Boron fractionation in soils

A chemical fractionation scheme for boron (B) developed previously for minerals and synthetic soils was used on twenty‐four soils with a wide range of physical‐chemical properties. The scheme differentiated soil B as readily soluble (0.01M CaCl2), specifically adsorbed (0.05M KH2PO4), oxide bound (0.20M acidic NH4‐oxalate), organically bound (0.02M HNO3 + 30% H2O2), and residual (HNO3 + HCl + HF). On the average, the readily soluble and specifically adsorbed B fractions accounted for less than 2% of the total soil B in a sequential extraction procedure. Mean contents of extractable B were 2.3% associated with various oxides and oxyhydroxides, and 8.6% in organically bound forms. The major portion of soil B existed in residual or occluded forms. The procedure was found to be satisfactory in terms of sensitivity and precision for the fractionation of B. A highly linear correlation coefficient of 0.972 was found between the sum of the five B fractions and total soil B content. Readily soluble B was positively correlated with both pH and cation exchange capacity (CEC), while negatively correlated with contents of DCB‐extractable aluminum (Al). Specifically adsorbed B was only correlated with the DCB‐extractable iron (Fe) content. Organically bound B was correlated with the contents of both clay and organic carbon (C). The general agreement between the separate and sequential extraction procedures suggested that the fractionation scheme was reasonably feasible.