Differential Adsorption of K, Ca, and Mg by Tropical Soils and Their Uptake by Kikuyugrass

Abstract

The differential adsorption of K, Ca and Mg at pH 5.0 and 6,0 by Maile, Hawi and Wahiawa soils representing an Inceptisol, a Mollisol and an Oxisol, respectively, was established at 25 C. The cationic concentration of the equilibrating solutions containing cationic pairs of K:Ca, K:Mg and Mg:Ca were adjusted to 0,1 N. The cationic ratios were 1:1, 1:2, 1:4 and 1:8, Equilibrium was achieved over a 12-day period. When equilibrating in 0.1 N solution chlorides of K, Ca and Mg, the three soils adsorbed more K, Ca and Mg, respectively, at pH 6,0 than at pH 5,0. At the same time, all the three soils adsorbed more Ca than Mg and then K at each pH level. The Malle and Hawi series preferentially adsorbed divalent cations (Ca and Mg) over monovalent cation (K) at both pH 5,0 and 6,0, Between the two divalent cations, Ca was preferentially adsorbed over Mg by the two soils. In the case of the Wahiawa soil, monovalent cation (K) was selectively adsorbed over divalent cations (Ca and Mg) at both pH levels. Between Ca and Mg, Wahiawa soil preferentially adsorbed Ca over Mg. The effects of various levels of K, Ca and Mg on dry matter yield, mineral composition and grass tetany ratioof kikuyugrass were investigated by a greenhouse experiment in two soils at pH 5.0 and 6.0. The two soils were Maile and Wahiawa soils representing an Inceptisol and an Oxisol, respectively. The greenhouse experiment was a 3x3x3 (KxCaxMg) factorial design with three replicates. The total dry matter production of kikuyugrass increased with increasing application of K and Mg in both soils at the two pH levels. Increasing application of Ca to Wahiawa soil significantly increased the dry matter production of kikuyugrass, however, in the case of the Maile soil, it was not affected significantly. Tissue K, Ca and Mg concentrations and uptake of these three elements by kikuyugrass increased with increasing applied R, Ca and Mg, respectively, in both soils at the two pH levels. Tissue Ca concentration of kikuyugrass decreased significantly with increasing applied K and Mg; however, tissue Mg level was not influenced by the application of Ca in Maile and Wahiawa soils at both pH 5.0 and 6.0. At the same time, the application of K decreased tissue Mg concentration of kikuyugrass grown in both soils at the two pH levels. The application of Ca and Mg to both soils did not produce any significant effect on the tissue K concentration of kikuyugrass at the two pH levels. The grass tetany ratio of kikuyugrass grown in the two soils Increased with increasing applied soil K. Such an increasing trend was enhanced by the lowering of the applied soil Mg level. Applied Ca did not affect the grass tetany ratio of kikuyugrass in both soils at the two pH levels. Concentrations of P, S and Zn in tissue of kikuyugrass decreased with increasing K in both soils at the two pH levels. In the two soils, tissue S concentration of kikuyugrass increased with increasing soil Mg at both pH levels.