3 minute read
Silicic Acid
BY TOM FORREST
Silicic Acid
Silicon (Si) is the second most abundant element present in the Earth’s crust. Soils generally contain up to 40% silicon (Kovda 1973) and plants use this key element in vast amounts. Recent advancements in the production of silicon fertiliser compounds and manufacturing techniques are giving farmers access to a new wave of silicate products.
The physically and chemically active Si substances in the soil are represented by monosilicic acids, polysilicic acids, and organosilicon compounds (Matichenkov and Ammosova, 1996). These forms of silicon are interchangeable with each other, as well as with other crystalline minerals and living organisms (soil microbes and plants). Silicic acid is the general name for a family of chemical compounds containing the element silicon attached to oxide and hydroxyl groups.
Monosilicic acid (HSiO) is the centre of these interactions and transformations. Monosilicic acid is the product of silicon mineral dissolution (Lindsay 1979), absorbed by plants and living organisms (Yoshida 1975). They can influence the chemical and biological properties of the soil, including significant interactions with phosphorus, aluminium, iron, manganese and other metal mobility.
Monosilicic acid is also essential in the formation of polysilicic acids and secondary minerals, however, plants and microbes can only absorb monosilicic acid (Yoshida 1975). Polysilicic acids will influence soil texture, water retention, cation exchange capacity, and soil erosion stability (Matichenkov et al, 1995).
Modern agricultural research and manufacturing in Europe have developed novel methods of blending and isolating these silicic acid compounds in a stable form (with a long shelf life; essential for successful product development). Brands are now available worldwide with extremely high concentrations of silicic acids.
There is an ongoing debate about the efficacy of monosilicic acid products vs older potassium silicate products. The benefits of silicic acid products over typical potassium silicate products are easy to explain. Silicic acid products (such as Super Si) contain just that: an absorbable form of silicic acid. Potassium silicate is a molecule that does not contain silicic acid, but silicate (SiO 2), which is not readily available for plants.
Scientists agree that for horticultural purposes, the amount of silicic acid that is yielded from dissolving potassium silicate into water is almost non-existent. Much like crude oil versus petrol, silicates must be “refined” before they can be used. However, this might not be a problem, because it is elemental silicon that plants need, not silicic acid per se.
Silicon-oxide bonds are very strong bonds that do not “magically” break up. Therefore, adding silicates (SiO 2 and such) to your plant’s diet does not help them in their need for silicon. Microbes or other chemicals must come into play so the plants can benefit from the silicates in the ecosystem. Adding silicic acid, on the other hand, will benefit your plants immediately in their need for silicon, because this form of silicon is readily available without the help of mineralization by microbes in the ecosystem. Having said that, potassium silicate products do still have an effect on plants because the silicates can be turned into absorbable molecules by organisms or other chemicals (e.g. strong acids).
Monosilicic acid is readily absorbable by the plant and can also make an aqueous solution more acidic, which increases absorbability of other minerals. Other forms of silicon in the soil are not easily assimilated by plants and only a small proportion is absorbed as H SiO.
Once absorbed by the plant, silicon polymerizes when it dehydrates and concentrates on the epidermis cells as biogenic amorphous silicon (SiO 2). This biological process illustrates that silicate compounds are essential to fight plant disease and for pest resistance (Epstein 1999).
Silicon can also alleviate various abiotic stresses including salt stress, metal toxicity, drought stress, radiation damage, nutrient imbalance, high temperatures, freezing and more. These beneficial effects are mainly attributed to the high accumulation of silica in the surface tissue.
Soil structure degradation occurs if a soil is lacking in silicon. This influences the decomposition of secondary minerals that control numerous growth factors and other soil structure properties. Silicon fertiliser also absorbs phosphorus and decreases leaching of the mineral by 40-90% (Matichenkov et al, 2000). It is noteworthy that the phosphorus is kept in a plant-available form.
