The chemical properties of a clayey oxisol from Amazonia and the attributes of its phosphorus fractions

Oxisols are the most commonly occurring soil types in the world, including in Brazil. For this reason and given the great agricultural potential of Amazonia, research on Oxisoils in the Amazon region is important. The objective of this study was to determine the following characteristics of a clayey Oxisol in the Amazon under different soil management systems: (i) its chemical characteristics, including pH, H + Al, Al, cation exchange capacity (CEC), organic matter (OM) and sum of all bases (S); (ii) its phosphorus fractions, including organic P and inorganic P, total P and available P; and (iii) the correlations between the P fractions. The areas selected for this study were located on a clayey Oxisol. Sample collection was performed at the following sites: forest, pasture and sites subjected to 4 years no-tillage (NT4), 8 years no-tillage (NT8) and 2 years of conventional tillage (CT2). The experimental design was completely randomised, involving five land use systems and three collection depths, according to a 5x3 factorial design with seven replicates. Analyses of pH, Al, H + Al and organic matter (OM) levels, the sum of all bases and soil CEC were performed. The pH values were naturally acidic, falling below pH 5, in the forest and under all three management systems. Decreased Al and H + Al values were also observed under the three management systems. The lime applications included in the soil management regimes influenced these results. The CEC and sum of bases were greatest in conventionally tilled soil in the 0.0-0.05 m layer, indicating the low mobility of the applied limestone in these areas. These two variables were not correlated with the OM content in the soil. The fraction of organic P was the most representative of the total P content, but the available P was not correlated with the OM content of the Oxisol. The investigated durations of no-tillage have not been sufficient to increase the OM content of the soil to a satisfactory level, as the climatic conditions of the region accelerate the decomposition of organic material.

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