Nature and mode of weathering of soil‐potassium reserves

Abstract

AbstractIn most partially weathered mineral soils only a small part of the total potassium is readily exchangeable to other ions; the bulk occurs in non‐exchangeable (or difficultly‐exchangeable) forms in potash feldspars, micas and micaceous clays, all of which are potential sources of potassium for plants.Some of the principles underlying the weathering processes in feldspars, trioctahedral and dioctahedral micas are discussed. The ease with which freshly cleaved potash feldspar releases potassium to water cannot be taken to indicate that the feldspar weathers easily; in soil the feldspar is protected by a surface covering of its own decomposition products. Trioctahedral micas are less stable to weathering than dioctahedral micas, particularly during the early stages of potassium depletion; the stabilities of these two main groups of micas differ more than can be accounted for in terms of ferrous iron contents. For micas and micaceous clays the relationships between parent lattices and weathering products must depend very largely on the extent to which layer‐charges have decreased on weathering. As more becomes known about the potassium‐containing minerals which occur in soils, we should be better able to understand the factors which influence the long‐term ability of soils to supply potassium to crops.