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Management
of Fertilizer Nitrogen in Arizona Cotton Production Felix Ayala, Research Assistant Thomas A. Doerge, Extension Soils Specialist
Nitrogen (N) is the nutrient that is required most consistently and in larger amounts than other nutrients for cotton production. Common rates of fertilizer N applied in Arizona cotton production systems range from 50 to over 300 lbs N/acre. The management of fertilizer N is critical, both for insuring optimum cotton yields, and minimizing the potential for environmental contamination. Soil N Reactions and Availability Soil N is composed of both organic and mineral forms but organic forms often comprise 80% - 90+% of the total soil N. Organic N is not immediately available for plant use but may be decomposed to available mineral forms (ammonium, NH4+; and nitrate NO3-) that are available through naturally occurring soil reactions. This decomposition process is generally referred to as mineralization. However, in most Arizona soils, the native organic matter (O.M.) levels are inherently low, and therefore, contributions of soil N for cotton production are usually negligible. As a result, most of the N needs of the cotton crop are supplied from crop residues and fertilizer. This includes N fertilizers applied to a crop plus residual N that remains in the soil from preceding crops. Some crops, particularly leguminous crops such as alfalfa, may leave relatively high amounts of residual N that are available for subsequent crops. Otherwise, N nutritional needs of the cotton crop must be supplied in fertilizer form. Common sources of fertilizer N used in Arizona are shown in the following table.
Table 1. Common nitrogen fertilizer
Important Soil Nitrogen Reactions Any fertilizer N applied as NO3-- N is readily available to the plant.
Ammoniacal forms (NH4+- N) of fertilizer N may be slightly available
for plant uptake initially, but after conversion through a two-step
biological process, become completely available as NO3-- N. The two-step
reaction is referred to collectively as “nitrification” and
occurs as follows:
The bacterial organisms responsible for carrying out these important
transformations are Nitrosomonas and Nitrobacter, for the two reactions,
respectively. Both organisms are naturally occurring and collectively
referred to as Nitrobacteria. There are several factors which affect
the activity of these organisms and resulting rates of nitrification.
Some of these factors include: soil reaction (pH ), soil temperatures,
as well as soil moisture and aeration. When soils are not saturated
due to recent irrigation (contributing to temporary conditions of poor
aeration), the conditions in most agricultural soils of Arizona are
conducive to nitrification. As a result, most fertilizer N applied to
a cotton crop any time during the growing season is converted rapidly
to a form readily available to the plant (nitrate). Nitrate - N is the most available for of N to the plant because it
is mobile in soil systems. Therefore, NO3-- N moves readily with soil
moisture and is available to the plant within the entire range of the
root system. Uptake of nutrients and water occur near the young root tips, particularly
the young root hairs. The NO3-- N present in the soil water moves to
the roots and is taken up with the soil water into the roots. An unfortunate
aspect of this phenomenon is that NO3- laden soil water will also carry
this N form below the root system if a wetting front pushes the soil
water below the active plant root system. Once NO3- is taken up by the roots, it moves in the transpirational
stream to the leaves. In the leaves, NO3- may either be stored in the
petiole or leaf cells ( within the vacuoles) or reduced and converted
into amino acids. The amino acids are then exported out of the leaves
to other organs (roots, stems, bolls, and young leaves) where they are
used as building blocks for proteins needed for growth. In older leaf structures, proteins may be broken down into amino acid
components and translocated within the plant to points of new growth
and development. The N in older, lower leaves on the plant is exported
first and will develop yellow or red coloration prior to abscising and
falling from the plant. This is a characteristic symptom of N deficiency.
N Functions in the Cotton Plant The use of N in amino acids and protein formation is an important function of N in the plant physiologically. Nitrogen is also an important component of nucleic acids, which serve in information and control systems in the plant. Enzymes represent an important class of proteins that are used in many of the plants’ biochemical systems. An enzyme responsible for converting CO2 from the atmosphere to carbohydrates is a major constituent of leaves and a major N - containing compound. If N deficiencies become severe, a resultant disruption of photosynthesis will follow, with a reduction in growth.
Maximum N demand by the cotton plant takes place during boll development.
