Effect of Farmyard Manure on Zn Sorption in Soils from Semi-arid and Humid Regions

The effect of farmyard manure (FYM) on sorption of Zn in three soil samples (i.e., one alkaline soil sample from a semi-arid region and two acidic soil samples from a humid region), which widely differed in their pH, CEC, clay mineralogy and free iron oxide, was investigated. The FYM treated (i.e., 0, 2.5 and 5%), homoionic (Ca-saturated) soil samples were equilibrated with solutions having a wide range of Ca: Zn at 10 mequiv. VI total chloride concentration. The pH of equilibrium soil suspension (pH_eq) was recorded and the sorption data were analysed, using equilibrium constant for Zn. (K_Zn) and standard free energy of sorption reaction(ΔG₀). The pH. decreased with the increasing proportion of Zn in equilibrating solutions. The pH_eq of alkaline soli sample decreased and those of acidic soil samples increased with the addition of FYM. At comparable FYM, ΔG₀ was maximum for highly weathered red soil from Beltola, followed by the rice growing acidic soil from Kuptipara and then alkaline soil from Hisar. The AGo increased from -7.66 to -4.48 kJ mol¹ in alkaline soil and decreased from 1.36 to -1.97 kJ mol⁻¹ and 5.08 to 2.04 kJ mol⁻¹ in acidic soils from Kuptipara and Beltola on FYM treatment. The negative values of ΔG₀ suggested thermodynamic preference for Zn over Ca in alkaline soil from Hisar and FYM treated soil from Kuptipara.