Field evaluation of soil tests for K: Quantity, intensity EUF‐K and other soil properties

Abstract

Ca⁺⁺ and Mg⁺⁺ dominate cation exchange in soils of temperate regions and, at soil‐solution concentrations, are ineffective in displacing K⁺. Thus, soil tests that strongly desorb K may misrepresent its availability to field crops. This study evaluated a range of contrasting procedures, viz., NH₄OAc, Mehlich‐2 and Morgan extracts, 0.01 M CaCl₂‐equilibration and associated K potential (?G) for the energy of K⁺ ‐ Ca⁺⁺ exchange, K activity or intensity: AR_e ^k or pK⁺ ‐ 0.5 p (Ca⁺⁺ + Mg⁺⁺), and electroultrafiltration (EUF) at 200 V, 20°C, 30 min and 400 V, 80°C, 5 min. Relationships between the methods are discussed. ?G or log 0.01 M CaCl₂ each explained 36.1% variance in K requirement of sugar beet over six soil series/associations; other methods explained from 24.1% to 27.5%. Prediction by regression model based on CaCl₂ (or ?G), fixation factor and %silt (i.e., K release) explained 51.3% variance. Partial K⁺ ‐ Ca⁺⁺ exchange achieved a better balance than did other methods in K desorption from sites of different bonding energy or K selectivity.