Quantifying mineral sources of potassium in agricultural soils

Soil potassium (K) reserves are crucial for crop quality and yield in low input agricultural systems. Soils contain varying amounts of K-bearing minerals and differ in their inherent K supply capacity. This thesis presents mineralogical methods for characterising and quantifying different K-bearing phases in soil and compares the results with K determined by common soil K extraction methods using 2 M HCl (KHCl) or aqua regia (KAqReg). The aim was to improve evaluations of potential long-term K supply of soils with different parent materials, particularly in grass/clover leys. Mineralogical budgeting based on X-ray powder diffraction was used to determine K distribution in K-bearing minerals in 20 Swedish and Scottish soils. KHCl and KAqReg were then tested against the concentrations of different mineral K pools and HCl effects on mineralogy were assessed through differential X-ray diffraction. Long-term Swedish cropping systems and soil fertility experiments were used to assess field K balances and long-term trends in exchangeable K (Kex) and KHCl and to estimate soil K delivery capacity. The K distribution between K-feldspar and different phyllosilicates varied significantly among soils, with Scottish soils showing greater variation in total K concentrations and K speciation than Swedish soils. HCl and aqua regia solubilised 1-17% and 4-60% of total K, respectively, depending on soil type and mineralogy, most derived from Fe-bearing di- and trioctahedral phyllosilicates. Conventional and organic cropping systems both demonstrated negative field K balances averaging 20-70 kg ha-1 yr-1 over 18 years, indicating a reliance on soil K delivery. This resulted in significantly decreased Kex and KHCl in two soils. Potassium weathering rates were 35-65 kg ha-1 yr-1 in silty clays and sandy loams, and 8 kg ha-1 yr-1 in loamy sands in insufficiently or non-K fertilised experimental plots. To compensate for net K output at field scale, the KHCl soil pool would have to be totally replenished within 30-300 years, depending on site. Although KHCl is a dynamic pool that is not exhausted until the least soluble K-bearing mineral has dissolved, replenishment rate kinetics ultimately limit K delivery from soil reserves. This demonstrates the significance of considering the size and properties of soil nutrient pools when using element balances to assess cropping system sustainability