Estimativa da disponibilidade de fósforo para a cultura do arroz irrigado em solos do RS

Abstract

The evaluation of the available phosphorus (P) is hindered in lowland soils flooded for the rice cultivation, since the alternation in the oxidation and reduction conditions determine modifications in the dynamics of this element. Several chemical methods of P extraction, that can be used to evaluate its availability to establish the need to apply P to upland crops, present low efficiency in lowland soils. Therefore, the objectives of this work were: a) to characterize the inorganic P fractions and the P adsorption capacity for different samples in lowland soils from Rio Grande do Sul state (RS); and b) to test the correlation between the phosphorus absorbed by rice and the phosphorus extracted by different methods of soil P analysis, and to propose a more appropriate methodology to predict P availability for flooded rice production. The soils samples were taken from the top 0.20 m in 16 locations in RS with wide diversity of chemical and physical characteristics. The samples were air-dried, crushed, and passed through a 2 mm sieve. To standardize the soil pH, limestone was applied in the equivalent amounts for the soil pH to reach 5,5, based on SMP index of each sample. After a 35-day incubation period, the pots for rice cultivation were setup. The pots were constituted of plastic buckets containing the soils and for each one of sampled soil were applied 3 levels of P: N0 (without P), N1 (1/2 of N2), and N2 (0,2 mg L-1 in the soil solution) based on the P adsorption capacity of each soil, with 3 replication, adding up 144 experimental units. The pots were positioned in the greenhouse in an entirely randomized experimental design. The rice was cultivated for 45 days with a water layer of approximately 5 cm. At the end of this period, the rice shoot was harvested to measure the adsorbed P. The methods Mehlich-1, Mehlich-2, Mehlich-3, Resin in sheets, Bray-1, Bray-2, Olsen, Morgan, Calcium lactate, Texas, Oxalate, EDTA, H2SO4, P-remained, and NaOH were tested and results were correlated with the P absorbed by rice. The results showed the wide diversity in the P adsorption capacity, total amount of P, and distribution of inorganic P fractions, but the Fe-P fraction was the most important in the labile-P before and after flooding. Among the methods, H2SO4, Calcium lactate, Texas, EDTA, Olsen, and Resin in sheets were similar to Mehlich-1 efficiency. However, there is not indication to change the actual method used in RS. Coupling the clay content in the P interpretation increase the predictability of some methods in some clay classes, specially to Mehlich-1, Bray-1, Bray-2, Calcium lactate, and P-remained