushels per acre; after
velvet beans 9.8 bushels; after beggarweed 4.8 bushels; and after
cowpeas 6.0 bushels. In 1927 on Section C increases in corn
yields due to the incorporation of leguminous summer cover
crops were as follows: Crotalaria striata 7.1 bushels per acre;.
velvet beans 7.3 bushels; beggarweed 1.6 bushels and cowpeas
4.1 bushels. In 1928 corn yields on Section D were not obtained
because of an exceptionally unfavorable season with regard to
both soil moisture and crop pests.
TABLE 5.-YIELDS OF CORN IN BUSHELS PER ACRE FOLLOWING THE IN-
CORPORATION OF SUMMER COVER CROPS.
Section D1 Section C2 Average Increase
S------ nnual due to
Summer cover Aver- Sections planted
crop 1924 1926 1925 1927 age D and C cover
S ... | Sec. C crop
"bu. bu. bu. bu. bu. bu. bu.
"Florida pusley" .... 8.1 6.9 16.0 3.3 9.7 8.7
Crotalaria striata 7.7 17.6 21.7 10.4 16.1 16.6 7.9
Velvet beans ............ 8.1 16.7 23.0 10.6 16.8 16.8 8.1
Beggarweed ........... 8.4 11.7 19.3 4.9 12.1 12.0 3.3
Cowpeas ............ 6.1 12.9 22.3 7.4 14.9 14.2 5.5
1 1924-Corn following a volunteer cover crop of "Florida pusley" com-
mon to all plots; 1926-corn following different summer cover crops grown
2 1925-Corn following different summer cover crops grown in 1924;
1927-corn following different summer cover crops for second time in 1926.
Effects of Summer Cover Crops on Crop Yields 13
Corn and sweetpotatoes were grown in two-year rotations
with "Florida pusley", Crotalaria striata, velvet beans, beggar-
weed and cowpeas, respectively, on a Norfolk medium fine sand
(deep phase). All of the summer cover crops grown in rota-
tion with corn and sweetpotatoes were leguminous plants except
the "Florida pusley". The leguminous summer cover crops re-
quired planting each year while "Florida pusley" reseeded
Average yields of top growth of cover crops expressed in
pounds per acre of air-dry material (8% moisture) for the
period of the experiment were as follows: "Florida pusley"
1,085, Crotalaria striata 4,552, velvet beans 1,323, beggarweed
648 and cowpeas 1,698. Except in the case of beggarweed,
the planted leguminous cover crops outyielded the volunteer
non-leguminous cover crop.
In the rotations of sweetpotatoes with the various summer
cover crops for the years 1925 to 1928, inclusive, average annual
yields of sweetpotatoes expressed in bushels per acre were as
follows: rotating with volunteer "Florida pusley" 20.8, with
Crotalaria striata 38.9, with velvet beans 26.9, with beggarweed
23.4 and with cowpeas 27.9. Average annual increases in yield
of sweetpotatoes due to the planted leguminous cover crop over
the volunteer non-leguminous cover crop, "Florida pusley", were
as follows: 18.1, 6.1, 2.6 and 7.1 bushels per acre, respectively,
for the cover crops mentioned in the above order.
In the rotations of corn with various summer cover crops
for the years 1925 to 1927 inclusive, average annual yields of
corn following the cover crops were as follows: after the volun-
teer "Florida pusley" 8.7, after Crotalaria striata 16.6, after
velvet beans 16.8, after beggarweed 12.0, and after cowpeas
14.2 bushels per acre, respectively. Average annual increases
in yield of corn due to the planted summer leguminous cover
crops over the volunteer non-leguminous cover crop were as
follows: Crotalaria striata 7.9, velvet beans 8.1, beggarweed
3.3, and cowpeas 5.5 bushels per acre, respectively.
14 Florida Agricultural Experiment Station
II. INFLUENCE OF SUMMER COVER ON NITRATE
AND ORGANIC MATTER CONTENT OF A POOR
GRADE OF NORFOLK SOIL
By R. M. BARNETTE and J. B. HESTER'
The arrangement and handling of the plots growing the dif-
ferent summer cover crops in rotation with corn and sweet-
potatoes made it possible to study the effect of these manuring
crops on the soil. Two types of observations were used to
determine this effect: (1) The periodic determination of the
accumulation of nitrates in the soil permitted a study of the
annual seasonal effect of the cover crops plowed into the soil,
and (2) the determination of the organic matter content of
the soil at intervals after the start of the rotations gave an
indication of the efficacy of the summer cover crops in build-
ing up or maintaining the organic matter content of the soil.
