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Clay-humus complexes, isolated from 5 soils (Entisol, Alfisol, Vertisol, two Mollisols), were extracted with 0.1 n citrate, EDTA, and oxalate at pH 7.0-10.0; amounts of Ca2+, Mg2+, Fe3+/2+, Al3+, as well as humic acid (HA) in the extract were determined. HA extracted increased with pH and varied with nature of ligand; largest amounts were extracted by EDTA at high pH. In the Entisol clay-humus extract, Ca2+ is dominant. In Alfisol sample, Ca2+ and Mg2+ have little role in clay-HA bonding; apart from monovalent cations, bonding is mainly through Fe3+/2+ and Al3+, which are well correlated to HA extracted. The extract from Vertisol sample contains little Fe3+/2+ or Al3+ and major bonding is through Ca2+. In Mollisol I and II, Ca2+, Mg2+, Fe3+/2+, and Al3+ are all involved in bonding and are highly correlated to extracted HA. Difference in mineralogy determines the difference in bonding strength between Alfisol and Vertisol complexes. DTA indicates dual bonding modes. A major fraction of HA (in clay-humus complexes) shows thermal destabilisation due to multiple attachments on the clay surface; a small fraction is also thermally stabilised by ionic bonding with Ca2+/Mg2+ and absence of ring strain in the complex. Only the Alfisol HA does not show thermal stabilisation in the complex.

Phosphate fixation in soils is a matter of concern in agriculture. Conventional application of phosphorus fertilizer suffers from low P use constraint, particularly in acidic soils. Rhizosphere centric slow release strategy bears tremendous prospects. In the present study, monocalcium phosphate (MCP) was impregnated in zeolite reinforced CMC-Na + -g-cl-Poly(Aam) hydrogel composites with aim to develop slow phosphate release device for soil application. X-ray diffraction, scanning electron microscopy, and Fourier transform-infrared spectroscopy confirmed the successful synthesis of slow release fertilizer formulations. Presence of zeolite in composite matrix during polymerization resulted in higher MCP loading. The "burst release" phenomena under neutral aqueous environment as compared to diffusion led slow release mechanism under acidic condition suggesting that phosphate release from developed composite matrix was pH responsive. The developed materials possess potential to serve as tool for improving phosphate use efficiency under resource stress agriculture.