Potassium release and fixation by some New Zealand soils as related to their clay mineralogy and degree of profile development

Abstract

The availability of potassium to pasture plants in New Zealand is dependent upon the mineralogy of the soil in many ways. It will depend first of all on the primary minerals present, mostly in the sand and the silt fraction of the soil, however, equally important is the amount and type of secondary clay minerals formed during pedogenesis. These minerals will govern the distribution of soil potassium into forms which are unavailable, potentially available and readily available to plants. For a soil under grazed pasture other important sources of available potassium are additions to the soil surface in fertilizer and/or in animal urine and dung. Where suitable clay minerals are present in the soil there may be a considerable release of potassium into a readily available form as this form is lowered by plant uptake. Some clay minerals may also convert any excess of available potassium into less available forms by the process of potassium fixation. This fixation may be beneficial if it decreases losses of soluble potassium due to leaching or to luxury uptake by plants, especially after top dressing or urine applications. For pastoral farming in New Zealand, exchangeable potassium is used as a guide to plant available potassium and is also related to dry matter responses to potassium in observational field trials, (Hogg, 1957). The aim of this study is to relate the availability of potassium to ryegrass, a major user of potassium in the pasture system, to the clay mineralogy of two sequences of New Zealand soils. In addition, the effect of clay mineralogy on the release and fixation of potassium by these soils is studies. Changes in soil potassium and clay mineralogy of both zonal and ash soils with increasing soil development are discussed.