In order to balance nitrogen (N) inputs with cotton (Gossypium Hirsutum L.) yield capability, it is necessary to know how well each cultivar can respond to N inputs and its tolerance to higher-than-average rates of N. This study was designed to 1) measure yield response of four cultivars to varying N rate and 2) develop soil and plant diagnostic criterion to improve the accuracy of N fertilizer recommendations for a clayey soil.

Field experiments were conducted from 1988 to 1990 on a Sharkey silty clay (very fine, montmorillonitic, nonacid, thermic, Vertic Haplaquept) at the Northeast Research and Extension Center at Keiser, Arkansas, to evaluate five N rates and four cultivars.

The experimental design was a 5 x 5 Latin-square with a split-plot arrangement of treatments. Nitrogen rates of O-, 50-, 100-, 150-, and 200-lb/acre were the main plots and cultivars the split plots. The experiments were repeated on the same plots each year.

Nitrogen was knifed-in using liquid N (16% N as urea and 16% N as NH₄NO₃). Nitrogen was applied sequentially as 50-lb/acre preplant, 50-lb/acre at pin-head square, and 50-lb/acre at pin-head square, and 50- or 100-lb/acre at early flowering until the total N needed for each treatment was attained. The cultivars Deltapine 20, Deltapine 50, Stoneville 453, and Stoneville 506 were planted on May 5, 1988; May 17, 1989; and May 25, 1990.

Recommended cultural practices were followed, including the application of harvest aids. Tensiometers were used to schedule furrow irrigations. Lint yields were determined by multiplying seedcotton yields by percent lint obtained for each cultivar from annual cultivar performance tests at Keiser.

Economic analysis was conducted utilizing a modified Spillman growth curve. The first derivative of this curve, marginal economic returns (i.e., lb lint per acre increase for a unit of N fertilizer input), was equated to the cost of fertilizer and the price of cotton.

Cultivars were found to respond similarly to increasing N fertilizer. In general for all cultivars, lint yield increased approximately 6 lbs per each lb of fertilizer N added up to the 100-lb-N/acre rate. Above the 100-lb N/acre rate, lint yields tended to level off.

The factor most affecting N requirement was year Yield did not increase significantly with N rates greater than 100-lb/acre in 1990, but continued to increase up to 200-lb/acre in 1988 and 1989. The lower N requirement in 1990 was probably related to the low yield potential, resulting from a late planting date. While in 1989, small frequent rains occurred producing conditions conducive to denitrification of N fertilizer on this clayey soil, thus a need for the higher N rate.

The maturity of the cotton crop as evidenced by first and second picking lint yield and proportion of first pick to total yield was found to depend on the cultivar or year and the N rate. As N rate increased, the lint in the first and second picking increased similarly for cultivars. This indicated that (1) more lint was available to be picked in the first picking due to improved N nutrition and (2) more bolls were set late and increased the second picking.

The result of economic analysis indicated that the break-even N rate varies from year to year and will also depend on N cost and cotton price. Break-even N rate ranged from 105 to 160 lb-N/acre depending on year and price of lint and fertilizer. The variation within a year arising from changes in prices of lint and/or N fertilizer was much less than that arising from different years.

Since cotton fertilizer management usually involves multiple soil N applications (including preplant and side-dress applications), the N rate may be modified with the post-emergent N applications to accommodate changes in N requirement in different growing seasons. Petiole monitoring is a tool the grower can also use to assist in evaluating N nutrition of the cotton plant during the season. If a N deficiency is indicated, supplemental N may be applied in season.

Some "best N management" practices for irrigated cotton production on clayey soil which can be inferred from this study are as follows: 1) avoid large denitrification losses by timing preplant applications as late as practical and applying one or more side-dress applications, 2) plan on applying at least the amount of N that results in acceptable returns in a bad year, and 3) in good cotton years, be prepared to add more N during fruiting and boll maturation periods.