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The forms, amounts and distribution of phosphorus (P) were determined in 28 calcare-ous soils from South-Western Australia to evaluate soil P status and their contribution to soil P tests. The soils under study exhibited wide ranges in content of total P (TP), organic P (org.-P), inorganic P (Pi) fractions and of P extracted by soil tests for plant available P. The values of TP ranged from 33 to 392 mg/kg with a mean of 176 mg P/kg soil. Org.-P constituted a considerable portion of TP (mean 35%). The mean ratio of organic C/ org.-P was 287. In general, the distribution of Pi fraction was Ca2-P, 15; Ca8-P, 27; Al-P, 26; Fe-P, 14; occluded-P (O-P), 20, Ca10-P, 16 mg P/kg soil, accounting respectively for 13%, 23%, 22%, 12%, 17%, and 13% of total Pi. The pattern of distribution of the Pi fractions, however, differed between virgin and cultivated soils. For virgin soils, the average relative abundance of Pi in the secondary Ca-P [(Ca2-P)+(Ca8-P)], nonoccluded Fe- and Al-P [(Al-P)+(Fe-P)] and acid extractable P (primary Ca phosphate (Ca10-P)) was 2.5:2:1 and in cultivated soils was 2:2.5:1, respectively. Statistical analysis indicates that the Ca2-P, Al-P, Fe-P and Ca10-P fractions made significant contributions to available P as estimated by the Olsen and Colwell methods. Stepwise regression showed that most of variation in Ol-sen-P (90%) and Colwell-P (82%) was accounted for by Ca2-P. Scanning electron micros-copy (SEM) showed that P was uniformly distributed in the soil matrix to the limit of the spatial resolution and sensitivity of this technique. There were no local concentrations of P or spatial associations of other elements with P.

Keywords: Inorganic P fractions, Available P, P in calcareous soils

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