Nitrogen (N)

Nitrogen serves several important roles within the plant due to its presence in protein molecules.

Nitrogen deficiency symptoms in Upland cotton.

The N element is found in many essential plant compounds including nucleic acids such as DNA, RNA, photosynthetic enzymes, and other compounds such as chlorophyll. Because of the critical nature of these proteins and enzymes proper N nutrition in the plant is important for maintaining plant health and proper growth and development. Nitrogen is the most commonly applied nutrient to a cotton production system. This is typically applied as an inorganic fertilizer source. The cotton plant can absorb nitrogen in only two forms, the nitrate ion (NO3--N), and ammonium ion (NH4+-N). The two forms of N are soluble and allow for movement into the plant through the root system along with the transpiration stream. Soil N is one of the most dynamic nutrients in a cropping system. Depending on the chemical and physical characteristics of the soil, N transformations may occur that render the applied nutrient unavailable to the crop.

An example of this would be the addition of a nitrate based fertilizer in a low-oxygen environment (i.e. waterlogged soil). The added fertilizer may be converted to gaseous forms of nitrogen and lost to the atmosphere through the following reaction:

2NO3- + 10e- + 12H+ → N2 + 6H2O

This is just one of many examples of N transformations that may occur in the soil, altering the availability of nutrients applied to the crop. Application of the nutrient under favorable conditions is critical to achieving maximum use efficiency by the crop.

Nitrogen uptake by the crop begins to increase rapidly as fruiting forms begin to develop on the crop.

Figure of nitrogen uptake patterns by cotton.

Uptake steadily increases through peak bloom and then begins to decline as the crop approaches cut-out. Applications of N fertilizer should be timed in an effort to coincide with maximum uptake and utilization by the crop.

It has been estimated that approximately 60-70 lbs of N is required to produce a 480 lb bale of cotton lint. This figure may be used to estimate the quantity of N fertilizer needed to produce a crop of a certain quantity of cotton. For example, a three-bale yield cotton crop will remove from the soil 180-210 lbs of N. This quantity of N may come from several sources including irrigation water, residual soil N, among others. Soil sampling and water testing will help to quantify the contribution from these sources and recommendations based upon this testing will vary across the cotton belt. Individual state resources for these recommendations may be found below. Let’s say for example that irrigation water testing revealed that it contained approximately 3 parts per million (ppm) nitrate-N. This equates approximately to 8 lbs nitrate-N per acre-foot of water applied (2.7*ppm = lbs/acre-foot of water). If a total of 5 acre-feet of water is applied during the course of the season we will have received approximately 40 lbs N/acre. If pre-season soil sampling revealed a concentration of 10 ppm nitrate-N this would equate approximately to 10-20 lbs nitrate-N in the top 6-12 inches of soil. This measurement is highly variable and may not contribute significantly to the overall season’s requirement of N. So in this example we will only include the contribution from irrigation water which is 40 lbs N over the season. This would indicate that approximately 140-170 lbs N would need to be applied to adequately supply a three-bale yield crop. Application recommendations in terms of timing and quantities for individual states may be found in the “Resources from Individual States” section.

Nitrogen cycle depicting all potential transformations of N in the soil, plant, atmosphere system.

Images of N deficiency symptoms

Nitrogen deficiency symptoms in cotton.

Nitrogen deficiency symptoms in corn.