Henry Sintim, Glendon Harris
Department of Crop and Soil Sciences, University of Georgia
Nitrogen (N) constitutes 78% of the earth’s atmosphere, and it is present as N2, an inert gas. By conversion, there is 117,000 tons/ac N in the atmosphere . However, plants can not assimilate N in the N2 form, and hence, the need to supply inorganic forms of N for crop production. Ammonium (NH4+) and nitrate (NO3–) are the inorganic forms of N readily utilized by crops. In moist, warm, and well-aerated soil conditions, as commonly found in Georgia soils, NO3– is more prevalent than NH4+. Also, the rate of NO3– uptake by crops is greater, but crops need to metabolize the NO3– to NH4+, followed by the conversion of NH4+ to amino acids, and the conversion of amino acids to proteins. The reduction of NO3– to NH4+ is an energy-requiring process. Two NO3– reductase enzyme molecules are needed for each NO3– reduced. Plants supplied with more NH4+ are reported to accumulate higher carbohydrates and protein than those supplied with more NO3–, especially at the early growth stage [1,2]. Overall, however, crops perform better under the combined application of NO3– and NH4+ sources of N . This is because plants can regulate intracellular pH more easily and store some of the N at low energy costs.
As N is susceptible to losses, especially under the prevailing conditions in the state, the University of Georgia Extension recommendation is to split-apply N in cotton. The total N rate for cotton is based on soil type, previous crop, growth history, and yield potential. The base N rate for 750; 1,000; 1,250; and 1,500 lb/ac lint yield is 60, 75, 90, and 105 lb/ac N, respectively, with the recommendation for 1,250 and 1,500 lb/ac lint yield assuming irrigated conditions. Recommended adjustment to the base rate is as follows:
|Factors to increase the base N rate by 25%||Factors to decrease the base N rate by 25%|
|· Farming in deep sandy soils||· Cotton following a leguminous crop such as peanuts or soybeans|
|· The previous crop was cotton||· Cotton following a good establishment of winter legumes, such as clover or vetch.|
|· There is a history of inadequate stalk growth||· There is a history of rank or excessive vegetative growth.|
The current recommendation is to apply between 25% to 33% of the total N rate at planting, and the remainder as side-dress between the square and 1st week of bloom stages. Make side-dress application closer to the square stage if the plant is showing stunted growth and the leaves are chlorotic (pale, yellow, or yellow-white). Side-dress application closer to the 1st week of bloom is suggested if the crops show good vigor with dark green foliage. If possible, a two-way side-dress application (half at the square stage and the remaining half between the 1st and 2nd week of bloom) would work fine. This would actually be beneficial in seasons of intensive rainfall conditions at critical stages, as has been the case this year. Several cotton fields were past the square stage before the current surge of thunderstorms, which will inevitably lead to the loss of some N, especially the NO3– forms of N. Recent studies conducted across six production conditions in Midville and Camilla showed cotton can tolerate early-season nutrient stress. Skipping initial fertilizer application until the square stage did not significantly affect cotton yield at all production conditions . The previous crop was peanut under four production conditions and corn under two production conditions. However, we do not recommend skipping at-planting N if the previous crop was not a well-established legume crop or the initial soil inorganic N (sum of NO3-N and NH4-N) level within one week of planting is not above 10 ppm. The figure below shows the seasonal N uptake of cotton in Texas (A) and California (B). As can be seen, N uptake is very marginal at the early growth stages and increases rapidly after the square stage. The ability to tailor N availability with crop demand will enhance N use efficiency.
Figure: Seasonal N uptake in cotton reported (A) Texas  and (B) California .
As previously noted, crops under the combined application of NO3– and NH4+ sources of N tend to perform better than the application of either source. Urea ammonium nitrate (UAN), ammonium nitrate, and urea are common sources of N for side-dress application. Due to potential hazards, ammonium nitrate is sometimes blended with other compounds, such as sulfur (S). Thus, there are various formulations of ammonium nitrate, which all tend to work fine. The choice of side-dress fertilizer should be based on accessibility, cost, efficacy, and ability to safely handle the product. As S application is routinely recommended, the value of S should be accounted for in determining the cost of side-dressed N sources.
Plant nutrient uptake is regulated by several abiotic and biotic factors. While not often, N deficiency could occur regardless of applying the recommendation rate. It is important to therefore monitor the nutritional health of the crop, especially between the square stage and 3rd week of bloom with tissue test analysis. The recommended tissue N level is between 3 to 4.5%. Low N rates can reduce yield and quality while excessive N rates can cause rank growth, boll rot, delayed maturity, difficult defoliation, and poor quality and yield. Thus, ensuring an optimum supply of N is critical. Nitrogen deficiency in cotton occurs as chlorosis of the leaves and red leaf blades when deficiency becomes severe. Deficiency appears first on older (lower) leaves, and plants show reduced growth, height, boll size, and an increase in boll drop.
Figure: Photos of N deficiency symptoms in cotton. Photo credits: Yara US (A and B)1, and North Carolina State University (C)2.
Henry Sintim at email@example.com
Glendon Harris at firstname.lastname@example.org
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- Sintim, H.; Amissah, S.; Ankomah, G.; Rogers, S.; Roth, R.; Lee, R. DAP and TSP for Phosphorus Nutrition: The Tradeoffs between Efficacy and Cost; University of Georgia, Athens, GA, USA, 2022;
- Marschner, P. Marschner’s Mineral Nutrition of Higher Plants; Marschner, P., Ed.; 3rd ed.; Academic Press, Waltham, MA, 2012; ISBN 9780123849052.
- Sintim, H.Y.; Amissah, S.; Agyei, B.K.; Hollifield, S.; Dowdy, M.; Sapp, P.; Harris, G. Nutrient Stress Effects on Cotton Productivity. In Proceedings of the 2022 Beltwide Cotton Conferences; The National Cotton Council: San Antonio, TX, January 4-6, 2022, 2022; pp. 27–30.
- Pabuayon, I.L.B.; Lewis, K.L.; Ritchie, G.L. Dry Matter and Nutrient Partitioning Changes for the Past 30 Years of Cotton Production. Agron. J. 2020, 112, 4373–4385, doi:10.1002/AGJ2.20386.
- Geisseler, D. Cotton Nitrogen Uptake and Partitioning; University of California, Davis, CA, USA, 2020;