Chemical fractionation of soil boron: I. Method development

A chemical fractionation scheme for soil boron was investigated by measuring the amounts of B extracted from minerals and synthetic soils amended with B. The scheme fractionated B into five fractions — readily soluble (solution plus non-specifically adsorbed), specifically adsorbed, oxide bound, organically bound and residual. These five fractions were extracted by 0.01 M CaCl2, 0.05 M KH2PO4, 0.2 M acidic NH4-oxalate, 0.02 M HNO3-30% H2O2, and 1:4:5 HNO3-HF-HCl, respectively. The methodology was tested on goethite and clay mica and three synthetic soils containing mixtures of goethite, clay mica, humic acid, calcite, and quartz sand and amended with three levels of B. Specifically adsorbed and oxide bound B were significantly correlated with the content of goethite in the synthetic soils, while residual B was significantly correlated with the content of clay mica. The recovery of extracted B, estimated as the percentage of total fractions over total B content in minerals and synthetic soils, ranged from 85 to 104%, with a mean of 97%. The overall results obtained with the fractionation scheme were deemed satisfactory as to sensitivity and precision for B fractionation. Key words: Boron, soil fractionation, sequential extraction