Changes in Clay Mineral Composition and Soil Potassium Pools Under 50 Years of Soybean–Wheat Cropping in an Alfisol

Abstract

Background: Potassium fertilizers are imported in India, and hence, potassium fertilization is a costly input. Resource poor cultivators variably use potassium fertilization for crop production which disturbs potassium dynamics in soil impacting soil health. Aim: A long-term negative potassium balance in the cropping system can result in the release of potassium from the non-exchangeable pool, ultimately resulting in changes in clay mineralogy, amounting to chemical degradation of soil. Method: Soils from a long-term soybean–wheat cropping system receiving variable amounts of potassium fertilizers, which was in the 50th cropping cycle, were used to investigate the potassium pools and clay mineralogy. Result: There was considerable reduction in yield both in control (78.7%) and 100% NP treatments (15.95%). The negative K balance followed the order 100% NP > 100% NPK > 50% NPK > 150% NPK. To meet the potassium requirement and negative potassium balances, potassium was being released from interlayers resulting in the annual loss of non-exchangeable K up to 2.42, 1.06, 0.74, and 1.34 kg ha−1 year−1 under 100% NP, Control, 50% NPK, and 100% NPK, respectively. The 100% NP treatment showed the smallest intensity of illite followed by 100% NPK, 50% NPK, Control, and 150% NPK. The illite intensity was reduced by 41.6%, 11.7%, 8.49%, and 1.6% in the 100% NP, 100% NPK, 50% NPK, and Control treatments, respectively, compared to 150% NPK. Conclusion: The correlation studies revealed a positive association between negative balance, non-exchangeable K, and illite, supporting the hypothesis that a reduction in non-exchangeable K under negative K balances results in alterations in the clay mineralogical composition. © 2025 Wiley-VCH GmbH.