Phosphorus availability in soils with flooded rice in Rio Grande do Sul, Brazil

In the paddy rice soils in the of Rio Grande do Sul State, Brazil, most inorganic P is bound to Fe oxides. It is expected that an extractant that has the capacity to extract P adsorbed to these oxides would be more efficient to evaluate the availabil-ity to rice plants. A study was carried out in a greenhouse, with five soils samples from the State of Rio Grande do Sul, to evaluate P availability to rice, extracted by the following methods: Mehlich-1, Mehlich-3, anion exchange resin, Olsen and P-oxalate. The soils were first incubated with three doses of triple superphosphate and then planted with rice, in under anaerobic conditions, during 38 days. The determination coefficient between P uptake by rice plants and soil-extracted P was: 0.83* for Mehlich-1, 0.81* for Mehlich-3, 0.88* for anion exchange resin, 0.85* for Olsen and 0.59* for P-oxalate. The efficiency of the tested methods to evaluate the availability of P to rice was improved by the separation of soils in two groups, according to the origin and contents of low-crystalline Fe oxides, extracted with oxalate at pH 6.

available phosphorus; anaerobic soils; extraction methods