Extratores de fósforo disponível para solos do Estado de Roraima

Abstract

The soils of the humid tropical region of Brazil are diversified because of its source material, climate and topography. They are mostly low fertility, with available phosphorus (P) in these soils generally low. The dynamic of soil P becomes a complex determination, especially by the distinct characteristics of the extractors. The objective of this study was to evaluate the maximum phosphorus adsorption capacity (MPAC) and the efficiency of Mehlich-1, Mehlich-3, Bray-1 and Resin extractors in the determination of available P in soils of the state of Roraima. Seven soils were collected in three regions of the state and determined its properties morphological, mineralogical, chemical and physical. To evaluate the MPAC of the soils, was determined the remaining P (P-rem) of the soil in triplicate for each soil in the laboratory and four replications. The P-rem of each soil was used to define the patterns of P in solution to be used in determining the MPAC by Langmuir isotherm. To test the efficiency of extracting, the experiment was conducted in greenhouse conditions. The design applied was in a randomized blocks design, in factorial scheme 5 x 7 with four replications. The first factor consisted of the seven soils (LA (ITA), LV (ITA), LA (SERRA DA PRATA), LA (CCA), PA (CCA), FT (BONFIM), SN (SURUMU)) and the second factor of five P rates estimated based on the MPAC of each soil (0, 25, 50, 75 and 100% of MPAC). The plot consisted of a black pot polyethylene with 5 dm3 of soil and five plants. The species used was corn, hybrid 30F35YH - Pioneer, grown in two phases of 30 day within interval of two days. The P was applied five days before planting using potassium dihydrogen phosphate (KH2PO4) as a source. Soil samples from each plot (200 g) were collected before planting to determine the available P using the extractors. At the end of each period, the plants were cut, packed in kraft bag and taking them to the forced circulation oven at 60 º C to obtain the dry mass. Correlation test for MPAC versus binding energy, pH, organic matter, clay and T were performed, in addition to dry matter x extractor and extractor x extractor. The MPAC is higher in FT (BONFIM) and LV (ITA) due to the type and content of clay fraction respectively. Considering the representative soils of the state, the MPAC is not correlated with any of the soil properties analyzed, obtaining strong relationship with the clay when listed only the Oxisols and Ultisols. The Mehlich-3 and Bray-1 extractors are more sensitive to changes in soil properties and are not suitable for heterogeneous groups of soils. The resin method exhibit the highest correlation with the different soils individually and grouped. This extractor is demonstrated appropriate in determining of the available P for plants to soils of the humid tropical region of the country.