Phosphorus fractionation under crops, trees and grasses

Soil samples were taken from the long-term fallow, pasture (grass), crop and tree field trials at Central Arid Zone Research Institute farm at Jodhpur in order to investigate soil P status and the distribution of various forms of P as well as the effect of rhizosphere on different P fractions. The soil P distribution was fractioned as inorganic (Pi) and organic (Po) by standard procedures and then step-wise fractionation deionised water (WSP) using sodium bicarbonate (bicarb-P), sodium hydroxide (hyd-P), and HCI (HCl-P). Finally, the soil was digested to obtain residual P (resid-P). A breakdown of the WSP, bicarb-P, hyd-P and HCl-P into inorganic P (Pi) and organic (Po) was indicative of considerable transformations within these P compartments compared with the initial values. The available P status was much less (0.3–0.9% of total P) in arid soil. Maximum P was present in NaOH extractable fractions (46.3–59.6% of total P) followed by HCI extractable fractions (24.4–41.2% of total P). Rhizosphere soils had significantly higher water soluble P and NaHCO₃ extractable P than non-rhizosphere soil. Pasture fields showed maximum P-build up as compared to crops, and trees. In general, considerable reduction in water soluble Po, NaHCO₃-Po and HCI extractable-Po was observed due to differences in vegetation.