Effect of Araxá rockphosphate and simple superphosphate on some physiochemical properties and on the available residual P of an Oxic Paleudalf

Abstract

The experiment was developed using surface soil samples (TRE, Oxic Paleudalf) from the Experimental Field Station of ESALQ, located in Piracicaba, SP, Brazil. This soil had received during the period al three years 6 ton/ha of Araxá rockphosphate (34,6% P₂O₅) or 1,2 ton/ha of simple superphosphate (20% P₂O₅) while three crops of beans (Phaseolus vulgaris, L.) were grown. This greenhouse experiment had the objective of evaluating the effect al these fertilizer applications on some physico-chemical properties of the soil, and also study the residual P effect through the Olsen and "A" value methods. Results indicate that Araxá rockphosphate (48.4% CaO), although applied at high rate, did not affect soil reaction. On the other hand, both fertilizers increased the exchangeable Ca²⁺ level approximately 0,5 mE/100 g. In short term (less than three years), the response of Araxá rockphosphate corresponded to its 2% citric acid soluble P₂O₅, content (4%) rather than to its total P₂O₅ content. Through "A" values (using KH₂³²PO₄) it was shown that the residual effect of simple superphosphate (108.6 kg P₂O₅/ha) was similar to the Araxá rockphosphate (136.6kg P₂O₅/ha), but these were significantly greater than the natural P content of the soil (54.0 kg P₂O₅/ha). The evaluation of available P in the soil (with and without fertilizer) by Olsen's method, had an efficiency of 64 and 94% (respectively) as compared to the "A" value method.