Effect of liming and phosphate application on sudangrass growth and phosphorus availability in two temperate acid soils

A pot experiment was carried out in the greenhouse with two loamy sand Dyst ric Cambisols derived from schist to investigate the effect of liming and p hosphorus (P) application on plant growth and P availability and its assess ment by four soil test methods: 0.01M calcium chloride (CaCl2), cation anio n exchange membrane (CAEM), Egner-Riehm, and Olsen procedures. Soils were f irst incubated for two weeks with lime at four levels, depending on their c ontent of exchangeable aluminum (Al). phosphorus was added at two rates (75 and 150 mg P kg(-1)) and the incubation proceeded for an additional two-we ek period. Sudangrass (Sorghum sudanenses cv. Tama) was then planted and ha rvested four weeks later. During incubation and plant growth, soils were ma intained at 70% of field moisture capacity. Although pH value and soil extr actable P in original soils were similar, the results showed a significant difference on the effect of liming and P application. Acidity was the major limitation for DM yield in the soil with the highest amount of exchangeabl e Al, while P availability was the main constraint in the other soil. Limin g above pH (0.01M CaCl2) 5.3-5.5 did not increase DM yield in either soil a nd showed a negative effect on one soil (9.7 to 6.9 and 10.2 to 7.8 g pot(- 1)). Phosphorus content and uptake by sudangrass increased with liming, rev ealing a positive effect of lime on the availability of P to plants. Added P showed a lower efficiency in the soil with highest amounts of Al compound s. Soil tests performed after the execution of the pot experiment showed va riable tendencies to predict P availability, according to the nature of the procedures and soils. Soluble P in 0.01M CaCl2 increased with the rise of soil pH. Extractable CAEM-P and Egner-Riehm-P also increased with liming, b ut reflected:the soil depletion caused by plant uptake. Extractable Olsen-P presented the most inconclusive results, suggesting the limitation of this method for acid soils which have been limed.