Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 6, 2003
Publication Date: June 1, 2004
Citation: Su, C., Suarez, D.L. 2004. Boron release from weathering of illites, serpentine,shales, and illitic/palygorskitic soils. Soil Science Society of America Journal. 68(2):96-105. Interpretive Summary: Elevated levels of boron in solution are toxic to plants. Boron toxicity is one of the major factors restricting reuse of agricultural drainage waters in the western U.S. Prediction of boron concentrations in soil solution requires consideration of input water concentrations, concentrating factors such as transpiration, adsorption of boron by soil minerals and release of boron from soil minerals. Boron release by dissolution of soil minerals has not been previously characterized. We identified illite and palygorskite as the source minerals for slow release of boron. We present predictive rate equations to describe the boron release with time, based on results from long term studies of up to one half year. This information can be used to predict which soils will be likely to undergo a rebound in soil boron after leaching soluble boron during reclamation. Utilization of this information increases our ability to predict soil boron concentrations and thus improve leaching and management practices on soils containing elevated levels of boron.
Technical Abstract: Toxic concentrations of B in salt-affected soils of arid and semiarid areas occur as a result of weathering of B-containing minerals or long term use of high B irrigation waters. Despite extensive research on B adsorption and release from soils, mineral sources of B within natively high B soils remain poorly understood. The objectives of this study were to identify source minerals contributing to the continued B release after extraction of soluble B and to estimate B release rate from weathering of B-containing minerals and soils. Two specimen illites (Morris and Fithian), two shales (Salt Creek and Moreno Gulch), a fresh and a weathering serpentine (antigorite) from the Coastal Range of California, a Traver silt loam (coarse-loamy, mixed, thermic Natric Haploxeralfs) and a Twisselman clay loam [fine, mixed (calcareous), thermic Typic Torriorthents] both containing illite and palygorskite were successively extracted 7 to 26 times following each 12-h equilibration in 0.1 M and 0.01 M CaCl2 solution until the supernatant solutions contained less than the detection limit of 0.001 mmol B L-1. Subsequently, more than 2 mu m and 2-20 mu m size fractions were separated and reacted in deionized water at pH 5, 7, and 9 adjusted with HCl and NaOH. The total B of the separated fractions ranged from 5.1 to 28 mmol B kg-1 and the surface areas from 5.7 to 126 m2 g-1. Boron release rates decreased with time and increasing pH. Average B release rates from 150 to 180 d ranged from 0.005 fmol m-2 s-1 for salt Creek shale (2 - 20 mu m) to 0.342 fmol m-2 s-1 for Traver sil (more than2 mu m) at pH 5, 0.004 fmol m-2 s-1 for salt Creek shale (2 - 20 mu m) to 0.060 fmol m-2 s-1 for Traver sil (more than2 mu m) at ph 7, and 0.002 fmol m-2 s-1 weathering serpentine (2 - 20 mu m) to 0.044 fmol m-2 s-1 for Traver sil (more than2 mu m) at pH 9. Nonstoichiometric dissolution was found for all materials at all pH levels. Illite, chlorite, and palygorskite were identified in the clay and silt fractions fo the soils. Boron release from the two soils was accompanied with high Mg release into the solution, suggesting palygorskite as a major source for B.