Thirteen Year Long Term Fertilization Effect on Soil Phosphorus Fractions of an Acid Inceptisol and Their Contribution to Phosphorus Uptake by a Double Crop of Rice under Sub-Tropical Climate

A 13 year old long term fertilizer experiment conducted on an acid soil with rice-rice system was used to study the impact of continuous application of fertilizer nutrients and amendments on changes in soil phosphorus fractions and their relative contribution to P nutrition. There was accumulation of total P on surface soil in all P fertilized treatments and depletion in P minus treatments. Depending on the P balance the treatments differed significantly with respect to P accumulation. 150%NPK treatment had highest P accumulation (841.92kgha-1) in surface soil. The high yielding treatments viz. 100%NPK+FYM and 100% NPK +FYM+Lime had relatively less P built up than 100%NPK. The total P comprised of inorganic P, 64.43-89.60% and organic P, 10.40– 35.57%. The inorganic fraction in terms of their abundance were in the order: Sal-P (42.13 kgha-1) <Occl-P (52.16 kgha-1) < Al-P (52.98 kgha-1) < Ca-P (55.44 kgha-1) < Red–P (134.22 kgha-1) < Fe-P (218.70 kgha-1). Olsen P which was significantly influenced by the fertilizer treatments had a very strong positive correlation (p<0.01) with Sal-P(r= 0.469***), Occl-P(r=0.639***) and Ca-P (r=0.739***). Among the inorganic P fractions, Ca-P contributed highest of 42.57% to P uptake, followed by Occl-P (28.59%), Sal-P (18.56%) and Al-P (7.63%). The fractions in order of their relative importance for P uptake under submerged rice-rice situation are Ca-P > Occl-P > Sal-P > Al-P > Red-P > Fe-P. The study indicated that 100%NPK+FYM and 100% NPK+FYM+Lime are the best treatments for maintaining more active fractions of P required for P nutrition of rice-rice system under the tropical acid situation.