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I I Boron I I I deficiency in crops and is not comprehen- sively summarized in this report. The crop literature has been summarized compre- hensively by Eaton (1935) and most recently by Perry et a1. (1994). Avian literature consisted mostly of studies done on mallards from the late 1980s to early 1990s. Some poultry literature was reviewed, but the bulk of this literature may have been missed since electronic literature retrievals do not date back further than the 1960s. Mammalian studies consisted mostly of laboratory studies done on rats, although some information was available for mice, rabbits, and other species. Available aquatic toxicity data for boron are limited. For aquatic species, the literature was composed primarily of freshwater laboratory studies. Fish, herptile, and invertebrate information was limited or lacking. The published scientific "white" literature was reviewed adequately, but the scientific "gray" literature, which includes government reports and unpublished data, was not. I I I I 1 I I Abiotic Factors Affecting Bioavailability Water The predominant species of boron in most freshwater systems (pH<9) is undissociated boric acid (Hem 1970; Maier and Knight 1991); the chemical form of boron found in water is dictated by pH and other constituents (Sprague 1972). Boron compounds are water soluble and tend to accumulate in aquatic ecosystems (EP A 1975). I I I I I I I' Soil In the United States, soil usually contains around 30 mg B/kg, dry weight (dw) (range 10-300 mg/kg). The precipitation:evaporation ratio of an area is a key factor in determining the degree to which boron can concentrate in soils and reach toxic levels (Butterwick et a1. 1989). The total boron content of soil is of little value for diagnosing boron status; experi- mental work by Gupta (1968) suggests that less than 5 percent of the soil boron is avail- able for plant uptake (Butterwick et al. 1989). In soils, boron may be found in four forms: organically bound, water-soluble, adsorbed, and fixed in clay and mineral lattices (Adriano 1986). Arid, saline soils generally contain the highest boron concentrations. In sandy soils, boron is leached more readily than in clay soils and is thus less likely to accumulate to toxic concentrations (Adriano 1986). Boron can react and bind with clays, suspended matter, and sediments of aquatic systems. Boron adsorbed onto clays accounts for a major proportion of the boron in many aquatic systems (Maier and Knight 1991). Biotic Effects Plants The environmental effects of boron are most noticeable in plants (Sprague 1972). Boron is an essential trace element for the growth and development of higher plants, for it plays important roles in the calcium cycle and in respiratory processes and the utilization of carbohydrates (Browning 1969). However, the range between insufficiency and excess is usually narrow. Gupta et al. (1985), for instance, found that some plants show signs of deficiency when boron concentrations in soil solution are <2 mg/L and show toxic effects at concentrations >5 mg/L. Other researchers report similarly narrow ranges of boron tolerance (Sprague 1972; Weir and Fisher 1972; Birge and Black 1977; Goldbach and Amberger 1986). The waterweed Elodea canadensis is sensitive to even very low ambient concentra- tions of boron; Perry et a1. (1994) reported that it showed a reduced rate of photosynthesis in water containing 1 mg B/L (28-day exposure). In addition, Hydrocotyle lImbeIlata, commonly found in the Southeastern United States, exhibited reduced growth and yellowing of the leaves when exposed to <1 mg B/L 0fJ