Assessing potassium reserves in northern temperate grassland soils: A perspective based on quantitative mineralogical analysis and aqua-regia extractable potassium

Increasing demand for locally produced farm products, with a lower input of mineral fertilisers, leads to a greater reliance on the inherent capacity of soils to supply plant nutrients. Typically 98% of all soil potassium (K) is found in K-feldspars and phyllosilicates, however the potential bioavailability of K from different mineral forms varies significantly. As a basis for improving sustainable nutrient management options for grassland systems we set out to determine 1) how soil parent material influences the amount of K in soils, 2) how the K is quantitatively distributed between mineral groups, 3) relationships between quantitative mineralogy (X-ray powder diffraction (XRPD)), total K (X-ray fluorescence) and the more widely available measurement of aqua-regia extractable K, and (4) how the studied soils could be ranked in terms of their long-term potential capacity to deliver K to plants. Soils representing eight of the most extensive soil associations (defined by different parent materials) under improved grassland in Scotland were analysed. Soil samples represented a wide range of total K concentrations, from 1.3 to 39 g kg− 1. Quantitative mineralogical data from XRPD were combined with mineral compositions to obtain the mineralogical apportionment of K. Apportionment of K varied significantly, from those soils with almost all K present in various phyllosilicates to others where almost all K is held in K-feldspars. Aqua-regia extractable K is related to K in phyllosilicates (excluding muscovite mica) and unrelated to K in K-feldspars. The studied soils are ranked in terms of their potential capacity to release K based on their mineralogical and geochemical characteristics.