Use of the quantity/potential relationship to provide a scale of the ability of extractants to remove soil potassium

SUMMARY Quantity/potential relationships, between gain and loss of K by the soil and K potential (RT In ακ/⊧αca+Mg) were determined on twenty-seven Rothamsted and Woburn soils. K extracted by neutral N ammonium acetate, by H-resin and 0·5 M sodium bicarbonate (pH 8·5) were also measured. The ability of an extractant to remove soil K is equated to a K potential (derived from the quantity/potential curve) which the soil attains on removing K equal to that taken out by extractant. Mean values for all soils were – 4995 ± 97 cal/equiv for the ammonium acetate, – 6081 ± 88 cal/equiv for the H-resin and – 4336 ±117 cal/equiv for the sodium bicarbonate extractants. For the first and last extractants the ability to remove K was less in rich than in poor soils. Varying the ammonium ion concentration from 0·1 N to 1 N in ammonium acetate/acetic acid mixtures, N in acetate ions, did not greatly affect their ability to remove K.