Release of interlayer potassium in Norwegian grassland soils

Potassium (K) release from sources that are not initially exchangeable is attributed to depletion of interlayer K of micas and clay minerals or weathering of feldspars. The aim of the present study was to estimate the K release from interlayer K. Soil samples from 17 field experiments in ley on a range of mineral soils in Norway were used in the study. The change in K‐fixation capacity was used as an estimate of depletion of interlayer K. It was assumed that the increase in K‐fixation capacity during 3 yr of cropping was equivalent to the amount of K depleted from the interlayer positions. Mean K fixation increased in the majority of the soils during 3 yr of grass cropping both with and without K application. The increase in K fixation indicated that without K application, the K uptake from interlayer K amounted to 43%, 28%, and 26% of the K yield for clay soils, high‐K sandy soils, and low‐K sandy soils, respectively. Including K uptake from exchangeable K in the topsoil and from subsoil, the explained K uptake amounted to 79%, 69%, and 81% for the three groups of soil, respectively. Simple linear‐regression analyses showed that the change in K fixation during 3 yr of grass cropping was best explained by the percentage of clay in the soil.