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 wid e range of physical-chemical properties. The scheme differentiated soi l B as readily soluble (0.01M CaCl2), specifically adsorbed (0.05M KH2 PO4), 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 l ess than 2% of the total soil B in a sequential extraction procedure. Mean contents of extractable B were 2.3% associated with various oxide s and oxyhydroxides, and 8.6% in organically bound forms. The major po rtion of soil B existed in residual or occluded forms. The procedure w as found to be satisfactory in terms of sensitivity and precision for the fractionation of B. A highly linear correlation coefficient of 0.9 72 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 conte nts of DCB-extractable aluminum (Al). Specifically adsorbed B was only correlated with the DCB-extractable iron (Fe) content. Organically bo und B was correlated with the contents of both clay and organic carbon (C). The general agreement between the separate and sequential extrac tion procedures suggested that the fractionation scheme was reasonably feasible.