Non-exchangeable K sustains plant K uptake in alkaline Mediterranean soils

Abstract Aims Potassium (K) fertilization recommendations commonly rely on exchangeable-K soil tests, assuming that this fraction represents the soil’s capacity to supply plant-available K. However, limited crop response to K fertilization at insufficient exchangeable-K concentrations suggests contributions from additional soil K reservoirs. This study aims to quantify the contribution of non-exchangeable K pools to plant uptake in Mediterranean alkaline soils with differing mineralogical composition, under intensive agricultural conditions. Methods A greenhouse experiment simulating intensive agricultural management was conducted using three Mediterranean alkaline soils differing in texture, mineralogy, and total K concentration. Alfalfa ( Medicago sativa L.) was grown for ~ 13 months, and harvested 16 times. Biomass production, K uptake, and temporal changes in exchangeable-K concentration were monitored. A mass-balance approach quantified K removal, and exchangeable-phase K/Rb ratios were used to trace K sources. Results Plant K uptake consistently exceeded predictions based on initial exchangeable-K, resulting in a negative K mass balance in all soils. Total uptake was 2.0–3.9 times greater than the estimated supply from exchangeable-K and natural inputs, with 50–74% derived from non-exchangeable pools. Yield response to K fertilization varied among soils, and was associated with mineralogical composition; clay-rich soils showed minimal response. Exchangeable-phase K/Rb ratios indicate K release from clay minerals. Conclusions These results indicate that non-exchangeable K pools can substantially contribute to plant K uptake in alkaline soils under intensive agriculture practices, with implications for K fertilization management in Mediterranean agroecosystems.