Differences in grain yield were not observed in experiments with paddy rice when comparing soluble phosphates and rock phosphate as phosphorus sources. However, chemical changes in flooded soils such as increases in pH and phosphorus and calcium levels in the soil solution probably decrease the dissolution of rock phosphates in flooded soils. The current work was therefore carried out with the objective of studying the solubilization of phosphate rocks in flooded soils. Two experiments, conducted in a random block design with four replications, were set up in two soil types (Albaqualf and Dystrudepts). The tested factors were phosphorus sources: (a) control treatment without P; (b) triple superphosphate; (c) "Patos de Minas" rock phosphate (Patos); (d) "Arad" rock phosphate (Arad). The pH values and the Fe, Mn, Ca, Mg and P concentrations were evaluated in the soil solution during 88 days of flooding. The pH values and the Fe2+ and Mn2+ contents in the Albaqualf and Dystrudepts solution were not affected by triple superphosphate nor by Arad and Patos rock phosphate. The phosphorus content in the soil solution was higher in the treatments with triple superphosphate than in those with phosphate rock, indicating that triple superphosphate is more soluble than rock phosphates in flooded soils. The comparison of rock phosphates reveals that Arad is more soluble than Patos phosphate only in the Albaqualf.
phosphorus; phosphate fertilizers; soil solution; irrigated rice
