Role of Organic Matter On Boron Adsorption-Desorption Hysteresis of Soils.

In this study we evaluated the boron (B) adsorption/desorption reaction of six soils and examined the extent to which organic matter content, as well as incubation time, affected B release.  Six soils varying in initial pH, clay content, and organic matter content were selected for the study.  Adsorption experiments were carried out to determine B adsorption isotherms using 0.01 M NaCl solutions containing 0 to 100 mg B kg^-1.  The soils were equilibrated for 23 hours.  Boron desorption was investigated after replacing aliquots with B-free 0.01 M NaCl solution three times.  Three of the soils were also spiked with 0-100 mg B kg^-1 and equilibrated for up to 23 months at field capacity water content in a temperature controlled room at 25^oC.  At various intervals subsamples were extracted with DTPA-sorbitol, boiling water, and 0.1 M NaCl.  The B adsorption/desorption behavior of all six soils was found to be nonhysteretic in the short-term experiment, regardless of organic matter content.  Apparent hysteresis of B adsorption can result from difficulties in the experimental procedure itself.  Some B adsorption hysteresis was found for two soils with all three extractants after long-term equilibrations of 1 to 23 months.  Our results do not support the hypothesis that B desorption hysteresis increases in soils with increasing organic matter content.