PLANT ABSORPTION OF SILICON
Tissue analyses from a wide variety of plants showed that silicon concentrations range from 1 to 100 g Si kg  1 of dry weight, depending on plant species . Comparison of these values with those for elements such as phosphorus, nitrogen, calcium, and others shows silicon to be present in amounts equivalent to those of macronutrients .
 Plants absorb silicon from the soil solution in the form of monosilicic acid, also called orthosilicic acid [H4SiO4] . The largest amounts of silicon are adsorbed by sugarcane (300-700 kg of Si ha  1), rice (150-300 kg of Si ha 1), and wheat (50-150 kg of Si ha  1) . On an average, plants absorb from 50 to 200 kg of Si ha  1. Such values of silicon absorbed cannot be fully explained by passive absorption (such as diffusion or mass flow) because the upper 20 cm soil layer contains only an average of 0.1 to 1.6 kg Si ha  1  as monosilicic acid . Some results have shown that rice roots possess specific ability to concentrate silicon from the external solution .
2 FORMS OF SILICON IN PLANTS
Basically, silicon is absorbed by plants as monosilicic acid or its anion . In the plant, silicon is transported from the root to the shoot by means of the transportation stream in the xylem. Soluble monosilicic acid may penetrate through cell membranes passively (. Active transport of monosilicic acid in plants has received little study.
 After root adsorption, monosilicic acid is translocated rapidly into the leaves of the plant in the transpiration stream . Silicon is concentrated in the epidermal tissue as a fine layer of silicon-cellulose membrane and is associated with pectin and calcium ions . By this means, the double-cuticular layer can protect and mechanically strengthen plant structures .
 With increasing silicon concentration in the plant sap, monosilicic acid is polymerized . The chemical nature of polymerized silicon has been identified as silicon gel or biogenic opal, amorphous SiO2, which is hydrated with various numbers of water molecules . Monosilicic acid polymerization is assigned to the type of condensable polymerization with gradual dehydration of monosilicic acid and then polysilicic acid :

SILICON FERTILIZERS
Although silicon is a very abundant element, for a material to be useful as a fertilizer, it must have a relatively high content of silicon, provide sufficient water-soluble silicon to meet the needs of the plant, be cost effective, have a physical nature that facilitates storage and application, and not contain substances that will contaminate the soil . Many potential sources meet the first requirement; however, only a few meet all of these requirements. Crop residues, especially of silicon-accumulating plants such as rice, are used as silicon sources either intentionally or unintentionally. When available, they should not be overlooked as sources of silicon. However, the crop demand for application of silicon fertilizer generally exceeds that which can be supplied by crop residues.
 Inorganic materials such as quartz, clays, micas, and feldspars, although rich in silicon, are poor silicon-fertilizer sources because of the low solubility of the silicon. Calcium silicate, generally obtained as a byproduct of an industrial procedure (steel and phosphorus production, for example) is one of the most widely used silicon fertilizers. Potassium silicate, though expensive, is highly soluble and can be used in hydroponic culture. Other sources that have been used commercially are calcium silicate hydrate, silica gel, and thermo-phosphate