Soil-Test Phosphorous in Fertilized Alkaline Soils as Influenced by Decrease of pH.

Because soil phosphorous (P) management has been required for both environmental and agronomic points of view, interest in the supply of soil P for plant growth with large amounts of residual P is increasingly stimulated. Soil pH is one of the main factors to determine the available P in previously fertilized soils. Objective of this study was to determine effects of decreasing pH in fertilized alkaline soils on extractable P levels measured by common soil tests. Four soils were amended with five levels of P (0, 50, 250, 500 and 1000 mg kg^-1). Mean soil clay content, organic carbon and pH ranged from 107.5 to 271.9 mg kg^-1, 4.7 to 32.1 mg kg^-1 and 7.4 to 8.0, respectively. Each P-amended soil sample was incubated for 180 days and then titrated to two target ranges of pH (5-6 and 6-7) using HCl for a stabilization period of 14 days. Three availability indices, Bray 1-P, Olsen-P and Mehlich 3-P, were determined in the P-amended soil samples for each target pH-range. Results indicated that three P availability indices valued increasingly with decrease of soil pH. This phenomenon was obviously enhanced by the presence of lime in alkaline soils. Lime could be neutralized by HCl added to decrease pH and the reaction of P with calcium was thus prevented from reduction of P availability. However, as iron and aluminum oxides were relative high in alkaline soils, decrease of pH might result in the reduced solubility of P due to the formation of iron and aluminum phosphate minerals. Moreover, at neutral to acid pH, organic matter maybe serves as a sink for surface adsorbed P. The alkaline soil with a large amount of organic carbon titrated to pH ranges of 5-6 and 6-7 was thus unfavorable to P availability.