Relation of Soluble Silica to Soil Fertility

Although silica is apparently not essential to the growth of most plants, it has been shown repeatedly that the addition of soluble silicate to soil or culture solutions had a beneficial effect when there was a deficiency of available phosphorus. It now seems clear that this is not because the plant utilizes silicate instead of phosphate ion, as first believed, but rather because silicate ion is able to displace phosphate ion from the surface of soil or colloidal material, thus increasing the ayailability of the small amount of phosphorus which is still present. 

For example, Sreenivasan reviewed the available information on the role of silicon in plant nutrition and concluded that silicate in the soil facilitates the uptake of 

In a study of the displacement of anions from soil by soluble silicate, Toth (131) showed that phosphate ion was released from the absorbed state only in slightly alkaline media, so thai t displacement s by hydroxyl or silicate ions rather than silicic acid. At about pH 7 soluble silica is essentially nonionized and has little tendency to displace phosphate ion. Definite increases in yield of barley and Sudan grass were noted when calcium or magnesium silicate was added to soil, these materials being apparently sufficiently alkaline to furnish some silicate ions. There was marked absorption of silica by rape, barley, and Sudan grass when grown in silicated soils. 

Duchon (135) concluded that the favorable action of colloidal silica on crop yields in sand cultures with insufficient phosphoric acid fertilization is due mainly to 

The amount of soluble silica available to plants in soil moisture is greatly affected by the chemical composition. Free oxides or iron or alumina absorb and insolubilize the silica. Oats take up silica at a rate that depends directly on the amount of silica in solution in the soil and the total. silica increases with the amount of water transpired. In the oat plant, silica thus plays a passive role from a biochemical standpoint (137).  

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