Soil potassium pools across depths limit crop response in sandy loam soil

Abstract Quantifying potassium (K) distribution across soil layers is crucial for understanding the often‐weak relationship between crop response to K fertilization and soil test K (STK). We evaluated exchangeable (STK Exch ), nonexchangeable (STK NonExch ), and semi‐total K (K SemiTotal ) at depths 0–20 cm, 20–30 cm, 30–40 cm, 40–65 cm, and 65–90 cm, along with the partial K mass balance after 41 years of a long‐term K fertilization study on a Coastal Plain sandy loam soil in North Carolina, USA. In 2023, soil was sampled from three K fertilization regimes: lower (K0), like crop removal (K1), and greater than removal (K2), with long‐term cumulative applications of 93, 1969, and 3917 kg K ha −1 , respectively. The 41‐year partial K balance was negative for K0 (−1615 kg K ha −1 ) and K1 (−136 kg K ha −1 ) but positive for K2 (1836 kg K ha −1 ). Although K2 received 42 times more K fertilizer than K0, its cumulative removal was only 22% greater, and yield response occurred in only 40% of the crops. Overfertilization (K2) enriched all soil K forms and promoted downward movement to 65–90 cm, where STK Exch doubled compared with K0 (55 vs. 26 mg kg −1 ). The K SemiTotal at 20–90 cm was higher under K2 (2579 mg kg −1 ) than K0 (1941 mg kg −1 ). The K SemiTotal depletion at 20–40 cm under K0 indicates that feldspar‐derived K buffers crop demand. These findings highlight that K contributions from less‐available K pools across soil layers can diminish crop responses to fertilization and weaken STK‐yield relationships.