Title: Vegetarian
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Title: Vegetarian
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Language: English
Creator: Horticultural Sciences Department, Institute of Food and Agricultural Sciences
Publisher: Horticultural Sciences Department
Place of Publication: Gainesville, Fla.
Publication Date: March 2010
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Vegetarian Newsletter

A Horticultural Sciences Department Extension Publication on Vegetable and Fruit Crops

Eat your Veggies!!!!!

Issue No. 555 March 2010

Nitrogen Fertilization of Sweet Corn in North-central


By: Lincoln Zotarelli, Assistant Research Scientist and
Michael D. Dukes, Associate Professor
Agricultural and Biological Engineering Department, Gainesville, Florida


In 2006, over 38,000 acres of agricultural land in Florida was planted with sweet corn (Zea mays
L.). This crop had a value of over 108 million dollars (USDA, 2008). Most sweet corn in Florida
is produced on coarse-texture soils, which are characterized by low inherent soil fertility and low
organic matter levels (Carlisle et al., 1988). Owing to the low soil water holding capacity of
these soils, poor irrigation management invariably results in inefficient nitrogen (N) fertilizer use
and farmers may opt to apply excessively high N rates to minimize the risk of yield reductions.
However, excessive N-fertilizer rates increase the potential of nitrate leaching and groundwater
pollution, especially in sweet corn production systems that require substantial inputs of N

In general, the crop N recommendation is the quotient of plant N requirement and the fertilizer
use efficiency. Crop N requirement is a physiological component, which is directly related to the
genetic potential of the crop and to plant growth conditions. This component is determined by
the overall crop N accumulation under optimum growing conditions. The fertilizer uptake
efficiency for a specific production system depends on several factors, including environmental
conditions, management, rate, timing and source of N. The actual recommendation of N-fertilizer
rate for mineral soils in Florida is 200 lb N/ac with part of this N applied at planting followed by
one or two sidedressings of the remaining N in the early part of the growth cycle (V6-V8 stage)
(Ozores-Hampton, 2010). Approximately 28-55% of the N applied is taken up by the crop
(Bundy and Andraski, 2005; Cherr, 2004). Fertilizer recovery typically decreases with an

increase in N-application rates and growers incur economic loss by applying more N than is
required to obtain a positive yield response (Macdonald et al., 1989). The objective of this study
was therefore to evaluate response of sweet corn to increased N-rate application in Florida sandy

Materials and methods

Field experiments were conducted at University of Florida, Plant Science Research and
Education Unit, near Citra, FL. The dominant soil type at this site is a Candler fine sand (Typic
Quarzipsamments, hyperthermic, uncoated), with more than 95% sand in the upper 1-2 m of the
soil profile (Carlisle et al., 1988). Overhead irrigation was applied at germination and thereafter
at 2-day intervals based on crop water requirements in the absence of rain.

Sweet corn var. Saturn Yellow was planted in spring of 2004 and 2006 using an in-row spacing
of 7 inches, a between-row spacing of 30 inches (29,582 plants/acre) and a seed depth of 1 inch.
Phosphorus and potassium were applied to all plots based on soil test recommendations (120 lb
P205 /ac in a single application during planting and 148 lb K20/acre applied in three splits
concurring with N applications). The N-rates tested were: 0; 60; 120; 180 and 240 lb/acre. The
fertilizer was applied as NH4NO3 banded on the side of the plant row. Each N rate was divided
into three applications: at emergence, and 28 and 42 days after emergence (DAE). Sweet corn
plants were harvested 70 DAE.

Plant biomass was determined at emergence; 14, 28, 42, and 63 DAE and also at final harvest
(70 DAE) for both years. Sweet corn yield was determined and ears were graded using USDA
standards (USDA, 1997). Plant dry weights were determined for sample tissues after oven drying
at 145 F for 96 h. Tissue samples were analyzed for total Kjeldahl N at the Analytical Research
Lab at Univ. of Florida. Nitrogen determined by the plant was calculated by multiplying weights
of stems plus leaves, and ear tissue by the corresponding N concentrations. The apparent N
recovery (ANR) was the ratio between the difference in the N accumulated in the fertilized and 0
N treatment.