Sufficient levels of N also must be present prior to boll filling to
prevent fruit abscission and abortion. Research results indicate that
approximately 55 to 60 pounds of N/bale of cotton lint is required for
production. Only a small amount of N is needed early in the season. Root systems
are small and recovery of fertilizer N applied early or preplant is
probably low, until plants develop a more extensive root system and
begin to fruit (figure 1 and 2). At the end of the cotton production season, Upland cotton plants lend
themselves best to chemical defoliation when petiole NO3-- N levels
have declined to levels below 2,000 ppm. A lowered N level in the plant
enhances maturity by promoting senescence and leaf abscission, and reducing
growth-stimulating hormones, all of which contribute to more complete
defoliation. It is important to manage N nutrition to lower petiole
NO3-- N levels late in the season without driving the plant into a N
deficiency that will diminish yield. Most Arizona cotton farmers have
learned that this can be done by reducing N inputs to the crop after
peak bloom periods. The actual fertilization program for any cotton crop is not always
a simple matter, depending heavily on the experience and skill of the
grower, which can be assisted through the use of several basic principles
and N management tools. The cotton plant has a distinctive growth pattern that is often characterized
in terms of the flowering pattern or “flowering curve”. This
curve is typified as being sigmoid or bell-shaped. The peak of the flowering
curve is followed by an event called “cutout,” which is a
temporary reduction in the flower production of the crop. The N requirements
and uptake patterns for cotton, closely resemble the growth pattern
expressed by the flowering curve. The total N uptake pattern is shown in Figure 1, and the daily uptake
pattern for N is shown in Figure 2. Review of these N uptake diagrams
taken form an Upland cotton research experiment in Arizona, reveals
that most of the total N needs of the crop are met approximately 100
days after planting (Figure1), which coincides very closely to the peak
daily uptake rate (Figure 2). If one were to overlay either of these
N uptake plots with a dry matter accumulation curve or a flowering curve,
the pattern and relationships in terms of rates and peaks would be very
similar. Figure 1 also illustrates the partitioning of N in the plant
into various plant parts over the course of the growing season. Of particular
interest here is that it can be shown how the cotton plant typically
allocates the largest portion of its total N to the seeds late in the
season (past 100 days after planting). This is obviously at the expense
of N in the leaves, which declines over time simultaneously as N in
the seeds increases. This is largely due to translocation and reallocation
of N within the plant to the seeds during boll development, which become
the strongest sink for N after peak bloom. The patterns of N use and needs by the cotton plant can be of specific
value in managing the N fertilization of a crop. It is obviously very
important to supply adequate, but not excessive amounts of N to the
crop at all times, but especially in the growth period prior to peak
bloom, when the demand is greatest. A deficiency during this period
may have a greater impact on yield than a deficiency at any other time.
To manage and plan for N fertilization at the beginning of the season,
one should obtain a soil sample from each field (preseason) and have
it analyzed for residual NO3-- N left in the soil form the previous
crop. The information provided in Table 2 can be referenced for interpretation
of these soil test results in terms of early season fertilizer N management.
To minimize the leaching potential of the NO3--N forms by early irrigations,
it is important to consider early season fertilizer N management in
terms of the needs of an active rooting system of the young plants.
This important consideration helps to improve the efficiency of N management
and fertilizer N recovery, but also helps in minimizing the potential
movement of NO3--N below the active rooting zone of the crop. It is recommended that split applications, by use of side-dress and/or water-run applications of fertilizer N prior to peak bloom periods in the crop, serve to improve fertilizer N efficiency, supply crop needs of N, and reduce losses of NO3--N due to leaching. Use of split applications in-season can be managed by considering the NO3-- N contributions from the irrigation water, as shown in Table 3, and by use of regular sampling and analysis of cotton leaf petioles for NO3-- N (Figure 3).
Table 3. Interpretation of irrigation wate
nitrates.
Interpretive and management guidelines for the N fertility of a cotton crop based upon petiole analysis have been in place for Arizona cotton production since the mid 1960’s (Ray, Tucker and Amburgey, Soil and Petiole Analyses Can Pinpoint Cotton’s Nitrogen Needs, Folder 97, The University of Arizona, College of Agriculture, 1964). More recent guidelines are available for Upland and Pima cotton in Arizona as shown in Figure 3 and Table 4 (Pennington and Tucker, The Cotton Petiole, A Nitrogen Fertilization Guide, Bulletin No. 8373, The University of Arizona, College of Agriculture, 1984).
Table 4. General guidelines for desirable
NO3 - N concentrations in petioles at
Analysis of cotton petioles – the stem tissue connecting the leaf
blade to the main stalk – for NO3--N helps determine the nitrogen
status of cotton at any time during the season. The petiole is selected
from the most recently fully-expanded leaves, usually a petiole of the
third to the fifth leaf from the terminal. Selection of the correct
petiole can substantially influence test results. Petioles from leaves
which are younger than the first fully-expanded mature leaf will have
NO3-- N values lower than those from the mature leaves. In general,
if any doubt exists about which petioles to use, collecting petioles
slightly older than the first mature fully-expanded leaf is better than
collecting younger petioles. About 25 to 30 petioles per sample are adequate for analysis. The number
of samples tested from each field depends on uniformity of the field.
Samples should be collected from uniform areas representing the largest
part of a field that can be treated separately. Samplings should be
made at one-to-two-week intervals through July. Nitrogen Management Using Petiole Nitrate Levels In Figure 4, three N management programs were employed in an experiment
with Upland cotton in Arizona. In the treatment designated as “deficient
N,” petiole NO3-- N levels were never brought above the warning
and deficient zones, resulting in comparatively lower yields. The “optimum
N” treatment provided adequate levels of N according to the N management
guideline chart, that were maintained through the peak bloom stage of
development. After peak bloom the petiole levels of NO3-- N were allowed
to decline, and no further N fertilization was applied. This management
strategy provided adequate N for excellent yield (4.1 bales/acre), and
allowed for a desirable N use pattern of the crop as previously described.
This strategy also allows for the natural senescence of the plant’s
vegetative structure, which accommodates late-season defoliation management,
without diminishing yield potential. The “adequate N” treatment in Figure 4 resulted in somewhat
excessive levels of N in the petioles in midseason. The ultimate result
of this treatment regime was a slightly more vegetative plant, and consequently
a slightly lower yield (3.9 bales/acre). Whenever petiole NO3-- N analysis is being used to manage the N fertilization program for a crop, one must also incorporate a “feel” and understanding for growth patterns, vegetative/reproductive balance, and the overall fruit load of the crop. Other management factors such as irrigation and insect, weed, and disease control must be properly practiced before N management will be completely effective. This will assist in developing a plant of N fertilization that is potentially more efficient, but also more complementary to the production potential of the crop.
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