Phosphorus sorption parameters after 35 years of fertilization to maize-wheat cropping system

Surface soil samples (0–15 cm) from 7 differentially fertilized plots (N₀P₀K⁰, N₁₀₀P₀K₀, N₂₀₀P₀K₀, N₁₀₀P₂₂K₀, N₂₀₀P₄₄K₀, N₁₀₀P₂₂K₄₁ and N₂₀₀P₄₄K₈₂) of a 35 years long-term experiment on maize-wheat sequence at PAU Research farm, Ludhiana were collected to study the effect of various fertilizer treatments on soil phosphorus (P) adsorption-desorption. These soils differed widely with respect to their P adsorption and desorption properties. Phosphate adsorption increased with increasing levels of added Pin all the soils. The extent of P adsorption was comparatively lower in plots receiving higher amounts of P. Phosphorus adsorption data was found to fit best to Langmuir (r² ≥ 0.94) and Frundlich isotherms (r² ≥ 0.92)for each on the seven soil samples. Soil P adsorption maxima obtained from Langmuir isotherm varied from 196 to 313μg g⁻¹ soil. The bonding energy values obtained from Langmuir isotherm plot were the lowest in control and N₂₀₀P₀K₀ treatments whereas these values tended to increase with P addition. Freundlich constants ‘a’ and’ ‘n’ (extent and rate of adsorption) calculated from the regression lines also showed similar trends. The plot of desorbed P/adsorbed P was linearly correlated for each of the seven soil samples. Computation of desorable P capacity (or desorption maxima, D_m) and desorption rate constant (K_d) from this relationship indicated higher D_m values in soil samples collected from control (N₀P₀K₀) and alone N fertilized (N₁₀₀P₀K₀ and N₂₀₀P₀K₀) plots as compared to treated with P fertilizers. The K_d values were higher in P fertilized plots than control (N₀P₀K₀) and N₂₀₀P₀K₀ treated plots.