Influence of continuous cropping and fertilization on adsorption and desorption of soil phosphorus

Surface soil samples (0–15 cm) from six differentially fertilized plots (N0P0K0, N120P0K0, N120P17.5K0, N120P35K0, N120P17.5K33.2 and N120P35K33.2) of a long-term experiment on maize-wheat annual sequence at Ludhiana (India) were collected after 11 years of continuous cropping and fertilization, to study 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 P in all soils. The extent of P adsorption was comparatively lower in the plots receiving P. Phosphorus adsorption data was found to fit best both to Langmuir and Freundlich isotherms for each of the six soil samples. Soil P adsorption maxima obtained from Langmuir isotherm varied from 123 to 498µg g−1 soil in the six differentially fertilized plots. The bonding energy values obtained from Langmuir isotherm plot were the lowest in control and N120P0K0 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 trend for the six soil samples. The plot of desorbed P versus desorbed P/adsorbed P was linearly correlated for each of the six soil samples. Computation of desorable P capacity (or desorption maxima, Dm) and desorption rate constant (Kd) from this relationship indicated higher Dm values in soil samples collected from check (N0P0K0) and N fertilized (N120P0K0) plots. This value tended to decrease in plots which received P during cropping. The Kd values were more in soil samples which were fertilized with P during croping and lower in check and N120P0K0 treated plots.