Plant Use of Nonexchangeable Potassium in Coarse and Fine Fractions of Granitic Soils in a Temperate Region

Nonexchangeable potassium (neK) derived from micaceous minerals is a key K source in less‐weathered granitic soils across the world, including Europe and eastern Asia. Plant use of neK may vary based on soil particle size, specifically coarse (> 20 μm) and fine (< 20 μm) fractions. This study quantified neK in both fractions of soils from different parent materials and evaluated its contribution to crop K uptake under varying K fertilization levels. In total, 55 soil samples were collected from vegetable fields with diverse soil groups or parent materials. Soils derived from granite and sedimentary rock, with high‐ and low‐neK content, respectively, were used in pot experiments with sweetcorn under four K fertilizer rates of 10%, 40%, 70%, and 100%. Soils were fractionated into coarse (> 20 μm) and fine (< 20 μm) fractions, and neK was evaluated using hot HNO 3 extraction. Compared with neK in the fine fraction, neK in the coarse fraction exhibited a stronger linear relationship with total neK (< 2 mm), indicating its greater influence on overall neK content. Granite‐derived soils showed relatively high neK levels, proportional to micaceous mineral content in the coarse fraction. In micaceous‐rich granitic soils, neK in the coarse fraction of 1066 mg kg −1 decreased by 8%, 9%, 11%, and 15% after the pot experiment under K fertilizer rates of 10%, 40%, 70%, and 100% (two‐way analysis of variance, p < 0.05), whereas fine‐fraction neK of 404 mg kg −1 did not decrease significantly. The reduction (⊿neK coarse ) correlated positively with K balance in granitic soils. In contrast, sedimentary rock‐derived soil had smaller neK levels (coarse‐ and fine‐fraction neK; 31 and 200 mg kg −1 , respectively), with no decline in either fraction after the same pot experiment. These findings suggest that coarse micaceous minerals serve as a geologically controlled neK source in soils. In high‐neK soils, the coarse fraction contained double the neK content of the fine fraction, leading to greater root accessibility and enhanced K release from neK through root uptake and exudates. In less‐weathered granitic soils, promotion of root elongation by basal K fertilization improves accessibility to the coarse fraction and may replace K topdressing by utilizing neK in the coarse fraction.