PHOSPHATE AVAILABILITY IN CALCAREOUS VERTISOLS AND INCEPTISOLS IN RELATION TO FERTILIZER TYPE AND SOIL PROPERTIES

The availability to plants of fertilizer phosphorus (P) applied to soi l, as measured by chemical extraction, is used to estimate P fertilize r needs. We studied the availability of P, applied as monocalcium phos phate (MCP) powder, ordinary superphosphate (OSP) granules and diammon ium phosphate (DAP) granules in 24 calcareous Vertisols and Inceptisol s of Andalusia, Spain, by using laboratory incubation techniques. The soils differed widely in their P adsorption- and Ca-phosphate precipit ation-related properties. For MCP, availability (defined as the propor tion of added P that is recovered by extraction with NaHCO3 or is isot opically exchangeable) decreased markedly with incubation time and inc reasing addition rate. The mean recoveries after 180 d of incubation a t field capacity at a rate of 246 mg P kg(-1) soil were 17% for Olsen P, 38% for Colwell P, and 16% for isotopically exchangeable P (IEP). I ncreasing the application rate to 2460 mg kg(-1) resulted in recoverie s of 6% for Olsen P, 25% for Colwell P, and 4% for IEP. While IEP-base d recovery was not significantly correlated to any soil property, that based on Olsen P (and, to a lesser extent, Colwell P) decreased sharp ly with increase in the ratio of clay (or Fe oxides) to total (or acti ve) calcium carbonate equivalent. Accordingly, Olsen P might overestim ate P availability in those soils relatively rich in carbonate and poo r in clay and Fe oxides. On the other hand, recovery of applied P from soils containing more clay and Fe oxides, by a sequential extraction (with H2O, two 0.5 M NaHCO3 treatments, 0.5 M HCl), was lower than 100 %, thereby suggesting phosphate occlusion by Fe oxides or clay. Availa bility of the fertilizers tested 90 d after application was found to d ecrease in the following order: MCP powder (rate, 246 mg kg(-1)) > DAP granules (rate, 547 mg kg(-1)) > MCP powder (rate, 738 mg kg(-1)) > O SP granules (rate, 308 mg kg(-1)). Differences between fertilizers ten ded to increase with increasing carbonate content in the soil. This ma y have been due to precipitation of Ca phosphates caused by the presen ce of Ca in the fertilizer and the high Ca- supplying capacity of the more calcareous soils.