The N status of the sweet corn crop was also measured by leaf light transmittance using a
handheld Minolta SPAD chlorophyll 502 m (Konica Minolta, Ramsey, NJ). Readings were
collected at the V8 corn growth stage (35 DAE). Readings were taken midway between the stalk
and leaf tip, and midway between the midrib and leaf margin. Readings were taken from the
uppermost leaf with the collar fully visible. Twenty-five plants were sampled per plot.

Sweet corn yield and plant N accumulation patterns

The sweet corn tests conducted in the spring of 2004 and 2006 showed similar yield patterns in
response to N-fertilizer levels. Fresh weight of marketable ears (total fresh yield minus cull
yield) in response to N-fertilizer rates is shown in Fig. 1. The N-rate of 60 lb/ac produced 3,758
lb/ac of marketable yields and with the N application of 120 lb/ac, marketable yield of sweet
corn increased to 12,821 lb/ac (Fig. 1). The marketable yield achieved with N rates of 180 and
240 lb/ac were 16,492 and 16,680 lb/ac, respectively, with no statistical difference between these
N rates. The increase in the N-rate application from 180 lb/ac to 240 lb/ac, represented an

increase of 25% in the applied N rate, but only 1% increase in sweet corn marketable yield.
Based on a cubic yield model (P < 0.0001), maximum fresh marketable ear yield was 17,808
lb/ac (434 crates/acre 42 lb/crate), which was achieved with a N rate of 203 lb/ac.
In addition, reduction in percentage of cull ears for N rates above 120 lb/ac was observed. The
percentage of culls was 90%, 27%, 9%, 7%, and 7% for N rates of 0, 60, 120, 180, and 240



Z 15000

3 12500

*L 10000
E 7500

Z 5000



180 Iblac 240 Ib/ac

400 X

120 Iblac 2
300 ,

60 Ib/ac CLE

V Y -41.4987 + 27.7106x + 1.0274x2-0.0036x
R2 = 0.96

0 25 50 75 100 125 150 175 200 225 250
N rate (Iblac)

Fig. 1. Average sweet corn marketable ears yield (Y) as a cubic function of supplemental N (x)
fertilizer for spring seasons of 2004 and 2006. The fertilizer was applied as ammonium nitrate
(NH4N03) banded on the side of the plant row. Each N rate was divided into three applications
at emergence, and 28 and 42 days after emergence (Fresh sweet corn box = 42 lb).

Sweet corn N uptake increased with applied N rate (Fig. 2 left). However, the increase of N rate
from 120 to 240 lb/ac did not result in an increase of end-of-season N uptake. The no fertilizer
and N rate of 60 lb/ac accumulated less N (P < 0.001) in the plant tissues than the treatments
fertilized with N rates above 120 lb/ac. There was no significant difference in N accumulation by
sweet corn plants (including ear and stover) between the N rates of 120, 180 and 240 lb/ac. With
an increase in N rate from 120 to 240 lb/ac, the ANR, which is an estimate of fertilizer uptake
efficiency, decreased from 0.72 to 0.45 (Fig. 1). Total crop N taken up from fertilizer was 98,
120, and 121 lb/ac for the N rates of 120, 180 and 240 lb/ac, respectively. The marginal crop N
efficiency [kg additional yield (Y) per unit extra fertilizer] dropped from 105 lb of fresh sweet
corn produced per lb of N applied for the N rate of 120 lb/ac to 91 and 69 lb of fresh sweet corn

produced per lb ofN applied for the N rates of 180 and 240 lb/ac, respectively. This result
underlines that maximizing production will invariably result in less efficient fertilizer use.

