Boron Release from Weathering of Illites, Serpentine, Shales, and Illitic/Palygorskitic Soils

ied in the central and south part of the western San Joaquin Valley of California. Alluvial deposits in the Despite extensive research on B adsorption and release from soils, valley derive primarily from Coast Ranges of California, mineral sources of B within natively high B soils remain poorly understood. The objectives of this study were to identify source minerals which are composed of several nearly parallel ranges contributing to the continued B release after extraction of soluble B of north-northwest trending mountains and intervening and to estimate B release rate from weathering of B-containing minervalleys (Presser et al., 1990). The Coast Ranges evolved als and soils. Two specimen illites (Morris and Fithian), two shales as a result of complex folding and faulting of geosyncli(Salt Creek and Moreno Gulch), a fresh and a weathered serpentine nal sedimentary rocks of Mesozoic and Tertiary age. (antigorite) from the Coastal Range of California, a Traver silt loam The geomorphic character of the alluvium laid down by (coarse-loamy, mixed, superactive thermic Natric Haploxeralfs) and streams draining the east-central Coast Ranges reflects a Twisselman clay loam [fine, mixed (calcareous), superactive thermic late Cenozoic uplift of the foothills and subsidence of Typic Torriorthents] both containing illite and palygorskite were sucthe valley (Lettis, 1982). The semiarid climate and the cessively extracted 7 to 26 times following each 12-h equilibration in geological settings of the valley have contributed to soil 0.1 and 0.01 M CaCl2 solution until the supernatant solutions contained less than the detection limit of 0.001 mmol B L 1. Subsequently, the salinization and toxic levels of B. 2m and 2to 20m size fractions were separated and reacted in The presence of excess soluble B in arid lands is deionized water at pH 5, 7, and 9 adjusted with HCl and NaOH. The usually attributed to the weathering of B-containing soil total B of the separated fractions ranged from 5.1 to 28 mmol B kg 1 minerals or the utilization of high B irrigation waters and the surface areas from 5.7 to 126 m2 g 1. Boron release rates (U.S. Salinity Laboratory Staff, 1954). Rhoades et al. decreased with time and increasing pH. Average B release rates from (1970) observed increased B concentrations in soil col150 to 180 d ranged from 0.005 fmol m 2 s 1 for Salt Creek shale umn effluents following a period of postreclamation (2–20 m) to 0.342 fmol m 2 s 1 for Traver silt ( 2 m) at pH 5, storage and termed the phenomenon “boron regenera0.004 fmol m 2 s 1 for Salt Creek shale (2–20 m) to 0.060 fmol m 2 tion,” which is attributed to the continued release of s 1 for Traver silt ( 2 m) at pH 7, and 0.002 fmol m 2 s 1 for sparingly soluble sources of B. Boron regeneration was weathered serpentine (2–20 m) to 0.044 fmol m 2 s 1 for Traver silt ( 2 m) at pH 9. Nonstoichiometric dissolution was found for all also reported in a high B soil reclaimed in the field materials at all pH levels. Illite, chlorite, and palygorskite were identi(Bingham et al., 1972). Postreclamation desorption of fied in the clay and silt fractions of the soils. Boron release from the B and redistribution of B by diffusion from bypassed two soils was accompanied with high Mg release into the solution, to leachable soil pore regions are additional mechanisms suggesting palygorskite as a major source for B. for regeneration (Peryea et al., 1985a, 1985b, 1985c). Boron is adsorbed by soil solids, and distributed between soil water and solid phases. A detailed review B is an element of great concern because for on the characteristics of B adsorption on different soil many plants the ratio of toxic to adequate B conconstituents is given by Goldberg (1993). Boron activity centrations is the smallest among the essential micronuin the soil solution is unlikely to be controlled by a solid trients. Boron concentrations in soil water in the range phase mineral. Tourmaline is often the only B mineral of 0.05 to 0.5 mmol L 1 have a deleterious effect on plant found in soils but it is extremely resistant to weathering; growth (U.S. Salinity Laboratory Staff, 1954; Gupta et whereas, hydrated B minerals form as evaporite deposits al., 1985). Toxic levels of B have been frequently refrom saline lakes and are too soluble to persist in most ported in the soils and irrigation waters of many arid soils (Goldberg, 1993). The readily soluble and specifiand semiarid regions of the world, including the irrigated cally adsorbed (inner-sphere complexed) B fractions areas of the western USA (Rhoades et al., 1970; Keren was reported to account for on average 2% of the and Bingham, 1985). Boron toxicity was reported to total B in 24 surface soils from Ontario, Canada (Hou overshadow other salt effects for several crops in a study et al., 1994). Recent infrared spectroscopic evidence of the long-term effects of using saline water for irrigasupports the formation of inner-sphere complexation of tion (Hanks et al., 1986). B with mineral surface functional groups (Su and SuSoil salinity and B toxicity have been repeatedly studarez, 1995). They postulated that adsorbed B might be responsible for controlling B activity in the soil solution. C. Su, USEPA, Office of Research and Development, National Risk Boron desorption has received less attention than B Management Research Laboratory, 919 Kerr Research Drive, Ada, adsorption, yet it is as important to B availability in soils. OK 74820; D.L. Suarez, USDA-ARS, George E. Brown, Jr. Salinity Many soils exhibit hysteresis in adsorption-desorption Lab, 450 West Big Springs Road, Riverside, CA 92507-4617. Received process. Desorption did not follow the adsorption iso6 Dec. 2002. *Corresponding author ([email protected]). Abbreviations: CEC, cation-exchange capacity; DI, deionized water; Published in Soil Sci. Soc. Am. J. 68:96–105 (2004).  Soil Science Society of America ICP-AES, inductively coupled plasma-atomic emission spectrometry; XRD, X-ray diffraction. 677 S. Segoe Rd., Madison, WI 53711 USA