A long-term field study was conducted to develop an efficient nitrogen (N) fertilization program on nonirrigated South Texas grown cotton. Weather records for the region have shown annual precipitation extremes varying up to 400 percent while growing season partitioned subtotals fluctuated 1800 percent. Since response to N is usually high dependent on available soil moisture, an effective N fertilization program which stresses N utilization efficiencies for rainfed conditions has to involve long-term studies and evaluations at varying levels of other growth limiting nutrients.

Using acid-base urea phosphate fertilizers, response to N was compared at 0, 20, 40 and So lb P₂O₅ N AC^-1. Nitrogen rates included 0, 20, 40 and 80 lb N Ac^-1. Cotton (Gossypium hirsutum L.) was grown from 1978 through 1990 on an Orelia fine sandy clay loam (Typic Ochraqualf). Lint yields ranged from 200 to 1000 lb AC^-1 during the term of the experiment largely reflecting wide fluctuations in seasonal precipitations. In seasons with ample precipitation, N response usually followed strong quadratic functions at the low and mid levels of applied P. In seasons when plants stressed for soil moisture, only slight yield increases and in extreme cases decreases in lint yields were measured especially with applied P fertilizer. Linear components of the response functions indicated lint to N conversions varying from 0.83 to 16. The only fiber property that appeared to be altered only slightly by N fertilization was strength.