Plant N uptake Apparent N recovery
140 100
120 Ear N a Apare N recovery
120 a
o -80 ,

ab a

so 40 -

2 20 P2

0 l-l --- 0
0 60 120 180 240 0 60 120 180 240
N rate (Iblac)

Fig. 2. The effect of N rates on sweet corn N uptake (left graph) in ears and stover and apparent
N recovery (right graph). Vertical bars labeled with different letters are significant at P < 0.05.

Use of chlorophyll meter to access sweet corn yield response

Chlorophyll meter readings (SPAD) and relative sweet corn yield showed high correlation (P <
0.001, R = 0.90) for measurements taken at 35 DAE (Fig. 3). SPAD readings below 40 units
were associated to the N treatments of zero and 60 lb /ac, which produced yields below 5,000
lb/ac. SPAD readings between 40 and 46 units were associated with intermediate N rates (120
and 180 lb/ac), while SPAD readings above 47 units showed that relative yields reached a
plateau, which indicates that chlorophyll meter reading is a poor indicator of excess plant
available N and luxury consumption at high N leaf concentration (Blackmer and Schepers, 1995)
and/or when factors other than N could be limiting sweet corn yield as well (Hawkins et al.,
2007). As soil N testing is not recommended for sweet corn in Florida soils because of the very
low N retention of sandy soils (Sartain, 2001; Simonne et al., 2005), a chlorophyll meter may
still be used as a complementary in-season tool for detecting sweet corn N stress and to fine-tune
supplemental N-fertilizer rate and/or timing.

Leaf Chlorophyll Meter



*l 60-
6 -

0 -




I -I I I I I I
20 30 40 50
Leaf Chlorophyll (SPAD readings)

Fig. 3. Relative marketable yield of sweet corn as a function of leaf chlorophyll meter (SPAD)
at 35 d after emergence for sweet corn fertilized with N-rates 0, 60, 120, 180 and 240 lb/ac.
Relative yields equal to measured yield divided by highest treatment mean. Relative marketable
yield corresponded to the average from 2004 and 2006.


The research finding from 2 years of experimentation showed that the current N-rate
recommendation of 200 lb/ac is adequate for north-Florida conditions. Increase of the N-
fertilizer rate above 200 lb/ac did not result in higher marketable yields. The increase of N rate
above 180 lb/ac reduced the apparent N recovery by sweet corn plants, resulting in higher risk of
N leaching. A chlorophyll meter was not a sensitive indicator of plant N excess occurring at N
rates above 180 lb/ac. A chlorophyll meter showed potential to be used as a complementary in-
season tool for detecting sweet corn N stress and to fine-tune supplemental N-fertilizer rate
and/or timing, however, further studies are needed to assess optimal timing of supplemental N


Bundy, L.G., and T.W. Andraski. 2005. Recovery of fertilizer nitrogen in crop residues and
cover crops on an irrigated sandy soil. Soil Science Society of America Journal 69:640-648.

0 0 Ib NIac
A 60 Ib N/ac
O 120 Ib Nlac
O 180 Ib Nlac /
V 240 Ib N/ac
A 0

Y= 4.76 x-128.37 if x< 46.8
(P < 0.001) R2= 0.90
0 Y= 95.50 if x> 46.8
, Aa/ (P< 0.001)

Blackmer, T.M., and J.S. Schepers. 1995. Use of a chlorophyll meter to monitor nitrogen status
and schedule fertigation for corn. Journal of Production Agriculture 8:56-60.

Carlisle, V.W., F. Sodek III, M.E. Collins, H. L.C., and W.G. Harris. 1988. Characterization data
for selected Florida soils. University of Florida-Institute of Food and Agricultural Sciences. Soil
Sci. Res. Report. Gainesville, FL. 88-1.

Cherr, C.M. 2004. Improved use of green manure as a nitrogen source for sweet corn, University
of Florida, Gainesville. http://purl.fcla.edu/fcla/etd/UFE0006501 (Last access 4 Mar. 2010).

Hawkins, J.A., J.E. Sawyer, D.W. Barker, and J.P. Lundvall. 2007. Using relative chlorophyll
meter values to determine nitrogen application rates for corn. Agron. J. 99:1034-1040.