Studies of the nitrate content of the soil were made during
the growing season of sweetpotatoes and corn following the
incorporation of the different summer cover crops in 1927 and
1928. Samples of soil for the determination of the organic
matter content were taken in 1926 and 1927 from the various
sections of the field on which the rotations were conducted. The
results of these studies will be discussed separately.
SEASONAL EFFECT OF INCORPORATING SUMMER COVER
CROPS ON NITRATE CONTENT OF THE SOIL
Methods of Sampling and Analysis: The nitrate content of
the soil of selected plots of the rotation experiment was de-
termined for three different depths after plowing in the cover
crops in 1927 (cover crop grown in 1926) and 1928 (cover crop
grown in 1927). The determinations were started two weeks
after the incorporation of the summer cover crops late in Jan-
uary and continued at semi-monthly intervals until October.
Representative plots of the field having the following rotations
were sampled in 1927 and in 1928.
"Florida pusley" "Florida pusley"
Sweetpotatoes Crotalaria striata Crotalaria striata
Sfeooatos Velvet beans Corn following Velvet beans
following --- Beggarweed Beggarweed
1Formerly assistant chemist, Florida Agricultural Experiment Station.
Effects of Summer Cover Crops on Crop Yields 15
Eight to 10 sub-samples at three depths, 0-9 inches, 9-21 inches
and 21-33 inches, were taken from each 1/30 acre plot. The
sub-samples were taken at different points over the plots and
in the middles between the rows after the land had been pre-
pared for planting. This procedure removed the area of sam-
pling as far as possible from the zone where commercial fer-
tilizers had been applied. All sub-samples from one depth were
thoroughly mixed and the composites sampled and analyzed.
The nitrate content of the moist soil was determined in a 1 to
5 soil-water extract on the date of sampling. The soil suspen-
sions were filtered through qualitative filter papers and the
nitrate determinations made on the clear' filtrates by the
phenoldisulphonic acid colorimetric method. The moisture con-
tents of the soils were determined by weighing out samples
and allowing them to air dry in the laboratory. The nitrate
content of the samples was calculated to milligrams of nitrogen
as nitrate per 100 grams of air-dried soil.
From the results of the determinations of the nitrate con-
tent of the soil of the plots of the different rotations, the fol-
lowing studies have been made: (a) The accumulation of the
nitrates in the different soil depths as influenced by the distribu-
tion of rainfall during the growing seasons of sweetpotatoes and
corn. (b) The influence of plowing under of different summer
cover crops on the accumulation of nitrates in the soil.
In making these studies the results of the nitrate determina-
tions of the soil samples from the corn and sweetpotato sections
have been averaged for each date of sampling. No attempt has
been made to determine the comparative effect of corn and
sweetpotatoes on the nitrate content of the soil, due to varia-
tions in the yields of the individual cover crops on the plots
of the two sections and to other uncontrolled factors. Graphs
have been used to show the nitrate content of the soil at the
different sampling dates since the actual data are very ex-
(a) Accumulation of nitrates in different soil depths as in-
fluenced by distribution of rainfall. The nitrate content of
the different soil depths of all plots for each date of sampling
in 1927 and 1928 have been averaged in these studies. The
average nitrate contents in three soil depths, 0-9 inches, 9-21
inches and 21-33 inches, for each date of sampling during 1927
and 1928 are shown graphically in Figures 2 and 3 respectively.
16 Florida Agricultural Experiment Station
RAINFALL BY DAYS AND PERIODS-1927 -
Iat- A / tA
o*. -, ft / ,
a / \ .
FE MARCH APRIL MAY JUNE I JULY 'I AUGUST SEPTENBERB OCTOBER I
e =.a- DEPTH DISTRIBUTION OF NITRATES -1927 19LECE
~ a- .. - -----------
FET'RUVARy MARCH I APRIL M AY JUNE 1 JULY AUGUST" EPTEM'BEl OCTOBER I
Fig. 2.-Relationship between the distribution of rainfall and the ac-
cumulation of nitrates in three soil depths from February to October, 1927.
