Some factors controlling potassium availability in Texas soils

Abstract

Field, greenhouse and laboratory studies were made to determine some of the factors controlling potassium availability in Texas soils, and to relate some selected soil chemical parameters to the nutrition of grain sorghum. In one portion of the greenhouse study, grain sorghum (var. RS 626) was grown to the full bloom stage in nutrient solution with graded levels of K. The K concentrations were 0.05, 1, 5, 50 and 200 ppm. Maximum yield was obtained at the 50 ppm concentration. The nutrient solution study indicated that grain sorghum leaves should contain about 1.5% K at the full bloom stage for maximum growth. The study also indicated that K deficiences are present in grain sorghum before they are visually evident. In another portion of the greenhouse study, the effects of K addition were studied on four important Texas soils containing different clay types. The soils used were; 1) Lake Charles clay (montmorillonitic) 2) Nacogdoches sandy loam (Kaolinitic), 3) Norwood silt loam (montmorillonite-mica) and 4) Miles loamy sand (mica-montmorillonite). Grain sorghum plants were grown on the soils with K application rates of 0, Æ°, 1, 2, and 4 times their original exchangeable K content. The soil solution K was found to diminish rapidly in the kaolinitic soil but the loss of solution K was not so rapid on the other three soils. Exchangeable K was depleted rapidly in the kaolinitic soil and slowly in the montmorillonitic soil. There was a rapid renewal of exchangeable K in the mixed clay soils resulting in little or no loss of exchangeable K with the growth of a grain sorghum crop. Grain sorghum growth was markedly increased with K additions on the Nacogdoches soil and there was a slight increase in growth at the low rate of K on the Lake Charles clay. Potassium additions to the Norwood and Miles soils reduced the yields of grain sorghum tops. In the field study, a survey was made of the cation content of grain sorghum leaves at the full bloom stage on eight different Texas soils. Cation determinations were also made on the eight soils in order to relate K and Mg uptake to the K and Mg found in the soil. The concentration of K in solution was found to be extremely low in the montmorillonitic soils. The equation: % K in leaves=0.866 + 1.835 soil solution K+ [log (1 + exchangeable K)] was found to be a good method of predicting the percent K in grain sorghum leaves at the full bloom stage (r= 0.890). There was an inverse relationship in all the studies between K in the plant growth medium and percent Mg in the plants. Based on the results of this study, the kaolinitic soild of East Texas require the addition of K fertilizer to maintain adequate plant growth because they have a low affinity for K, they contain small amounts of exchangeable K and the rate of K renewal is slow. Montmorillonitic soils of the Blackland Prairie and ...