In order to evaluate the direct and residual effects of ordinary granulated superphosphate as compared to magnesium thermophosphate (Yoorin), supplied at four rates (0,50,100 and 200 ppm), seven successive croppings were carried out in the glasshouse. Available phosphorus was assessed after the third, fourth and seventh harvest by using Mehlich 1, Bray 1, anión exchange resin and isotopic dilution (E value). The highest correlation coefficients between available P and either total dry matter or total P content was found to occur with data provided by Mehlich 1 and resin. Phosphorus extracted by Bray 1 as well as that estimated by the E value showed lower "r" coefficients.
phosphate fertilizers; extracting agents; available phosphorus
