 
Nitrogen Fertilization
More than any other nutrient, N can
increase or decrease yields of cotton. Apply too
little N, and yields drop sharply. On the other hand, apply too much N or
apply at the
wrong time, and plants will be rank, slow to fruit, more attractive to
insect pests, late to
mature, more difficult to cover with crop protection chemicals, quick to
develop boll
rot, more troublesome and expensive to defoliate and control regrowth,
and more
likely to have grade reductions from bark.
N Rate
The recommended rate of N ranges from 50
to 70 pounds N per acre. The best rate
for a particular field depends on soil texture, the previous crop,
expected rainfall
patterns or irrigation, and grower experience in that field. Without
knowledge of the
field and the specific management practices used, it is difficult to give
specific
recommendations, but some guidelines are available. Uptake studies across
the cotton
belt suggest that cotton needs 60 pounds N per acre to produce one bale
of lint.
Numerous N rate studies in North Carolina and Georgia show that
unfertilized plots
can supply 40 to 70 pounds of available N from organic matter, subsoil
storage, and
rainfall. Soil N supplies are generally higher on the more productive
loamy soils. A
good crop of soybeans or peanuts will supply an additional 20 to 30
pounds N per
acre. Thus, the recommended rates are consistent with a range of total
available N
from 110 to 170 pounds per acre following peanuts or soybeans, or from 90
to 140
pounds per acre following other crops. In general, soils with more than
16 inches to
the subsoil will require the highest rates of N, while loams and finer
textured soils will
require the lowest rates of N. Following soybeans or peanuts, use a total
N rate of 40
to 50 pounds per acre.
Deficiency
Nitrogen deficiency symptoms first
appear on the lower leaves. They become a pale
yellowish green, fading with age first to hues of yellow, then variously
tinted shades of
red, and finally brown as they dry up and are prematurely shed. Deficient
plants are
stunted and generally unthrifty in appearance, and fruit set is poor.
If a deficiency develops, nitrogen can be
applied to the soil until the second or third
week of bloom. Beyond that point, soil applications become questionable.
Foliar
applications can increase yields at this stage of crop growth when plants
are deficient.
(See Foliar Fertilization.)
If nitrogen is leached out of the rooting
zone, it should be replaced. Adjustments for
replenishment are suggested in Table 1.
Table 1. Adjustments for Leaching Loss of Nitrogen
| Inches of Excess Water* |
Fraction of Total Nitrogen
Used to Be Replaced** |
1
2
3 or more |
1/5
1/3
1/2 to 3/4 |
* Inches of water that entered the soil in the 4- to 5-day
period in excess
** Extra nitrogen needed through third week of fruiting
|
Timing
Timing is important for cotton. Unlike
crops such as corn and tobacco, only a small
portion of the N is taken up before fruiting or flowering. About 45 days
after
emergence, nutrient uptake increases rapidly until it reaches a prolonged
peak about
two weeks after first bloom, when the processes of flower production,
boll filling, and
boll maturation create a heavy demand for nutrients. All too frequently,
all the N is
applied early in the season, or even before planting. While this may be
the most
convenient means of application, it makes little sense in an area subject to
unpredictable leaching rains. The applied N remains exposed to leaching
rains for more
than 60 days before demand begins to peak. Leaching losses during this
period will
need to be accounted for and replaced to attain optimum yield. Heavy
applications
early in the season can lead to excessive vegetative growth and delayed
fruiting.
Cotton needs about 20 to 25 pounds N per
acre to get the plant through sidedress
time. If following peanuts or soybeans, no initial N may be required. The
ideal time to
sidedress would be just prior to first bloom, but uncertainty with
rainfall dictates that N
should be applied between first square and first bloom. On sandy soils
subject to rapid
leaching, the sidedress N can be split, with half applied about four
weeks after
emergence and the remainder in three to four weeks.
Sources
Of the many N sources available for cotton
fertilization, no one source has proven to
be superior to others. Nitrogen solutions, ammonium nitrate, ammonium
sulfate urea
and anhydrous are most frequently used because of their high analysis.
Sodium nitrate
and calcium nitrate can be used, but have no proven benefit over
ammonium-type
fertilizers and are more expensive per pound of N applied. Conversion of
ammonium
forms to nitrate occurs very rapidly under warm, moist conditions. The
choice should
be based on price, convenience, and availability of equipment. Liquid N
solutions are
very convenient and exhibit little volatile loss when dribbled beside the
row, even
without cultivation. Anhydrous is a very economical source of N, but requires
specialized handling equipment There is a temptation with anhydrous to
apply all the
necessary N prior to planting. The best results are still obtained when
sidedress
applications are knifed-in after squaring begins. Care should be taken to
avoid root
pruning. Urea is also a suitable N source, but surface-applied sidedress
application
should be lightly incorporated on light, sandy soils.
