Fertilizer Research 40: 109-119, 1995. 109 @ 1995 KluwerAcademicPublishers. PrintedintheNetherlands. Phosphate availability in calcareous Vertisols and Inceptisols in relation to fertilizer type and soil properties B. Castro & J. Torrent Departamento de Ciencias y Recursos Agrfcolas y Forestales, Universidad de C6rdoba, Apdo. 3048, 14080 C6rdoba, Spain Received5 July 1994;accepted in revisedform28 November1994 Key words: diammonium phosphate, fertilizer rate, monocalcium phosphate, ordinary superphosphate, P availability Abstract The availability to plants of fertilizer phosphorus (P) applied to soil, as measured by chemical extraction, is used to estimate P fertilizer needs. We studied the availability of R applied as monocalcium phosphate (MCP) powder, ordinary superphosphate (OSP) granules and diammonium phosphate (DAP) granules in 24 calcareous Vertisols and Inceptisols of Andalusia, Spain, by using laboratory incubation techniques. The soils differed widely in their P adsorption- and Ca-phosphate precipitation-related properties. For MCR availability (defined as the proportion of added P that is recovered by extraction with NaHCO3 or is isotopically exchangeable) decreased markedly with incubation time and increasing addition rate. The mean recoveries after 180 d of incubation at field capacity at a rate of 246 mg P kg -1 soil were 17% for Olsen R 38% for Colwell R and 16% for isotopically exchangeable P (IEP). Increasing the application rate to 2460 mg kg-1 resulted in recoveries of 6% for Olsen R 25% for Colwell P, and 4% for IER While IEP-based recovery was not significantly correlated to any soil property, that based on Olsen P (and, to a lesser extent, Colwell P) decreased sharply with increase in the ratio of clay (or Fe oxides) to total (or active) calcium carbonate equivalent. Accordingly, Olsen P might overestimate P availability in those soils relatively rich in carbonate and poor 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 H20, two 0.5 M NaHCO3 treatments, 0.5 M HC1), was lower than 100%, thereby suggesting phosphate occlusion by Fe oxides or clay. Availability of the fertilizers tested 90 d after application was found to decrease 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) > OSP granules (rate, 308 mg kg-1). Differences between fertilizers tended to increase with increasing carbonate content in the soil. This may have been due to precipitation of Ca phosphates caused by the presence of Ca in the fertilizer and the high Ca- supplying capacity of the more calcareous soils. Introduction Recommended rates of fertilizer phosphorus (P) are frequently based on the values of a soil P test. If a test has been calibrated for a certain soil and crop soils test- ing below the critical level (viz. the minimum soil test level that meets crop needs) must be fertilized. Hence, it is useful to know the proportion of fertilizer P added to soil that is converted into available P, as measured by the soil P test. It is then possible to estimate the amount of fertilizer P needed to reach (or exceed) the required critical level. "Campifia" soils, which are calcareous Vertisols (Haplo- and Calcixererts) and associated Inceptisols (Xerochrepts) developed on Tertiary marls, are the most fertile soils for rainfed agriculture in the Riv- er Guadalquivir Valley, Andalusia, Southern Spain. Similar soils are encountered in various areas of the Mediterranean region. Campifia soils differ widely in their clay and calcium carbonate contents, and have been characterized for P fractions (Solfs and Tor-