SUBSOILING AND POTASSIUM PLACEMENT EFFECTS ON WATER RELATIONS AND YIELD OF COTTON

Deep placement of K fertilizer may alleviate late-season K deficiency of cotton (Gossypium hirsutum L.) on soils adequate in surface soil K but low in subsoil K. This 2-yr study in Alabama evaluated effects of deep tillage (in-row subsoiling) and K fertilizer placement on yield, leaf K deficiency, soil water depletion, and stomatal conductance of cotton grown on a soil with a root-restricting hardpan. The Norfolk sandy loam (fine-loamy, siliceous, thermic Typic Kandiudults) soil tested medium for K in the top 15 cm and low at greater depths. Treatments were: (i) no K, no subsoiling; (ii) no K, subsoiled; (iii) surface application (84 kg K ha(-1)), no subsoiling; (iv) surface application, subsoiled; and (v) deep placement in the subsoiled channel. Surface application without subsoiling resulted in the greatest soil water content (0- to 80-cm depth); deep placement, the lowest. Stomatal conductance was highest with no-K, no subsoiling and lowest with K (surface or deep), subsoiled. There was no evidence of K or drought stress-induced stomatal closure, and stomatal closure was not related to severity of leaf K deficiency. All three K treatments increased leaf K concentration at early bloom. Subsoiling without K fertilizer increased plant size and severity of leaf K deficiency; with surface K, subsoiling more than doubled total leaf area but did not affect leaf K deficiency. Within subsoiled treatments, leaf K deficiency was more severe with deep placement of K than with surface application. Subsoiling, especially with K fertilizer, maximized seed cotton yield in both years (avg. 3261 kg ha(-1)) but reduced stomatal conductance. Stomatal closure and premature leaf senescence are not the likely mechanism for late-season leaf K-deficiency in cotton. Although subsoiling was necessary to maximize cotton yields on this Coastal Plain soil with a root-restricting hardpan, deep placement of K fertilizer was not superior to surface application.