Ozores-Hampton, M, V.M. Stall, S.M. Olson, S.E. Webb, S.A. Smith, R.N. Raid. 2010. Sweet
corn production in Florida, p. 273-284. In S. M. Olson and Santos, B., eds. Vegetable Production
Handbook for Florida 2010-2011. IFAS, Gainesville.
http://www.hos.ufl.edu/vegetarian/10/Jan/VPH%202010-2011/Chap%2021.pdf (Last access 4
Mar. 2010).

Sartain, J.B. 2001. Soil testing and interpretation for Florida turfgrasses, pp. 6. Soil and Water
Science Department, Florida Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville, FL.

USDA. 1997. United States standards for grades of sweet corn for processing. USDA,
Washington, DC. http://www.ams.usda.gov/AMSvl.0/getfile?dDocName=STELPRDC5050409
(Last access 4 Mar. 2010).

USDA. 2008. U.S. sweet corn statistics. National Agricultural Statistics Service, USDA.
http://usda.mannlib.cornell.edu/MannUsda/viewDocumentInfo.do?documentlD=1564 (Last
access 4 Mar. 2010).

Vegetarian Newsletter

A Horticultural Sciences Department Extension Publication on Vegetable and Fruit Crops

Eat your Veggies!!!!!

Issue No. 555 March 2010

Cold Weather Protection for Vegetables and Tropical Fruits
in South Florida

By: David D. Sui', Jonathan H. Crane2, Alan L. Wright3, and Ronald. W. Rice1
SPalm Beach County Extension, West Palm Beach, FL
2 Tropical Research and Education Center, Homestead, FL
3 Everglades Research and Education Center, Belle Glade, FL

Winters in south Florida (the part of the state extending south from West Palm Beach) are
generally mild and pleasant with day/night temperatures typically in the 70sF/50sF. While
many other states are contending with snow and ice during this time of the year, south Florida
agriculture is in full swing with winter vegetable production, as well as some tropical fruits, and
winter market prices are generally favorable. However, south Florida is not free from frosts and
freezes, thus vegetable and tropical fruit growers should have a cold protection plan in place to
deal with the sporadic arrival of cold fronts. For two years in a row, January 21-22 in 2009 and
January 4-13 in 2010, vegetable and tropical fruit growers in south Florida have confronted
severe cold weather and suffered substantial crop losses. With cold protection strategies in place,
crop losses can often be minimized.

Figure 1. Unprotected
Photo by David Sui.

amato: Total Loss from freeze events in January 2010.

Figure 2. Bell Peppers Protected by Freeze Cloth during several freeze events in January
2010. The Foam Cups Provided Non-Abrasive Support of Freeze Cloth. Photo by David Sui.

,1 .1"--

Many different cold protection methods have been used by vegetable and tropical fruit growers
in south Florida. Every cold front is different and each has specific weather characteristics that
work together to cause cold damage. Furthermore, each cold protection method has its own
strengths and weaknesses. Farms should be prepared to have multiple cold protection methods in
place and be able to choose the proper approaches according to each unique cold situation.
Adverse weather is forecast by many sources. For south Florida, National Weather Service-
Miami, which provides forecasts for a week ahead (http://www.srh.noaa.gov/mia), and the
University of Florida's Florida Automated Weather Network (FAWN), which provides instant
temperature recordings at various sites throughout the state of Florida (http://fawn.ifas.ufl.edu),
are two most reliable free-of-charge sources on the Internet. Growers should regularly check the
websites for weather updates.

Once a cold freeze or near freeze weather is in the forecast, growers should plan well ahead in
order to avoid frantic last-minute decision-making as temperatures start dropping. According to
the crops a farmer grows, farms usually have a predetermined threshold temperature upon which
action must be immediately taken.