Nitrification inhibitors are
recommended in some parts of the country to slow the
change of ammonium N to nitrate. These materials are most effective in
cold, wet soils.
The ammonium form is less subject to leaching because it is bound by soil
clays and
nitrate is not. Because cotton is generally planted in warm soils, these
compounds
seldom show beneficial results in on-farm tests. The best way to reduce
leaching losses
is to sidedress the major portion of the nitrogen once squaring has begun.
Nitrogen, Pix and Irrigation
The potential to reduce vegetative growth
with Pix has led some growers to increase N
rates with the hope of increasing yields. On-farm tests in North Carolina
consistently
show that cotton yield response to nitrogen is not affected by Pix
applications.
Additional nitrogen is not justified just because Pix will be applied.
Furthermore, where
excessive rates of N are used, and soil moisture is good, Pix will not
adequately
control rank growth at labelled rates.
Under irrigated conditions, yield
potential on some soils can sometimes approach three
bales. Higher N rates (90 to 120 pounds N per acre) may be justified in these
situations. When high N rates are planned for irrigated cotton, split N
applications to
provide the bulk of the N as flowering begins. Plan on using Pix to help
control
vegetative growth, but be aware that primary control depends on
maintaining high
square retention and a heavy fruit load.
Source: Cotton Fertilization, Steven C. Hodges,
Soil Science Extension Specialist,
North Carolina Cooperative Extension Service, 1995.
Nitrogen Management for Cotton Following
Cotton on Coastal Plain Soils
Due to recent market situations, the
practice of planting
cotton following cotton instead of following peanuts, corn or soybeans is
increasing in Georgia and other parts of the Southeast. The objective
of this research was to determine the optimum N rate for cotton
following cotton on Coastal Plain soils.
A field study was conducted from 1993
through 1996 at the University
of Georgia Coastal Plain Experiment Station in Tifton, GA. Cotton
plots ( 8 rows by 40 ft) were established on a Tifton loamy sand soil
following peanuts and fertilized with 0, 20, 40 , 60, 80, 120 and 160
lb N/a. The same plots were maintained and fertilized with the same N
rates each year of the study. The cotton variety was Stoneville/GA
King and the study was irrigated. Ammonium nitrate was used as the N
source and was split applied, half at planting and half at first
square. The center 2 rows were mechanically harvested and a subsample
was ginned for turnout. The experimental design was a randomized
complete block with four replications.
Previous crop is a significant factor to be considered when
determining N rates for cotton on Coastal Plain soils. The
recommendation of 30 to 40 lb. N/a for cotton following
peanuts was verified in this study. For first year cotton
following cotton, the optimum N rate was 80 lb./a. However,
under more normal growing conditions and yield levels,
60 lb. N/a may still be enough on these soils. The optimum N
rate for the third and fourth years of continuous cotton was
100 lb. N/a. Yields even dropped back down for the N rate
above 100 lb./a during the fourth year. This indicates a possible
buildup of residual soil N and detrimental effects due to history
of excessive N applications on this soil. With more cotton
following cotton, growers should be prepared to adjust N rates
according to previous crop history as well as soil type, growing
conditions and yield goals.
|
Cotton lint yield response to N fertilizer rates applied four
consecutive years. |
| N Rate |
Year |
| 1993 |
1994 | 1995 |
1996 |
| lb / a |
-------------------------
lint yield (lb / a) ------------------------- |
| 0 | 996a |
686a | 1083a |
832a |
| 20 | 1133b |
1010b | 1330b |
1070b |
| 40 | 1157b |
1192c | 1440c |
1233c |
| 60 | 1117b |
1287d | 1433c |
1257c |
| 80 | 1136b |
1423e | 1497c |
1367d |
| 100 | 1178b |
1490e | 1679d |
1456e |
| 160 | 1157b |
1495e | 1718d |
1387d |
| Average | 1125 |
1226 | 1454 |
1228 |
| Significance | * |
** | ** |
** |
| CV (%) | 6.3 |
5.6 | 5.2 |
3.0 |
* and ** indicate significance
at the 0.05 and 0.001 probability levels, respectively.
Values followed by the same letter are not different at the
0.05 level of probability. |
|
Lint Yields 1993 - 1996
Click on any bar graph to view enlarged or to print: |
|
|
Source:
Glen Harris and Shelby Baker,
Extension Agronomist and Research Scientist
University of Georgia,
Tifton, GA
Return to Cotton Fertility: Week 1
|
