- The role of silicon is complex and varied.
- The importance and benefits of silicon are species (and even cultivar) dependent.
- The importance and benefits of silicon are dependent on the background levels of silicon in the growing media.
|Before (above) and after (below) pictures for grape plants receiving silicon treatment.
PHOTOS COURTESY NATIONAL SILICATES
Historically, the latter point on background levels of silicon has been particularly contentious given that silicon is present in all soil and cannot be completely eliminated. There was a need to establish what represented a low or deficient level of silicon. Researchers recognized that the level of background silicon could be better controlled if a crop was grown in a nutrient solution. The switch from soil- to soilless-grown horticultural crops further accelerated research into the application of silicon. With the possible exception of rice, the greatest amount of research on the application of silicon has been with soilless-grown crops.
The growing acceptance of the importance of silicon-based fertilizers led to the need for regulatory approval. Under the classic definition of a nutrient, silicon meets most but not all the definition requirements. However, for certain plants, silicon meets all the requirements of being a nutrient. Helping to resolve the debate of “is it or isn’t it” a nutrient was the classification of silicon under categories such as “beneficial” in Brazil, or “supplement” in Canada. The Canada Fertilizers Act defines a supplement as a product that improves plant growth and/or yield. The CFIA has recently approved a potassium silicate-based product as a source of potassium and available silicon.
Silicon exists in nature in a myriad of shapes and forms with a similarly complex nomenclature. Silicon is almost always associated with oxygen. It is also found in nature as a silica or silicate. Examples of silica include quartz and diatomeous earth. Examples of silicate include clays, feldspar, talc and mica.
For silicon to be taken up by roots it needs to be in the form of silicic acid. Available silicon therefore refers to silicon in the form of silicic acid. In soils, the level of silicic acid is largely dependent on the mineral form of silicon. Typically, silicates provide higher levels of silicic acid than silica. Any silicon-based fertilizer would need to supply available silicon in a pure, consistent, ready-to-use form.
A number of studies have examined the benefits of silicon to plants grown in soilless medium. Benefits are greatest in greenhouse crops under conditions of abiotic stress. Abiotic stresses are non-living factors such as salinity, shade and imbalanced nutrient levels.
As available silicon is taken up by the roots, it is deposited in cell walls as a silica gel, also referred to as biogenic opal. Once it has been incorporated into cells in this form it is immobile and cannot be redistributed around the plant. In the cell walls, silicon plays a role as a structural component that increases cell rigidity. Plants grown with higher levels of silicon are less prone to lodging and tend to be better positioned for interception of light. Older, basal leaves have consistently been shown to have much higher levels of silicon than younger leaves due in part to a plant’s tendency to compensate for lower levels of light. Silicon-deficient plants are more prone to wilting and earlier senescence.
The deposition of silicon also plays a role in helping regulation of water. Under conditions of salt stress (or drought), the hydrophilic nature of deposited silicon helps to better retain water within the plant. The result is increased turgor in conditions of salinity. Saline conditions also elevate damaging peroxides within the plant. A key defence against free radical oxygen is the presence of antioxidant enzymes such as superoxide dimutase. Conditions of saline (or other abiotic) stresses decrease antioxidant enzyme activity. Available silicon has been shown to better maintain enzyme activity, thereby reducing and detoxifying oxygen free radicals. Results of detoxification include a higher enhancing chlorophyll content and improved photochemical efficiency.
DISTRIBUTION OF METALS
Metals such as manganese and zinc are essential for plant growth; however, it is easy to exceed ideal concentration in a nutrient solution. Excess metals express themselves as necrotopic spots and a decrease in yield. Adding available silicon does not diminish metal contents in leaves, but does distribute the metals more evenly. As with conditions of excess salinity, elevated silicon levels help mitigate the effects of oxidative damage that are associated with high metal levels.
Our understanding of the various roles of available silicon in maintaining plant health and facilitating plant growth has now developed to the point where its benefits are increasingly recognized by regulatory bodies around the world. Likewise, the value of augmentation of available silicon can now be realized by commercial growers.