Home About Us Our Services Publications Testing Submissions Contact Us Links

The Loam Ranger – Is silicon a plant nutrient?

Dear Loam Ranger,

I’ve heard that I should be adding silicon fertilizer to my soil. But isn’t all soil made of silicon? Why should I pay money for something that’s already present? Is this a scam?

The short answer is that it appears that plants benefit from silicon (Si). So it’s probably not a scam. The long answer has many ifs and buts.

Is it essential?

Two key criteria determine whether an element is an essential plant nutrient. The first is that absence of the element prevents the plant from growing (and the element doesn’t simply rectify a chemical or microbial imbalance in the soil). The second is that the element must form part of an essential molecule in the plant.

In the case of the first criterion, it is extremely difficult to exclude Si from the growing medium. Si is so common as a contaminant of other chemicals that is it present even when steps are taken to exclude it. It’s even present in glassware. And although under strict controls it’s present in only small amounts, these amounts might be enough to meet plant requirements. So it has not been proved beyond doubt that plants cannot grow without Si.

In the case of the second criterion, Si is not known to form part of essential molecules in plants. On the other hand, it does form part of the structural components of plant cells. So it’s still not clear whether it’s essential.

Is it beneficial?

Nevertheless, plants benefit from Si. At the extreme, plants called horsetails (Equisetum spp.) cannot grow without Si, which forms up to 10% of the plant by dry weight. (These plants are rare in Australia and occur only as noxious weeds.) More commonly, however, grasses contain 1% to 5% Si by dry weight. Si supplementation of grasses, particularly rice, grown in Si-poor soils can double yields. There appear to be several reasons:

• First, Si is deposited in cell walls, where it provides mechanical strength at less than 10% of the energy cost of lignin and cellulose. Sturdier plants resist lodging better and can direct their leaves towards the sun better. In particular, sturdier turf resists wear better, although constant mowing may negate any beneficial effects of Si.
• Second, this physical reinforcing also provides mechanical resistance against fungi and insects. Plants supplemented with Si are more resistant to diseases and pests than plants deficient in Si.
• Third, the physical barrier created by Si in leaves reduces water loss, and therefore water usage.
• Fourth, Si can rectify imbalances between other nutrients. It has been shown to overcome simultaneous zinc deficiency and phosphorus excess, not by substituting for either, but by somehow altering the supply of both to plants. Additionally, it has been shown to mitigate the effects of iron, manganese and aluminium toxicity, possibly by binding to the ions.
• Fifth, it is present in plants at concentrations characteristic only of macronutrients. So if macronutrients (such as nitrogen, phosphorus and sulphur) are present at concentrations of 0.1% to 8%, then Si, present at up to 10%, should also be a macronutrient, no? Yet without clear evidence that plants cannot grow in the complete absence of Si, we cannot be certain.

Don’t all soils have silicon?

Actually, no. Most soils have Si, but some organic soils (such as those derived from peat) and some tropical soils subjected to regular heavy rain have little or no Si. Ironically, plants growing in sand may actually have very low levels of available silicon, because sand, being quartz, is insoluble, and is the most stable component of soils. Crops on such soils have benefited enormously from applications of Si fertiliser. In particular, rice is often grown on such soils in the tropics, and could benefit substantially from Si fertiliser.

What sort of fertiliser should I use?

First, you need to establish whether your plants even need Si fertiliser. Given the ubiquity of Si in most soils, the answer is probably no. However, if you are keen to find out, send us some samples and we’ll tell you the concentration of available Si, which will then help you decide whether you could boost it.

If you decide to buy and apply Si fertiliser, the form of the Si is critical. Surprisingly, Si is soluble in water. It is typically found at concentrations common to calcium, potassium and sulphate in the soil, and much higher than phosphate. The crucial point is that silicic acid (H₄SiO₄) and silicates (e.g. K₂SiO₃) are soluble, but silica (SiO₂) is not. (Quartz is silica. So is opal.) If the fertiliser label states that the Si is present only as silica, it’s probably not useful. (It’s not enough to conclude that it can’t hurt; this is just throwing money away.) Look for silicic acid or a silicate such as potassium silicate. If the Si is not in soluble form, it won’t help.

Silicon facts

• Si is the most abundant and stable element in the Earth’s crust. Because of its ubiquity, it is likely that it will one day be found to be essential.
• Si must be soluble to have any effect. Its solubility varies with pH: Si is more soluble in alkaline conditions (pH > 7).
• The presence of Al, Ca, Fe, P and organic matter affects the solubility of Si.
• Quartz (SiO₂) is the most resistant mineral in soils, with a hardness approaching that of sapphire. So any sand-based product will not supply soluble Si.
• Si may be passively taken up by the plant, purely reflecting water flow, or it may be actively absorbed; we’re not sure.
• Some plants (notably monocots) seem to benefit from Si.
• Some plants (notably dicots) actively prevent the uptake of Si.
• Si is an essential nutrient for humans and other animals and may be linked with several disorders.
• ... but high consumption of Si in plants by ruminants may result in a disorder called siliceous renal calculi.
• Inhalation of high levels of Si-bearing dust results in silicosis, a chronic lung disease.
• ... but Si as quartz poses no health hazard.
• The presence of tiny opal particles in pasture plants can result in considerable wear on sheep’s teeth, since opals are harder than teeth.
• For an accurate laboratory determination of Si in plant tissue, the plant tissue must be free of dust or soil particles.

Further reading

Epstein E. 1994. The anomaly of silicon in plant biology. Proc Natl Acad Sci USA 91: 11–17.

Hull RJ. 2004. Scientists start to recognize silicon’s beneficial effects. Harsco Minerals International, Sarver, PA, USA.

Download as a PDF