The potassium status of soils: Significance of the “Italian ryegrass test”

Abstract

There is a simple relationship between the amounts of K extracted from soil by ammonium acetate or sulphuric acid and by Italian ryegrass provided that the grass is grown on a small amount of soil and that the time required for K exhaustion by the grass is short and that exhaustion is achieved as indicated, for example, by the absence of regrowth. In general, ryegrass can extract more K from grassland soils than can chemical reagents, even sodium tetraphenylborate. The pool of soil K exploited by ryegrass although larger than that obtained by acetate or sulphuric acid, does not include it completely. The reagents remove from the soil a little K which the roots cannot reach but do not extract the whole pool actually available to the roots. The maximum quantity of K extractable by ryegrass can be deduced from the K content of the herbage after three weeks test cropping even when conditions are not strictly controlled. This applies only if the ryegrass roots colonise the soil quickly and completely as when the volume of soil is limited so that growth is entirely dependent on the amount of available K.