ABSTRACT

Objectives in a five-year field study were to identify crop growth and yield responses to applied nitrogen (N) and provide in-formation to improve fertilizer N management using soil residual N estimates. Responses of Acala cotton (Gossypium hirsutum L.) to a range of applied nitrogen treatments were investigated in a multi-site experiment in California's San Joaquin Val-ley. Objectives Baseline fertilizer application rates for the lowest applied nitrogen treatments were based on residual soil nitrate-N (NO3-N) levels determined on soil samples from the upper 2 ft of soil collected prior to spring N fertilization and within about one to two weeks post-planting each year. Results have shown positive cotton lint yield responses to increases in applied N across the 50 to 200 lb N/ac range in only 41 percent (16 out of 39) of the test sites. Soil NO3-N monitoring to a depth of 8 feet in the spring (after planting) and fall (post-harvest) indicate most changes in soil NO3 occur within the upper 4 feet of soil. However, some sites (those most prone to leaching losses of soluble nutrients, water) also exhibited net increases in soil NO3-N in the 4 to 8 foot zone when comparing planting time versus post-harvest data. The lack of yield responses and soil NO3-N accumulations at some sites indicate that more efforts should be put into identifying the amount of plant N re-quirements that can be met from residual soil N, rather than solely from fertilizer N applications.