Mineralogical differences in a temperate cultivated soil arising from different agronomic processes and plant K-uptake

Potassium (K) is an essential plant nutrient mainly present in the crystal structure of K-bearing soil minerals (K-feldspars, micas). To assess the evolution of (clay) mineralogy due to K+ release from these minerals in continuously cultivated soils, samples were collected from long-term (1904-2014) field experiments submitted to contrasting crop rotations and amendments, with and without fertilisation. Soil samples were size-fractionated and mineralogy of the silt fraction and of clay subfractions was determined quantitatively by modelling X-ray diffraction (XRD) patterns. Clay subfractions were also analysed for their cation exchange capacity (CEC). Mineralogical data indicate the stability of clay mineralogy and the increased abundance of the finest clay subfraction at the expense of coarser ones, regardless of agronomic practices or amount of K removed by plants. This increase is accompanied by an increase in the bulk clay CEC owing to the major contribution of the finest clay subfraction to this parameter. Mineral dissolution, rather than alteration of mineral phases from coarse clay subfractions, most likely supplements K for plant nutrition in this soil. Dissolution is favoured over cation exchange owing to the dioctahedral character of micas and to their fine-grained granulometry. The observed longterm mineralogical resilience also indicates that part of plant nutrition is obtained from subsoil in a non K-limited context, whereas K supply through K-feldspar dissolution appears marginal.