The effects of four irrigation methods and five nitrogen fertilization rates were investigated on the growth, development, and yield of cotton from 1982 to 1984, and 1986 and 1987. The four irrigation methods studied were a high frequency overhead sprinkler, low frequency overhead sprinkler, furrow flow, and a dryland control. Irrigation was begun when tensiameters registered threshold values of vacuum pressure at predetermined depths, and ended on August 15 (Low Frequency and Furrow Flow) and September 7 (High Frequency). Five urea-nitrogen fertilization treatments were tested within each irrigation regime. Rates of 0, 30, 60, 90, and 120 lbs N/A were split and applied half preplant and half when the crop reached first square stage. The nitrogen treatments were surface broadcast and incorporated by shallow tillage at both applications. The soil at the study site was an Hebert silt loan (fine-silty, mixed, thermic Aeric Ochraqualfs), and had previously been cropped to soybeans.

Significant yield responses ( a=0.05) to the irrigation methods were evident all five years of the study. Overhead irrigation, low frequency and high frequency, was found to produce significantly greater yields than the other irrigation methods the first two years of the study. Verticillium wilt drastically reduced yields in all irrigated regimes in 1984. Most severely affected by the damage were the high frequency irrigation plots which yielded significantly less than all other irrigation methods. Conversely, the dryland block had the least disease pressure and the greatest yield. The furrow irrigated block was the highest yielding irrigation method both in 1986, a growing season characterized by drought stress and dramatic response to irrigation, and in 1987 under more optimum growing conditions. Although verticillium wilt again affected the high frequency irrigation regime in 1987, the disease was not observed in other irrigation blocks.

Preliminary soil testing of the study site in the spring of 1982, before initial application of N treatments, showed residual NO₃- -N to be high (40 lbs N/A). Soil NO₃- -N was found to be uniform both laterally throughout the test site and vertically. The high N status of the soil influenced the first two years (1982 and 1983) of response to N fertilization under all irrigation regimes. During these years, crop response to the N fertilization treatments was minimal. No differences were significant ( =0.05) in 1982 as a result of the N treatments. The 1983 results show trends of increased yields with N rates within the dryland, low frequency, and furrow irrigated regimes indicating that the residual N was being depleted by continuous cropping. Verticillium wilt drastically reduced yields under all irrigation regimes in 1984, except the dryland plots. The combinations of the high frequency irrigation method and higher N fertilization rates produced the least lint due to increased virulence of the disease. The dryland plots were the only ones not affected by the verticillium, and the only ones to positively respond to N fertilization. The 1986 cropping season was extremely dry, and positive response to N was observed in all irrigation regimes except the dryland control. The dryland results were due to water becoming the most limiting growth factor rather than N. Yields in 1987 were less dramatically influenced by N in the low frequency and high frequency center pivot methods, but slightly more influenced by N in the furrow irrigated and dryland blocks.