Response of Recent Acala Cotton Varieties to Variable Nitrogen Rates in the San Joaquin Valley of California

Abstract

Nitrogen fertilizer is routinely applied to crops grown in rotation with upland cotton ( Gossypium hirsutum L. ) in the San Joaquin Valley (SJV) of California. However, increasing N fertilizer costs, the potential overuse of N resulting in excessive vegetative growth and harvest delays, increasing pest pressure, and concern for nitrate contamination of groundwater support a reassessment of current N fertilization practices. The primary goal of this research was to provide information that would assist SJV cotton growers in updating and improving N management practices. Plot site selection included two university field stations and six on‐farm locations representing all SJV cotton‐producing counties. Nitrogen treatments of 56 to 224 kg N ha −1 were over a 5‐yr period. Cotton lint yield responded positively to increasing N applications in only 41% (16 out of 39) of the test sites. Yield response to fertilizer N was related to residual soil N in the upper 0.6 m of soil as follows: below 70 kg ha −1 residual NO 3 –N, 9 of 17 sites responded positively to increasing applied N; at 70 to 125 kg ha −1 , 5 of 11 sites responded; and at greater than 125 kg ha −1 , only 2 of 11 sites responded. Changes in soil NO 3 –N levels from postplanting to postharvest were generally larger within the upper 1.2 m of soil than at lower depths. However, net increases in soil NO 3 –N also occurred in the 1.2‐ to 2.4‐m range at sites prone to leaching.