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 h. 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 to 100 mg B kg(-1) and equilibrated for up to 23 months at field capacity water content in a temperature-controlled room at 25 degrees C. At various intervals, sub-samples 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.