Relative contribution of different sized soil separates to inorganic P fractions in a Typic Ustochrept of N-W India

Abstract

The contribution of different sized soil separates to various inorganic P fractions was investigated after 32 years of long-term fertilization under maize-wheat-cowpea fodder crop rotation. Phosphorus was applied as single super phosphate. A major portion (71.5%) of Olsen-P was extracted from sand size particles, and only 11.4% and 17.1% was present in silt and clay size separates of the surface soil layer, respectively. Unlike Olsen-P, Al-P and Fe-P in soil were mainly present in the clay fraction of the soil. Percent contribution of clay, sand and silt particles in the surface layer was 61.5, 27.4 and 11.1 to Al-P, and 66.4, 21.6 and 12.0 to Fe-P, respectively. However, Ca-P in surface soil layer was mainly (68%) present in sand size soil separates followed by silt (28.5%) and clay (3.5%) size soil separates. Percent distribution of P forms in different sized soil separates of the subsurface layer also followed the same pattern, but there was little decrease in the contribution of sand with a corresponding increase in the contribution of silt and clay size soil separates. X-ray diffractograms specified the precipitation of residual fertilizer P as octa-calcium phosphate, hydroxy apatite and variscite, after reacting with calcium and aluminum in the soil. The peaks recognizing the presence of these reaction products were observed only in sand and silt size soil separates. However, no peak was found in clay sized soil separates.