Phosphorus Availability Indices in Relation to Phosphorus Fractions in Some Himalayan Acid Soils of North-west India

In a study on the indices of phosphorus availability in some Himalayan acid soils of NW India, it was found that Olsen's extractant removed most of its P from Al-P fraction (r=0.811**) followed by Ca-P (r =0.474*). The inclusion of other forms e.g., saloid P, Fe-P, reductant soluble and organic P did not alter this situation to any great extent. Multiple regression analysis showed that about 65.7% variation in Olsen P was accounted for by the variation in Al-P fraction alone. Bray's procedure derived its P mainly from Al-P followed by Fe-P and Ca-P almost in equal proportion and the extent of variability explained by these fractions was up to 50.5%. The most important source of P for Bray P₂ extractant was Ca-P and about 36.9% variation could be attributed to that in this fraction. Inclusion of other variables could hardly increase the predictability to a significant level. North Carolina, Williams and Truog's extractants removed P mostly from Ca-P and organic P fraction and the maximum variation in P removal due to the changes in P fractions was 79.9, 79.6 and 45.4%, respectively. Based on contribution of P fractions to its availability indices, the methods of Olsen et al., North Carolina and Williams et al., were found promising.