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

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.