The use of chemical extractions and continuous cropping to measure the potassium status of soils

Abstract

Soil potassium, measured by simple chemical extractions, was compared with potassium availability assessed during continuous cropping of perennial ryegrass in pots, on 12 surface soils representing four Tasmanian soil groups.

For three soil groups with mainly kaolinitic clay, potassium uptake by the plant (the main measure of potassium availability) was related to a decrease in the potassium extracted from the soil by boiling lM nitric acid during the experiment. The decrease in soil potassium extracted by repeated nitric acid digestions averaged 95% of the total potassium uptake. However, a large and rapid uptake from the illitic yellow podzolics was only partly assessable by chemical extractions. For all soils, much of the large amount of potassium taken up early was stored in the plant roots and transferred to the tops much later.

Plant yields were related to exchangeable potassium only within soil groups; over most soils they were related to nitric acid-soluble potassium. Yield responses to added potassium usually occurred when exchangeable potassium became depleted to a critical level of 0.2–0.25 m-equiv./100 g or nitric acid-soluble potassium to 0.4–0.7 m-equiv./100 g. The critical level of potassium in ryegrass tops was 0.5–0.8%.

On average nearly two-thirds of the potassium released from soils came from exchangeable sources. Potassium initially non-exchangeable was differentiated into quickly released 'intermediate' potassium and slowly released 'constant rate' potassium. Release of intermediate potassium predominated from the yellow podzolics and from one non-calcic brown soil with much randomly interstratified material in its clay.