Silicon: A Plant Nutritional Enigma.

Silicon is very common.  Most soils are silicates of one kind or another.  The element is the second most abundant element in rocks and the soils derived from them. The soil water in most soils, the soil solution, commonly has concentrations of silicon, mainly as silicic acid, H₄SiO₄, on the order of 0.1 to 0.6 mM, or roughly 100 to 1000 times those of phosphate.  The chemistry of soil silicon has been amply studied by soil scientists and is quite well understood.  These statements, however, cannot be made in regard to the plant nutritional aspects of silicon - quite the opposite.  The element is largely disregarded in studies of plant physiology, even plant nutritional ones, and it is not included in the formulation of the commonly used nutrient solutions.  This, despite the fact that all soil-grown plants contain it in appreciable amounts.  In fact, the most important staple food crops, wheat and rice and corn, may have in their dry matter more silicon than any other mineral element.  The reason for the poor status of silicon in plant nutritional science is twofold.  It is, first, historical.  It was not till the mid-1860s that two German investigators, J. von Sachs and W. Knop, devised the solution culture method of growing plants.  They promptly found that they could grow plants without adding silicon to their nutrient solutions, and omission of silicon from the formulation of solution cultures has been routine ever since.  The implication is that silicon is not an essential element and hence is of no significance. The second feature causing silicon to be enigmatic is fundamental.  Regardless of plant physiological ideology, there is abundant evidence that in the real world of nature and of agriculture adequacy or dearth of silicon can make the difference between thrifty plants and their loss due to biotic or abiotic stresses.