INFLUENCE OF CONTINUOUS CROPPING AND FERTILIZATION ON ADSORPTION AND DESORPTION OF SOIL-PHOSPHORUS

Surface soil samples (0-15 cm) from six differentially fertilized plot s (N0P0K0, N120P0K0, N120P17.5K0, N120P35K0, N120P17.5K33.2 and N120P3 5K33.2) of a long-term experiment on maize-wheat annual sequence at Lu dhiana (India) were collected after 11 years of continuous cropping an d fertilization, to study soil phosphorus (P) adsorption-desorption. T hese soils differed widely with respect to their P adsorption and deso rption properties. Phosphate adsorption increased with increasing leve ls of added P in all soils. The extent of P adsorption was comparative ly lower in the plots receiving P. Phosphorus adsorption data was foun d to fit best both to Langmuir and Freundlich isotherms for each of th e six soil samples. Soil P adsorption maxima obtained from Langmuir is otherm varied from 123 to 498 mu g g(-1) soil in the six differentiall y fertilized plots. The bonding energy values obtained from Langmuir i sotherm plot were the lowest in control and N120P0K0 treatments wherea s these values tended to increase with P addition. Freundlich constant s 'a' and 'n' (extent and rate of adsorption) calculated from the regr ession lines also showed similar trend for the six soil samples. The p lot 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, D-m) and desorption rate constant (K-d) from t his relationship indicated higher D-m values in soil samples collected from check (N0P0K0) and N fertilized (N120P0K0) plots. This value ten ded to decrease in plots which received P during cropping. The K-d val ues were more in soil samples which were fertilized with P during crop ing and lower in check and N120P0K0 treated plots.