Boron, Silicon, Molybdenum, Arsenic, and Selenium

Si(OH)4 is a very weak acid, pKj — 9.1. It polymerizes and precipitates as amorphous silica when concentrations reach 10 3 M. Such high soluble silica concentra- 

Evolution has taken little advantage of silicon s ubiquity and relatively constant solubility. Silicon is a useful strengthening agent of the plant s cell wall and forms a cast of the cell wall s morphology. These can remain intact in soil as phytoliths after the plant decays. Silicon is essential for animals and only in trace amounts. 

Silicon reactions are central to rock weathering and soil development. Silicon is the soil component lost in greatest amount from rock minerals during weathering, and the transformations of silica into secondary minerals are the major reactions of soil development. The sand fraction of soils is usually 90% quartz (SiC 2), the most prevalent form of Si in soils. Highly weathered soils may contain as little as 20% Si (Table 2.1a). A1 and Fe ore deposits are essentially highly weathered soils from which most of the Si has been lost. 

Secondary silicates form as clay minerals in soils after weathering of the primary silicates in igneous minerals. The secondary silicates include amorphous silica (opal) at high soluble silica concentrations and the very important aluminosilicate clay minerals kaolinite, smectite (montmorillonite), vermiculite, hydrous mica (il-lite), and others. Kaolinite tends to form at the low silicate concentrations of humid soils, whereas smectite forms at the higher silicate and Ca concentrations of arid and semiarid soils. The clay fraction of soils usually contains a mixture of these day minerals, plus considerable amorphous silicate material, such as allophane and imogolite, which may not be identifiable by x-ray diffraction. 

Borates, molybdates, selenates, and arsenates occur in such trace quantities in soils that they probably exist only as impurities in major soil particles and on particle surfaces rather than as separate minerals. Soils in humid regions sometimes benefit from borate and, less often, molybdate additions. The range between deficient and excess is narrow, so spreading a few kg ha-1 (a few Ibs/acre) uniformly over the soil is difficult unless the borate and molybdate salts are mixed in with other fertilizers or inert materials. 

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