Avaliability of potassium in surface horizons of some Brazilian soils

Abstract

Surface horizons of eighty one Brazilian soils were used to study potassium supplying power through extraction by chemical methods. The forms of potassium determined were total, "available", and exchangeable. The “available" potassium was obtained by normal nitric acid solution, sulphuric acid (10 ml concentrated acid + 25 ml water), and sulphuric acid (1 ml concentrated acid + 25 ml water). Correlations among the extracting methods were obtained. The most significant correlations were obtained between H₂SO₄ (10 ml) x HNO₃ and between H₂SO₄ (1 ml) x exchangeable, with correlation coefficients of 0,9738⁺⁺ and 0,7956⁺⁺, respectively. Within each soil class, variation in the amount of total potassium was generally observed, the greatest amounts of total potassium were obtained in the samples from low rainfall areas. The Lithosol, Vertisol, Non-Calcic Brown Soil, Brunizem Regosolic Latosol, Regosol, and Red-Yellow. Podzolic soils studied probably contain adequate potassium for plant growth. Of the mechanical separates of the soils studied only the silt fraction (0.05-0.002 mm) correlated with the amount of potassium extracted by the acid nitric method (r = 0.4697⁺⁺). It is suggested that the H₂SO₄ (10 mI/25 ml water) method be used to determine the "available” potassium due to it being more practical than the nitric acid method.