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Nitrogen

Nitrogen fertilization of cotton is complicated and involves a variety of factors, including: (1) yield potential, (2) soil type, (3) weather, (4) sources of nitrogen, and (5) timing of application. Nitrogen fertilizer rates vary from farm to farm and from field to field within a farm. Nitrogen rates should be based on yield potential, history of rank growth in a field, soil type, and level of management.

As a general guideline, approximately 50-60 pounds of nitrogen fertilizer are needed to produce a bale of cotton on light-textured soils; 60-70 pounds of nitrogen fertilizer are needed to produce a bale of cotton on medium textured soils; and 70-80 pounds of nitrogen are needed to produce a bale of cotton on clay and clay-loam soils. Therefore, if a medium-textured soil (C.E.C. = 15) has a yield potential of two bales per acre (good soil, irrigation, etc.), the rate of nitrogen to use is 120-140 pounds per planted acre. Reduce nitrogen rates if a field has been in soybean production, corn rotation, or has a history of rank growth.

Weather, particularly intense rainfall, has a great influence on the efficiency of applied nitrogen fertilizers. Nitrogen can be lost through leaching, which occurs in sandy soils. Leaching means that nitrogen is moved downward by water through the soil and possibly out of the effective rooting zone of the soil. This prevents nitrogen from being taken up by the plants.

Another form of nitrogen loss is denitrification, which occurs in heavier textured soils. When these soils are saturated with water, bacteria break down nitrate, and the nitrogen is released into the atmosphere as nitrogen gas. Heavy and prolonged periods of rainfall can result in nitrogen losses severe enough to require additional nitrogen applications to correct the problem.

All sources of nitrogen are considered equal in their ability to provide nitrogen to cotton. No one form or source of nitrogen is superior to another if all are applied correctly. Base choices on price, availability, and ease of application. Solid urea requires special consideration when applications are made to cotton. If dry urea is applied to the soil surface in hot, dry weather, the rate of nitrogen loss can be high unless it is incorporated into the soil by tillage, rainfall, or irrigation within 2-3 days. If urea is incorporated into the soil by any of these methods within 2-3 days, or if the temperature is less than 75 F, losses are minimal. When left on the soil surface during midsummer for 5-7 days, in the most severe cases 50 percent or more of the nitrogen within the urea can be lost. Losses of over 30 percent in these situations are more common.

Some growers apply nitrogen in split applications. The decision to use split applications, as opposed to all pre-plant, should be based on the rate of nitrogen used and whether or not irrigation is possible. If you will use more than 100 pounds nitrogen per acre, you should split the rate because of the danger of salt damage. A split of one-half pre-plant, one-half side-dress, or two-thirds pre-plant, one-third side-dress can be used. Research at the Delta Research and Extension Center has shown a yield increase in irrigated cotton by applying one-half the total nitrogen pre-plant and one-half the nitrogen at first bloom. Daily use rates of nitrogen are relatively low until squaring. During square set, daily use rates of nitrogen begin to increase, and during bloom and boll fill, daily use rates of nitrogen become high (provided there is adequate moisture available for uptake and respiration). Split applications of nitrogen tend to increase the chances of providing nitrogen to meet crop demands during peak demand periods.

Another consideration for irrigated cotton is to apply one-third of the total nitrogen at planting, one-third at late square-early bloom, and one-third at near-peak bloom. This late application would have to be aerially applied and must be done only where irrigation is used.

If you plan to use less than 100 pounds nitrogen per acre on non-irrigated cotton, applying all the nitrogen pre-plant is as good as split applications in most cases. If you plan to use more than 100 pounds nitrogen per acre on non-irrigated cotton, apply one-half to two-thirds pre-plant and the remainder between first square and first bloom. Where pre-plant nitrogen is applied, broadcast before rows are formed; apply no more than 40 pounds, because salt injury may occur. The remainder could then be applied as a side-dress application.

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