ABSTRACT

In South Carolina, potassium recommendations (based on soil test) used for irrigated cotton (Gossypium spp.) were developed sometime ago for dryland cotton. Many feel that the higher yield levels of today's cotton production, especially irrigated cotton, necessitate higher levels of fertilization. Eight on-farm experiments were conducted in 1991 and 1992 under center pivot irrigation systems to answer this question. Cultivar selection, tillage, weed and insect control, irrigation scheduling and other management differed among farm cooperators but were within recommended practices. Soil series on which the experiments were conducted were typical of the region, Norfolk (Fine-loamy, siliceous, thermic Typic Kandiudult) and Faceville (Clayey, kaolinitic, thermic Typic Kandiudult) loamy sands.

For Treatment 1, P and K fertilization based on soil test recommendations, was applied preplant by the growers. Shortly after plant emergence, treatment 2 was established: 50 lb P205 and 100 lb K20 in excess of that recommended by soil test, applied in a narrow band over the row. Soil samples, in 6-inch increments to a depth of 36", were taken from treatment 1 at early bloom and analyzed for Mehlich I-extractable P and K.

There were no effects of additional P and K on any of the factors measured. Leaf P and K were not affected by the treatments and were within current sufficiency ranges, albeit levels established with dryland cotton. Yields ranged from 804 to 1395 lb lint/acre across locations and averaged 1078 lb lint/acre, but additional P and K did not increase yield. Fiber properties-- micronaire, strength, length, and uniformity-- were identical between treatments. We concluded that, although yield levels attainable with irrigated cotton are greater than those obtained dryland, the currently established soil test recommendations and tissue sufficiency levels are satisfactory for producing high yield irrigated cotton. There were four replications of each treatment in a randomized complete block design. Plots consisted of 6 or 8 rows 35' long.

Uppermost fully expanded leaf blades were obtained from bloom until cutout at two week intervals and analyzed for P and K content. At maturity, approximately a 1/1000th acre area was hand harvested from the center row. The seed cotton was ginned and weighed. The fibers were subjected to HVI analysis. Data was subjected to analysis of variance procedures to determine treatment effects.