Water has the highest thermal capacity amongst all materials that nature provides. Thus, the
most widely used cold protection method by growers by far is water application. Elevating the
water table is the first line of defense when dealing with freezing weather. Most vegetable and
tropical fruit farms in south Florida are connected to irrigation/drainage systems that use farm
canals and field ditches to regulate seepage irrigation, often with the help of pumps.
Manipulating water table levels is the least expensive but most effective way to battle the cold. It
is important to raise the water level several days before the freeze since it takes time for the sun
to heat up the extra volume of water in the soil profile and for thorough capillary seepage to
occur. Water-filled soil-pore space buffers against cold temperatures much more effectively than
air-filled soil-pore spaces. Once the ground is moistened, it radiates heat and keeps the near
ground temperature warm, thus protecting vegetable and tree roots from freeze damage. For near
freezing temperatures in the lower 30sF, elevating water tables alone may be sufficient for
protection. One weakness of this method is that cold-stressed plants may suffer a higher
probability of plant disease development due to prolonged wet soil conditions. It is advisable to
drain the fields and return to a normal water table level once the freeze danger has passed.

Freezing and subfreezing weather in the forecast calls for additional cold protections, such as
freeze cloth, air mixing, microjets, in-tree sprinklers, and soil banking.

Freeze cloth is another effective method that can be used in addition to an elevated water table.
Use PVC pipes, aluminum pipes, or any smooth arches to support the freeze cloth above the crop
for low tunnels. For plants that are already fairly tall-statured and staked, pipe extensions on the
stakes can be arranged to create high tunnels. To prevent "tip burns", make sure the freeze cloth
is supported so that it does not come in contact with foliage, otherwise plant tissues and freeze
cloth can actually get frozen together, causing crop damage.

Figure 3. Basil Difference: Protected (two beds in the middle) vs. Unprotected (beds on the
left and right) following the freeze events in January 2010. A prior freeze has also killed
plants and left large skips from the unprotected bed (left). Photo by David Sui.

Air mixing is effective to prevent radiation frost, which tends to form when a clear sky and calm
winds (less than 5 mph) allow an inversion to develop, and temperatures near the ground drop
below freezing. Wind machines that used to be used in citrus groves are less used these days due
to their high fuel cost and initial investment. Helicopter flight has become more popular recently
for large acreage operations, but this is an expensive option, and pilots are generally paid a fee to
simply be on call for possible night-time flights. The weakness of air mixing is its ineffectiveness
against advective freezing, which happens under cold air and windy conditions.

Microjets are popular cold protection strategies for tropical fruit trees, and are used to protect
root systems and lower trunks. In-tree sprinklers are installed to protect limbs and canopy. Care
must be taken to make sure that the system is operational and spraying water through the night
until just past dawn or when ice/frost has melted away. Unintentional stoppage (pump runs out of
fuel, intake pond dries up, etc.) may quickly result in evaporative cooling, and subsequent freeze
damage could be more severe than the scenario where no cold protection measure had been
implemented. Before the freezing weather comes, check that the equipment is working, the water
supply (pond) is at least sufficient to last well into next morning, the in-tree sprinkler system is
on in all zones and not on zoned rotation, and to use diesel pumps (rather than electric) for better
dependability in case electrical problems develop during severe weather or power rotation
between municipalities.

Similar to trunk wrapping, soil banking of fruit trees can protect the basal trunk from freeze
damage and this has been applied to early stage sweet corn, in some cases, where cultivation
hilling buries the growth point of young seedlings below the soil surface.

Finally, all of these practices involve careful planning and timing. There is no one-size-fits-all
method in cold protection against freeze damage. Attention to preparation, designing sensible
cold protection protocols, and having all materials in inventory long before cold weather
approaches should help farms escape the brunt of many cold-weather systems, ensuring a more
stable supply of fresh produce for a demanding market once the cold weather has passed.


John L. Jackson, Kelly Morgan and William R. Lusher, 2009. Citrus Cold Weather Protection
and Irrigation Scheduling Tools Using Florida Automated Weather Network (FAWN) Data.

Katharine B. Perry. 1994. Frost/Freeze Protection for Horticultural Crops.

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