Some rocks used in fertilizer production contain low water-soluble impurities, such as iron and aluminum phosphates. The industrialization processes of these rocks may generate simple superphosphate with an insufficient solubility degree to meet the requirements established by Brazilian fertilizer law. If applied to waterlogged soils, reduction reactions can increase the solubility of such fertilizers. The objective of this study was to evaluate phosphorus release from these fertilizers from flooded lowland soils of Rio Grande do Sul State. Six lowland samples were collected: two Albaqualf (Santa Maria and Pelotas); two Alfissol (Santa Maria and Cachoeirinha) and two Molisol (Santa Vitória do Palmar and Uruguaiana). Each soil received two phosphate sources - simple superphosphate and superphosphate with iron phosphate impurities - and a treatment without phosphorus addition. In the treatments with phosphorus, an amount equivalent to 100 mg kg-1 P was applied. Soils in 8 L plastic buckets were flooded and a 5cm water layer was maintained over the soil surface, in a complete randomized design with three replications. Soil solution was collected weekly and, immediately, the pH and redox potential (Eh) measured; afterwards, P and Fe were extracted in the lab. Results show that Eh decreased and pH, P and Fe content increased in the soil solution in all tested soils. The P increase in soil solution was independent of the phosphate source, indicating simple superphosphate with iron phosphate impurities for use in lowland soils where flooded rice is grown in Rio Grande do Sul, even though it does not comply with the solubility requirements established by the Brazilian fertilizer legislation.
Fertilizers; phosphorous; impurity; flooded rice
