Distinguishing Boron Desorption From Mineral Dissolution in Arid Zone Soils.

Boron release from six arid-zone soils from the San Joaquin Valley of California was investigated as a function of reaction time, solution pH, and suspension density.  A multiple batch extraction experiment was carried out for 362 days to distinguish B desorption from mineral dissolution.  Amounts of B released decreased rapidly and reached a low constant value after about six months of reaction time.  Slopes of the B release curves for the last eight extractions approached zero indicating that this B release was from mineral dissolution.  Various B soil tests were evaluated for their ability to measure native adsorbed B.  The DTPA-sorbitol extract was the best measure of plant available native B but still only extracted 48% of the amount of B desorbed in the long-term experiment.  The greatest amount of B was extracted at the lowest suspension density for DTPA-sorbitol (92%) and phosphate buffer pH 6.6 (95%).  These extractants are recommended for quantifying native adsorbed B which must be considered in application of chemical speciation transport models to describe B movement in soils.  Incorporation of our results will allow improved predictions of soil solution B concentrations under diverse agricultural and environmental conditions.