Phosphate fractions in calcareous Vertisols and Inceptisols of Spain

Calcareous Vertisols and associated Inceptisols are abundant around the River Guadalquivir Valley, southern Spain. They support a profitable rainfed agriculture; little is known, however, about their phosphate status and fertility. For this reason the phosphate fractions in the Ap horizons of 24 of such soils were studied. The calcium carbonate equivalent of the samples ranged between 46 and 628 g kg-1 and the citrate-dithionite-bicarbonate (CBD)-extractable Fe (Fed) between 1.1 and 11.6 g kg-1 soil. Total P ranged from 331 to 1051 mg P kg-1 soil; organic P represented usually less than 10% of total P. Of the inorganic P, the HCl-P (Ca phosphates) fraction was the most abundant (39-77% of total P). The fraction extracted specifically by NaOH (nonoccluded Fe- and Al-bound P) was present in very small amounts. The citrate-bicarbonate (CB)-extractable P (P sorbed on active surfaces) ranged from 13.1 to 66.1 mg P kg-1 soil and had values similar to the resin-extractable (labile) P. The CDB-P (P occluded in the Fe oxides) was correlated with Fed (r = 0.92) and ranged from 7.8 to 52.0 mg P kg-1 soil. The changes in labile P associated with cropping with wheat (Triticum aestivum L.) or incubation with added P could be accounted for by changes in the CB-P fraction. The CDB-P and HCl-P fractions also changed, albeit less markedly; absolute changes were similar for both fractions but relative changes were much higher for the CDB-P. This suggests that Fe oxides (mainly as goethite) are very active in releasing and fixing P, and that occluded P is relatively mobile. The values of the Olsen and Colwell tests were correlated with labile P. The fraction of labile P extracted by these tests was negatively correlated with Fed. Consequently labile P can be predicted, in practice, from P tests and Fed values.