The milligrams of nitrogen as nitrates per 100 grams of air-
dried soil are shown vertically and the time (dates of sampling)
horizontally to scale. In addition, graphs which show the daily
amounts of rainfall by bars and the total amounts of rainfall
between the sampling dates by lines are given for the sampling
periods of 1927 and 1928. By this means the nitrate accumula-
tion in the three soil depths at the different dates of sampling
may be compared with the distribution of rainfall during the
period of sampling.
RAINFALL BY DAYS AND PERIODS 1928 / ---...
-i /i "
FEBRUARY I MARCH I APRI Y JI E LY A GUST I BR
Z DEPTH DISTRIBUTION OF NITRATES- 1928 LEGEND
s IARYI MARCH I APaRI I' MAY "'I JUnt JULY ; I AuGust mlSEPTEMBERT
Fig. 3-Relationship between the distribution of rainfall and the ac-
cumulation of nitrates in three soil depths from February to October, 1928.
In general, for the two years 1927 and 1928, an inverse re-
lationship was observed between the total amounts of rainfall
between the dates of sampling and the nitrate content of the
Effects of Summer Cover Crops on Crop Yields 17
three soil depths. The periods between samplings which had
high total rainfall, had a low nitrate content in the three soil
depths at the end of each period. Conversely, drought periods
brought about a rapid accumulation of relatively large quanti-
ties of nitrates in the three soil depths. Only traces of nitrates
were found in all depths of the soil investigated on dates of
sampling following periods of excessive and leaching rainfall.
In addition, evidence of the leaching of nitrates from the
upper to the lower depths of soil was obtained. For instance,
there are four dates of sampling in 1927 and six in 1928 when the
nitrate content of the lower soil depth was equal to or exceeded
that of the depth above it. If the daily rainfall record of the
periods preceding this apparent leaching of nitrates into the
lower depths is examined, such periods are found to be char-
acterized by a number of light rains just before the date of
sampling or else by one heavy rain in an otherwise dry period.
In general, the nitrate content of the 0-9 inch depth was found
to be higher than that of the 21-33 inch depth for most sampling
r 4u1R y MWCH AP'IL I MAY JUN *. 'I A61UST I sEpTtM)' I OCTOB I
S // \ NITRATE ACCUMULATION IN SURFACE SOIL (0-9") FOLLOWING
192"/ \ INCORPORATION OF COVER CROPS__ LE_-__-
F CROTALRIA STRATA
E AR MARCH APRL MAY JUNE JULY I AUGUST SE PTEMBER
Fig. 4.-Accumulation of nitrates in the surface soil (0-9 inches) after
the incorporation of different summer cover crops in 1927 and 1928.
(b) Influence of plowing under summer cover crops on ac-
cumulation of nitrates in the surface soil. The results of the
nitrate determinations on the 0-9 inch depth of soil of the
plots of the sweetpotato and corn rotations for each date of
sampling have been averaged to make this study. The results
of the averages for the sampling periods of 1927 and 1928 are
given graphically in Figure 4.
18 Florida Agricultural Experiment Station
The summer leguminous cover crops grown in 1926 and plowed
under early in 1927 gave a higher nitrate content in the 0-9
inch soil depth from February to June than did the non-legumin-
ous volunteer "Florida pusley". During this period, the surface
soil in which Crotalaria striata had been incorporated showed
by far the highest nitrate content for each date of sampling.
'The velvet beans and cowpeas plowed into the surface soil gave
a lower nitrate content than did the Crotalaria striata but higher
than the beggarweed and "Florida pusley". The rotting of these
two leguminous summer cover corps developed about the same
quantities of nitrates in the soil from early February to late
June. During most of this period the incorporation of the lower
yielding but leguminous beggarweed gave slightly higher nitrate
nitrogen contents in the surface soil than did the plowing under
of "Florida pusley".
