The uptake of initially available soil potassium by ryegrass

SUMMARY The potassium potential which limits uptake of K by ryegrass, ‘the uptake potential’, was derived from the uptakes of K and from the quantity/potential relationships of a number of soils by a method which avoids exhausting the soil by cropping. This potential was – 5600 cal/equiv in Rothamsted soils, but the method was unsuitable for Woburn soils. In Rothamsted soils uptake of K was more closely related to quantity than to potential of K, but in Woburn soils it was equally well related to both. From 28 days until 82 days, when the initially available K had been used in most soils, diffusion in the soil seemed to control rate of uptake of K. On longer cropping, much initially nonavailable K was released, the grass still grew and the mean K potential had fallen to only –5189 cal/equiv in Rothamsted and –5336 cal/equiv, in Woburn soils.