A study examined clay-humus complexes (COMF) in tropical soils (Inceptisol, Alfisol, Vertisol, Mollisol). High temperatures and leaching in these conditions challenge soil organic matter. Stabilization involves physical, chemical, and microbial mechanisms. Sustainable practices like no-till farming and organic matter additions can help. The study aimed to identify bridging cations (Ca2+, Mg2+, Fe3+/2+, Al3+) in COMF. The treatments comprised of control (no fertilization), 100% NPK (100% of recommended N, P and K through fertilizer), 50% NPK + 50% of recommended N supplied through either farm yard manure (FYM) or cereal residue (CR) or green manure (GM). Bridging cations like Ca2+, Mg2+, Fe3+/2+, Al3+ were extracted with 0.1 N citrate, 0.1 EDTA, and 0.1 ammonium oxalate at pH 7. Ca2+ is dominant in the Inceptisol clay–humus extract. In Alfisol sample apart from monovalent cations, bonding is mainly through Al3+. The extract from Vertisol sample contains little Fe3+/2+ or Al3+ and major bonding is through Ca2+ in Mollisol Ca2+, Mg2+ and Fe3+/2+ are all involved in bonding. Irrespective of all the soil types EDTA showed better extraction of Ca2+ and Mg2+ compared to citrate and ammonium oxalate whereas ammonium oxalate showed better extraction of Al3+. In Inceptisol and Alfisol Al3+ content was the highest under NPK + CR and lowest under control. Application of NPK + GM or NPK + FYM or NPK+CR showed better bridging cations over control in COMF.

Clay-humus complex, Cation Bridging, Organic Manure, FYM, EDTA, Ammonium oxalate, Citric acid

Long-term application of inorganic and organic sources like FYM, crop residue and green manure significantly increased the bridging cations in clay humus which might lead to higher carbon stability in these treatments. Among the cations Fe and Ca concentrations was higher and they might play key role in clay humus bridging. Knowledge of the binding bridging cations and clay mineral contributes to our understanding of the sequestration process of SOM and clay humus stability.

Binder Singh, Nayan Ahmed, Abhishek Das, Prasenjit Ray, Kavitha P. Jadhav and Mohan Kumar K.T. (2023). Impact of Long-Term Fertilization and Manuring on Cation Bridging in Clay-Humus Complex under Four Soil Orders. Biological Forum – An International Journal, 15(11): 30-35.