Studies on soil potassium. III. The intensity of soil potassium following exhaustion by different plants

Abstract

Exhaustive cropping of four soils by four species revealed only small differences in the limiting intensity (expressed as the equilibrium activity ratio, ARo) of soil potassium, for which the mean value was 5.0 x 10-4 mole1/2 L 1/2. Replanting of depleted soils led to slightly lower values. With a fifth soil, which contained significant amounts of potassium felspar in both clay and sand fractions, grasses reduced the intensity to a lower level (3.8 x 10-4 mole1/2 L 1/2) than did legumes (21.0 x 10-4 mole1/2 L 1/2), and these values were substantially reproducible with replantings of the same species. It was suggested that the peculiar behaviour of this soil may have been associated with the differential ability of the various plants to use potassium from orthoclase. The intensity at which potassium was first absorbed from sources not initially in equilibrium with 0.001M calcium chloride varied from 8 to 29 x 10-4 depending on the soil, and, in the case of the felspathic soil, depending on the plant.