Sensitivity of cationic micronutrient extractors to pH elevation in cerrado soils

Abstract

The application of acidity correctors to soils redistributes geochemical species of Zn, Cu, Mn, and Fe between fractions available and unavailable to plants, through processes affected by pH variation, such as precipitation by hydrolysis, dispersion of organic matter, adsorption desorption dynamics and changes in the interfacial properties of colloids. However, the sensitivity of traditional chemical extractors to these geochemical changes has not been sufficiently elucidated, especially in highly weathered systems. The aim of this study was to evaluate the sensitivity of different extractors to the increase in soil pH induced by liming and to identify changes in the chemical species of cationic micronutrients. The experimental design followed a 2 × 7 factorial scheme, involving two types of soil representing the main pedogenetic classes in Brazil (Latossolo and Neossolo) and seven levels of correction with CaCO₃ (0-, 0.25-, 0.50-, 1.0-, 1.5-, 2.0- and 2.5-times the H+Al content of the soils). The treatments were arranged in randomized blocks with three replications. After stabilizing the active acidity of the soil (pH), the Cu, Fe, Mn and Zn contents were determined using four extractants (Mehlich-1, Mehlich-3, KCl and DTPA). The behavior of the extractants varied due to their different mechanisms for accessing the chemical species. Mehlich-1 proved to be insensitive to an increase in soil pH because it is acidic and the pH of the extracting solution does not undergo significant changes after contact with the soil, leading to the dissolution of hydrolyzed species, while DTPA was sensitive to an increase in pH, probably because its complexation constants with nutrients did not exceed the precipitation-dissolution constants, limiting the dissolution of precipitated species. KCl showed low efficiency in recovering Zn, Cu, Mn and Fe, probably due to the predominance of non-exchangeable species, which makes it apparently insensitive to pH changes. The variable behavior of Mehlich-3 stems from its multiple extraction mechanisms. To assess the availability of nutrients in the short term, DTPA stood out as the most suitable extractant for quantifying the available levels of Zn, Cu, Mn and Fe for plants.