After the latter part of June it is apparent that the different
summer cover crops plowed under in January had no specific
effect on the nitrate content of the surface soil during the re-
mainder of the 1927 sampling period. The high nitrate con-
tent of the surface soil in August is correlated with a very dry
period preceding the date of sampling (see Figure 2). During
the fall period, from the middle of September to the latter
part of October, the nitrate content of the soil was low on each
date of sampling. This low nitrate content is without doubt to
be attributed to the growth of a volunteer fall cover crop after
During the 1928 sampling period, from the first part of Feb-
ruary up to the middle of June, the nitrate content of the soil
in which Crotalaria striata had been plowed under was definitely
higher than that of the soil in which the other summer cover
crops had been incorporated. Cowpeas plowed into the surface
soil gave the second highest content for each of the sampling
dates up to the middle of June. Velvet beans plowed into the
soil gave a lower nitrate accumulation in the surface soil than
did the Crotalaria striata and the cowpeas, but a slightly higher
nitrate content than that of the soils in which beggarweed and
"Florida pusley" had been incorporated. The soil in which
beggarweed had been incorporated had a slightly higher nitrate
content than the soil into which the non-leguminous "Florida
pusley" had been plowed.
As in 1927, after the middle of June cover crops plowed under
in January had no definite individual effect on the nitrate con-
Effects of Summer Cover Crops on Crop Yields 19
tent of the surface soil. The early fall period was again marked
by a very low nitrate content in the soil.
The results of the periodic determinations of the nitrate con-
tent of the soil in which different cover crops had been incor-
porated indicate a relatively rapid decomposition of the cover
crops immediately after their incorporation. The period of
maximum decomposition of the cover crops after their incor-
poration with the soil lasts about five months, or from early
spring until the middle of June. The nitrate content of the
deep sandy soils following the plowing under of cover crops
depends upon a number of factors, chief among which is the
amount and distribution of rainfall. These same factors which
influence the accumulation of nitrates in the soil without doubt
largely influence the availability of the nitrates to plants.
"ORGANIC MATTER CONTENT OF SOIL FOLLOWING THE
INCORPORATION OF DIFFERENT SUMMER
Samples of soil were taken from the plots of Sections A and
,C in March 1926. These sections had been planted to the different
summer cover crops in 1924 and to sweetpotatoes and corn, re-
spectively, in 1925. The soil samples were taken just prior to
the planting of the summer cover crops in 1926. The soils of
-the plots of Sections B and D were sampled in June 1927. These
:sections were planted to sweetpotatoes- and corn, respectively,
,in 1924; to the different summer cover crops in 1925 and to
sweetpotatoes and corn in 1926.
Soil samples from the plots planted to the different summer
cover crops were taken from the 0-6 inch and the 6-12 inch
,depths. Eight sub-samples were taken from the two depths
of soil to form a composite sample for each of the 1/30 acre
plots. A soil tube was used for sampling.
The soil samples were taken to the laboratory and air-dried.
"They were then screened with a sieve having 2 mm. round
holes to remove the rough undecomposed organic matter, char-
coal and small stones. Then the samples were ground finely.
The loss on ignition and the nitrogen and carbon contents of
the soils were determined on the finely ground samples by
standard methods of analyses. The loss on ignition is a satis-
factory method for determining the organic matter content of
.sandy soils. Samples were not taken from the area at the be-
20 Florida Agricultural Experiment Station
ginning of the experiment in 1924, and the experiment was dis-
continued in 1928.
Variations in results of analyses of the soils of the plots of
the two rotations were very small. The results of the loss on
ignition, carbon and nitrogen determinations on the soils of
the plots growing the different summer cover crops have been
averaged and are given in Table 6. Very small differences in
the organic matter (loss on ignition), carbon and nitrogen con-
tents of the soil were found, indicating little or no gains of
these components in the soil which might be attributed to the
planted summer cover crops.
TABLE 6.-AVERAGE LOSS ON IGNITION, CARBON AND NITROGEN CONTENTS,
OF THE SOIL OF THE DIFFERENT COVER CROP PLOTS.
