Forms of phosphorus in virgin and fertilised calcareous soils of Western Australia

Abstract

Total phosphorus (P), inorganic P (P_i), organic P, and several P_i fractions were determined for 8 fertilised calcareous soils under agriculture and their virgin analogues under natural bush to ascertain changes due to agricultural development. The relationships between soil properties and forms of P were also determined. In general, agricultural development of soils resulted in increases in total P (average 105% increase), P_i (154%), organic P (49%), Olsen P (200%), Colwell P (100%), and all P_i fractions compared with their virgin analogues. For the virgin soils, the abundance of the P_i fractions was in the order: Al-P>O-P (occluded P)>Fe-P>Ca₁₀-P = Ca₂-P>Ca₈-P, which changed to Al-P>Ca₈-P>Ca₂-P>Ca₁₀-P>Fe-P>O-P for fertilised soils. The average contribution of each fraction to the increase in total P_i was Al-P (29%), Ca₈-P (26%), Ca₂-P (18%), Fe-P (13%), Ca₁₀-P (13%), and O-P (4%). The change in Ca₈-P was closely correlated with the content of the active fraction of calcite in the soil (ACCE). The increase in Fe-P associated with agriculture was highly correlated with citrate-dithionite-bicarbonate (CDB) extractable Fe (Fe_d) and acid-ammonium oxalate extractable Fe (Fe_o). The increase in Al-P was correlated with the ratio of acid-ammonium oxalate extractable Al (Al_o) to Fe_o. Both Olsen and Colwell NaHCO₃-extractable P were highly correlated with Ca₂-, Al-, Fe-, and Ca₁₀-P, and total P values. Multiple regression analysis indicated that Ca₂-P and Ca₁₀-P were major contributors to available P as determined by the Olsen and Colwell soil tests.