ABSTRACT

Deep placement of K fertilizer has been suggested as a means of alleviating late season K deficiency on soils testing adequate in surface-soil K but with a low level of K in the subsoil. However, the comparative and interrelations effects of deep tillage and K fertilizer placement have not been elucidated. This field study evaluated the relative effects of deep tillage (subsoiling) and K fertilizer placement on yield, leaf K deficiency symptoms, soil water depletion, and stomatal conductance of cotton grown on a soil with a root-restricting hardpan. The study was conducted for two years (1990-91) on a Norfolk sandy loam (fine loamy, thermic siliceous Typic Kandiudults) in east-central Alabama. Treatments selected for study from a randomized complete block of four replications were: 1) no-K, no-subsoiling; 2) no-K, in-row subsoiled; 3) surface application of 75 lb K/acre without subsoiling; 4) surface application of 75 lb K/acre plus in-row subsoiling; and 5) deep placement of 75 lb K/acre in the in-row subsoiled channel. Deep placement of K fertilizer resulted in the greatest soil water depletion below the 8-inch depth while surface K application without subsoiling resulted in the least soil water depletion below the 8-inch depth. Stomatal conductance both years was highest in the no-K nonsubsoiled treatment and lowest where K fertilizer was applied in conjunction with subsoiling. There were no indications of stress-induced stomatal closure and stomatal closure was not related to severity of leaf K deficiency symptoms. Leaf K concentrations at early bloom were increased by K application and reduced by subsoiling. Subsoiling and surface application of K resulted in the greatest seed cotton yield, highest leaf K concentration at early bloom, greatest leaf area, and lowest stomatal conductance. Our results suggest that stomatal closure and premature leaf senescence is not the most likely mechanism for the appearance of leaf K deficiency symptoms. Although subsoiling is necessary on Coastal Plain soils with root-restricting hardpans, surface application of K fertilizer was superior to deep placement of K on these soils.