Summer cover Depth L. I. C N C/N
inches percent percent percent
"Florida pusley" .... 0- 6 0.899 0.437 0.034 12.8
6-12 0.831 0.409 0.031 13.2
Crotalaria striata 0- 6 0.953 0.488 0.036 13.6
6-12 0.878 0.437 0.032 14.0
Velvet beans ............ 0- 6 0.948 0.482 0.035 14.3
6-12 0.820 0.423 0.030 14.6
Beggarweed ......... 0- 6 0.859 0.442 0.032 13.8
6-12 0.876 0.423 0.030 14.5
Cowpeas .................. 0- 6 0.954 0.457 0.035 13.3
6-12 0.827 0.411 0.029 14.3
Average ................. 0- 6 0.923 0.461 0.034 13.6
6-12 0.846 0.421 0.030 14.1
1 The loss on ignition is a measure of the organic matter in sandy soils.
That additions of large quantities of summer cover crops add
only slightly to the content of well-decomposed organic matter
in the deep phases of sandy soils has been observed in other
studies. Results of analyses of soil from; summer cover crop
studies in a pineapple orange grove have been reported in Flor-
ida Experiment Station Bulletin 253. The analyses show no
specific effect of any of the summer cover crops on the well-
decomposed organic matter of the soil. The residual effect
of the summer cover crops on the sandy soils under the climatic
conditions of Florida evidently must be sought in the undecom-
Effects of Summer Cover Crops on Crop Yields 21
posed residues of the plant parts persisting in the soil after
the decomposition of the more succulent parts (i. e. leaves and
small stems). These undecomposed residues are mostly woody
plant parts, not accounted for by the method of study used here.
The carbon-nitrogen ratio of the soil has not been definitely
influenced by any of the summer cover crops. The average
C :N ratios for the 0-6 inch and the 6-12 inch depths were
found to be 13.6 : 1 and 14.1 : 1, respectively. These are slightly
higher than the ratio usually obtained for cultivated soils (8 :1
to 12 : 1). This higher ratio may be due to the presence of
charcoal in the soil. The annual burning over of the forest
and cut-over lands has been practiced in Florida for years. The
charcoal left from burning of surface vegetation and roots be-
comes gradually incorporated with the soil.
SUMMARY (PART II)
Nitrates were determined in three depths of a Norfolk medium
fine sand (deep phase) at intervals of two weeks from February
to October, during 1927 and 1928. Different summer cover
crops had been plowed into the soil previously. The different
summer cover crops plowed into the soil were "Florida pusley"
(volunteer non-leguminous cover crop), Crotalaria striata, vel-
vet beans, beggarweed and cowpeas. Sweetpotatoes and corn
were grown on the land after the incorporation of the different
cover crops. A study of the effect of the amounts and distribu-
tion of rainfall on the accumulation of nitrates in the three
soil depths was made. Results of these studies are summarized
1. The accumulation of nitrates in the three soil depths of
this deep phase of Norfolk medium fine sand was found to de-
pend primarily upon the amounts and distribution of rainfall.
Samples taken following periods of excessive and leaching rain-
fall showed only traces of nitrates in all soil depths. Samples
collected during drought periods showed higher concentrations
of nitrates in all soil depths. On most dates of sampling, the
0-9 inch depth had a higher nitrate content than the 9-21 inch
depth, which in turn had a higher content than the 21-33 inch
depth. Following periods with a number of light rains just
previous to sampling or dry periods with one heavy rain, some
of the nitrates of the upper soil depths were leached into the
22 Florida Agricultural Experiment Station
2. From the first of February to the middle of June there was
a definite effect of the different summer cover crops plowed into
the soil on the nitrate content of the soil. During this period
the soil into which the planted leguminous summer cover crops
had been turned showed a higher nitrate content than the soil
in which "Florida pusley" had been incorporated. Crotalaria
striata plowed into the soil gave the highest nitrate content in
the soil for this period, both in 1927 and in 1928. The nitrates
formed from the decomposition of velvet beans and cowpeas
were about the same in 1927, but cowpeas gave a higher nitrate
content in the soil in 1928 than did velvet beans. The decomposi-
tion of beggarweed gave rise to a lower nitrate content in the
soil than did the other planted leguminous cover crops.
3. After the middle of June there was apparently no specific
effect on the accumulation of nitrates in the surface soil at-
tributable to the plowing under of the different summer cover
crops. The early fall period of both years is marked by a low
nitrate level in the soil.
Soil samples were collected from the 0-6 inch and the 6-12
inch depths of the different cover crop plots in 1926 and 1927.
The samples were air-dried, screened through a 2 millimeter
round-holed sieve, finely ground and analyzed for their content
in organic matter, carbon and nitrogen. The results of these
analyses are summarized as follows:
4. The different summer cover crops plowed into the soil had
no specific effect on the content of well-decomposed organic mat-
ter, carbon and nitrogen in the soil.
5. The average C : N ratio' of the soil was found to be 13.6
for the 0-6 inch depth and 14.1 for the 6-12 inch depth.