Soil phosphorus fractions in a typic haplustept as influenced by organic sources after soyabean (Glycine max L.) – wheat (Triticum aestivum) cropping sequence in semi arid condition*

Rock phosphate enriched biogas slurry, pressmud and farm yard manure alone and in combination with DAP in different ratios (1:3, 1:1 and 3:1) were evaluated in field with soybean-wheat cropping sequence at the IARI, New Delhi on a sandy loam soil. Soil samples (0–15 cm) after the harvest of wheat were taken to examine the residual effect of various treatments on available, inorganic and organic P pools in soil. Application of manure showed significant increase in available P over control. Among the fractions of phosphorus Fe-P, Ca-P and organic P fractions in soil remained almost unaltered while Saloid-P and Al-P fractions were increased in comparison to their initial concentration noted before sourcing the soybean. The extent of increase was more at higher rates of P application to soybean. Application of rock phosphate enriched biogas slurry in the ratio of 1:1 and 3:1 of DAP and manure on P₂O₅ basis gave the highest increase in the P fractions of soil. Therefore, a change in different inorganic and organic P fraction in soil can be influenced by the manipulation of sources and levels of phosphorus.