SELECTIVE SEQUENTIAL DISSOLUTION TECHNIQUES FOR TRACE-METALS IN ARID-ZONE SOILS - THE CARBONATE DISSOLUTION STEP

Dissolution capacity and kinetics of carbonates by sodium acetate (NaOAc)-acetic acid (HOAc) at various pHs were studied. A comparative study of the selectivity, specificity, and effectivity of NaOAc-HOAc solution on carbonate bound fraction during the sequential selective dissolution procedure was conducted by comparing the dissolution of major and trace elements from arid zone soils by this buffer solution at various pHs. The effect of the pH of NaOAc-HOAc solution on the following fractions in the sequential selective dissolution procedure was also studied. NaOAc-HOAc solution at pH 5.5 at a soil to solution ratio of 1:25, can dissolve all the carbonate from calcareous soils with .10-20% of carbonate; at pH 5.0 it can dissolve all the carbonate in soils with about 30-50% calcium carbonate (CaCO3). A second era-action with fresh buffer solution at pH 5.0 is required for soils with more than 50% of carbonate. Six hours of extraction time is generally sufficient for complete carbonate dissolution. For most of agricultural soils in arid and semi-arid zones, the attack of the buffer solution at pH 5.0 on other solid-phases seems to be limited. But the buffer solution at pH 5.5 would be better for some forest soils with low carbonate content and high organic matter content. The part of carbonate fraction not be dissolved in this step is released in the following steps: easily reducible oxides fraction (ERO), organic matter fraction (OM), and reducible oxides fraction (RO), leading to gross misinterpretation of the elemental partitioning in arid zone soils.