DTPA and Mehlich-3 micronutrient extractability in natural soils

Most soils of the humid area in the Iberian Peninsula are characterized by acid, desaturated, organic rich surface horizons. In this study micronutrients (Cu, Fe, Mn and Zn) were extracted using DTPA and Mehlich-3. Total contents were also determined by digestion with nitric acid. Seven natural soil profiles, representing major acid soil types developed over a wide variety of parent materials were collected. In addition, three soil profiles containing carbonates and/or a high base saturation were sampled. The organic matter content when all samples were included in the data set ranged between 0.37 and 26.51%. For the acid set of samples, Mehlich-3 extractable concentrations of Cu, Fe, Mn and Zn were higher than contents extracted by DTPA. However, DTPA extracted higher concentrations of Cu, Fe and Mn than Mehlich-3 in near neutral or calcareous soils. Correlation coefficients between DTPA and Mehlich-3 extractable amounts were in the range from 0.49 to 0.96 for the set of acid soil samples and in the range from 0.71 to 0.92 for non-acid horizons. However, for acid and non-acid samples combined, correlations between micronutrient contents extracted by both solutions were much weaker and not significant. Results illustrate difficulties for selecting a single multielement extractant for different soils of the studied region. Interactions between micronutrient extractability, pH and organic matter content were evaluated. In all of the studied soil profiles, there was a decrease in the extractable Cu, Fe, Mn and Zn contents with increasing soil depth.