Assessing the availability of potassium in soils

Abstract

Two forms of depletion cropping were examined. Perennial ryegrass was continuously cropped on two contrasting soils in pots. Simultaneously, repeated crops of ryegrass, subterranean clover, and marrow-stem kale were grown by a modified system of depletion cropping in which the whole plant was harvested and the soil sampled before being repotted for further crops. Fractions of soil potassium determined by chemical extractions were compared with the results from depletion cropping. Although the modified system needed added attention, particularly between crops, the available potassium was well exploited because of the thorough mixing of the soil. A further benefit was that the depletion of potassium in soils and plants could be followed progressively. Different root systems of the plant species greatly influenced the total potassium uptake. Reserves of initially non-exchangeable potassium contributed much to the uptake while decrease in soil potassium measured by repeated nitric acid extractions ranged from 63 to 80% of the uptake. Ryegrass with its dense root system was able to take up more non-exchangeable potassium from a yellow podzolic soil than clover and kale.