ABSTRACT

The primary sources of boron in most soils are boron — containing minerals of either the very insoluble tourmaline and the volatile emanations of volcanoes or the very soluble hydrated minerals (Chesworth, 1991). Other sources of B in soils include organic matter, irrigation water and fertilizers as well as the numerous difficultly quantifiable anthropogenic sources. Major soil minerals-containing B are (1) for borates (hydrous), Borax — Na2B6011 . 5H20; kemite-Na2B4O7.41120; Colemanite — Ca2B6011 5H20 and (2) for borates (anhydrous): Ludwigite — Mg2FeBO5; Kotoite-Mg3(B03)2and (3) for complex Borosilicates: Tourmaline and Axinite (Chesworth, 1991; Dana and Dana, 1997; Krauskopf, 1972; Mortvedt, 2000). The most abundant and most common boron mineral is tourmaline, an aluminium borosilicate that contains about 3.1% boron (Muettertes, 1967) and widely distributed as a minor component of rock. In igneous, metamorphic and sedimentary rocks, B occurs as borosilicates which are very resistant to weathering and not readily available to plants. Mobilization of the immobile or unavailable forms of rock B occurs by weathering in the pedosphere, which includes soil reactions of acid-base, oxidation-reduction and dissolution—precipitation. Several anions of B such as B02 , B207 2, B033 , H2B03 and B (OH) 4are formed in soil solution. The dominant species in the soil when B from primary silicate goes into solution are apparently its un-dissociated acid, H3B03and, in part B(OH) . Only at pH above 7 are other anions, such as H2B07 and B4032 , are likely to occur in soil solution. The boric acid form of B in soil solution is mobile and easily lost by leaching especially in coarse-textured soil in

 humid regions. In soils, whether boron is released from soil minerals, mineralized from organic matter or is added to soil by means of irrigation or fertilizer, part of it remains in the soil solution and can be taken up by plants while part of it is adsorbed by soil particles of organic matter or mineral fraction. Soil organic matter adsorbs B more than mineral soil constituents. The concentration of boron in the soil solution, which is the amount of water solution B available for plant uptake, is generally controlled by adsorption reaction. Since plants roots absorb B mainly from soil solution (Oertli and Grgurevic, 1975) and an equilibrium exists between the soil solution and adsorbed B (Russell, 1973), then the adsorbed pool of B, perceived as nontoxic to plants (Keren et al., 1985a,b), acts as a buffer against sudden changes in the level of boron in the soil solution (Hatcher et al., 1962),It is therefore important to know how boron is distributed between the solid and the liquid phases of the soil.