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Plant, Human & Animal biology
Terrestrial Vegetation
Terrestrial vegetation contains, on average, 0.15% of silicon by weight. The silicon content varies greatly according to the species of plant. It is high in plants of the graminae family (except corn) whose ashes can contain 30 to 60 % of Si O2, and it is low in leguminous plants and plants containing potash (potato).
One hectare of leguminous plants extracts 10 kg of silica from the ground per year, beech 63 kg and wheat 105 kg. Silica represents more than 50 % of minerals extracted from the soil by corn. Silicon is especially localised in the oldest leafs.
Equisetum arvense L (horsetail) is particularly rich in silica and ashes of this primitive species can contain up to 80 %.
In fact, Silicon is an essential fertilising agent. But, just like for the animal species, its essentiality is not easy to prove: no farming system (in-soil or out-of-soil) is ever completely free from silica (rock salt, air or water always contain significant traces).
The seeds of graminaceous plants (wheat, oats, millet, barley, rice, except for corn), do not grow well if placed in an environment or soil without silica, and yields are clearly improved by the addition of silicates. Tomatoes are also thought to be sensitive to the addition of sufficient amounts of silica.
Silica thus also plays a support role and, combined with calcium, it affects the coefficient of rigidity of the plant. Moreover, there is often a balance between the Silica and Calcium contents. The leaf of the chestnut tree , for example, contains 1 to 5 % of silica, that of reeds 66% of silica.
Marine biology
Considering the recent Progress made in the estimation of geo-chemical and biological flows, the CNRS (France) has just published a new total assessment of the silicon contained in the oceans, covering the past 10,000 years
The geo-chemical balance between the net additions and removals of silicon is estimated at 6,100 billion moles of silicon per annum.
The silicic acid added to the oceans is mainly of fluvial origin (82%), in particular in the tropical and equatorial zones.
The residence time of Silicon in the surface layers of the oceans is 400 years. Silicon passes 40 times through the phases of biogenic silica production / dissolution of diatoms (siliceous algae), before leaving the ocean to be accumulated in the sediments.
The silicon cycle remains dominated by biology, however, because biological flows are much more intense than geo-chemical flows. This biological flow is currently of the order of 240,000 billion moles of silicon per annum.
Human & animal Biology
The significant lack of information regarding the biology of silicon comes without doubt from the difficulty of measuring it, so that many historical results are of such doubtful validity that they must be considered with circumspection.
It would not be superfluous to start again using the methodological precautions of modern assaying systems.
The relative scale of previously estimated tissue concentrations is undoubtedly correct, but the absolute values should be revised downwards.
The total silicon content of the human body is estimated at 7 grams, mainly in inorganic form. Only the organic fraction has a biological function.
This form decreases notably as from the age of ten years old. The blood concentration is about 10 µg/ml in man (1/10 of the concentration of calcium).
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