Abstract
ABSTRACT: Extraction of K and Mg with boiling 1 mol L-1 HNO3 in an open system for predicting K and Mg uptake by plants is a method of low reproducibility. The aim of this study was to compare the extraction capacity of different acid methods relative to hydrofluoric acid extraction for K and Mg. A further objective was to develop a chemical extraction method using a closed system (microwave) for nonexchangeable and structural forms of these nutrients in order to [...]
Highlights
There is a dynamic balance among K forms in the soil
The EPA 3051A method can be used to estimate the total content of K in the clay fraction of soils developed from carbonate and phyllite/mica schist rocks
Some studies have shown the presence of interstratified mineral mica-smectite in silt and clay fractions with basal reflections (001) in 2.4 nm, a value obtained by the regular interstratified layers of 1.0 and 1.4 nm (Havlin et al, 1985)
Summary
As plants take up K from the soil solution, there is buffering of dissolved K by exchangeable, nonexchangeable, and structural forms of K (Kirkman et al, 1994). The release of structural forms of Mg to plants depends on exchange reactions and weathering of Mg minerals in the soil (Christenson and Doll, 1973). Biotite (K(Mg2Fe)AlSi3O10(OH)2), containing about 180 g kg-1 MgO, may be an important source of structural Mg in soils (Jackson, 1979). This nutrient is found in the soil only in the structural, exchangeable, and solution forms, differing from K, which may present a nonexchangeable form as well. The term “nonexchangeable K” used by several authors refers only to K reserves potentially available to plants, as the extraction method (boiling HNO3) normally dissolves structural forms of K
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