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Boron release from six arid-zone soil samples 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 performed for 362 d to distinguish B desorption from B released from mineral dissolution. Amounts of B released decreased rapidly and reached a low constant value aft er about 6 mo of reaction time. Slopes of the B release curves for the last eight extractions approached zero (constant release rate), indicating that this B release was from mineral dissolution. We also did a shorter term experiment that contained added Mg and silicate in solution. These experiments were conducted to evaluate the contribution of B release from dissolution of B-containing Mg silicate minerals. We determined that most of the B released from the soils can be attributed to release from sorption sites rather than resulting from dissolution of B-containing Mg silicates. Various B soil tests were evaluated for their ability to measure native extractable adsorbed B. The diethylenetriaminepentaacetic acid (DTPA)-sorbitol extract was the best measure of available adsorbed B but still only extracted 48% of the amount of B released in the long-term experiment. The greatest amount of B was extracted at the lowest suspension density for DTPA-sorbitol (92%) and phosphate buff er pH 6.6 (95%). These extractants are recommended for quantifying native extractable 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.