ABSTRACT

Nitrogen (N) is frequently the most expensive plant nutrient applied to cotton (Gossypium hirsutum). A single, inexpensive soil-test that would predict the availability of N from the soil during the growing season is desirable. However, because N is chemically dynamic in the environment, no single soil-test accurately fulfills this need. Therefore, the amount of supplemental, fertilizer N required to achieve optimum cotton yields cannot be readily determined by soil testing alone. Consequently, cotton's annual N needs are often estimated by means of a material-balance analysis. Research suggests that cotton removes about 50-55 lbs. of N/bale as seed plus lint. Thus 2- and 3-bale crops would require about 100-110 and 150-165 lbs. N/acre-year, respectively. However, not all of the N taken up by cotton must be applied as fertilizer, since there are several environmental sources of N. Rainfall and/or irrigation water often supply 20 lbs. N/acre-year. Also, nitrate-N in the soil profile, above the 4-ft. depth, should be readily accessible to cotton roots. Recent research in California and Texas suggests that 50 lbs. N/acre are usually present in the 4-ft horizon, and depending on the previous crop and winter rainfall, larger amounts are frequently found. About a half-ton of organic N is present in the first half-foot of soil for every percent of organic matter in the soil. While few estimates of the rate of turnover of soil organic N are available for cotton production systems, an estimated conversion of 30- 50 lbs. N from organic forms to nitrate-N per 1% organic matter per year is probably conservative. With the present lack of yield increase in cotton, the trend to increasing rates of N fertilization is probably unjustified. Rather, growers are encouraged to make realistic estimates of crop yields, utilize knowledge of cropping history and recent rainfall, and use spring soil nitrate tests, as appropriate, to adjust rates of N fertilizer. Recent research suggests that N rates greater than 75-100 lbs. or 120-150 lbs. N/acre for 2 or 3-bale crops, respectively, infrequently cause yields to increase above the levels achieved by these rates. In fact, when fertilizer N is applied in excess of such rates, cotton management costs may increase due to increased vegetative growth, increased susceptibility to certain insects, delayed maturity and/or difficulty in preparing for harvesting.