Effect of Farmyard Manure on Ca-Zn and Ca-Cd Exchange Equilibria in Soil

Ca-Zn and Ca-Cd exchange equilibria were studied in the surface (0–20 cm, S) and associated sub-surface (40–60 cm, SS) soil samples of a sandy loam soil collected from two adjoining fields-one receiving farmyard manure (FYM) @ 45 t ha⁻¹ a⁻¹ (FI) and another not receiving FYM (FO). The per cent adsorption of added Zn (% Ads. Zn) and selectivity coefficient (K_Zn) in SFI and SSFI increased, remained constant and then decreased when the equivalent fractions of Zn (C_Zn) in the equilibrating solution increased from 0.0005 to 0.04, 0.04 to 0.1 and 0.1 and 1.0, respectively. In SFO and SSFO samples, % Ads. Zn and K_Zn increased gradually when C_Zn increased from 0.0005 to 0.01 and then decreased with further increase in C_Zn from 0.01 to 1.0.

The % Ads. Cd and K_Cd in SFI and SSFI samples increased when CCd increased from 0.0005 to 0.008 and then decreased with further increase in CCd. In SFO and SSFO samples, % Ads. Cd and K_Cd increased when C_Cd increased from 0.0005 to 0.004 and from 0.005 to 0.008, respectively. The standard free energy change of Ca-Zn exchange showed higher thermodynamic preference of Zn over Ca in the sub-surface over the surface soil samples. For the Ca-Cd exchange reaction, soil samples from no FYM treatment showed similar trend but in FYM treated samples the trend reversed.