The mechanism of silica
High concentrations of silica not only give plants mechanical strength but can also help protect them against disease, pests and environmental stress.
ALL FACTS SUGGEST that Si-rich materials can be used for reducing - “P” - “TIE UP” and keep applied PHOSPHORUS in plant available form.
CaHPo4 + Si (OH) 4 = CaSio3 + H2O + H3 Po4 Calcium / Phosphate + Silica Acid = Calcium Silicate + water + Phosphate
2Al (H2Po4) 3 + Si (OH) 4 + H = Al2Si2O5 + SH3PO4 + 3H2O Aluminium Phosphate + Silica Acid + Hydrogen = Aluminium Silicate + Phosphate + Water
2FEPo4 + Si (OH4) + H = FeSiO4 + 2(H3PO4)
Iron Phosphate + Silica Acid + Hydrogen = Iron Silicate + Phosphate
FERTILISERS that are high in soluble silica – particularly in the form of mono- silica acids, such as Madura Guano Sulphur Gold® - react in the soil solution to release Si(OH)4. Whilst the benefits of silica [Si] for plants are well documented, the method of uptake is less understood.
Plant- available silica enters the plant the same way as water. Silicon is taken up in the form of silicic acid. Two different modes of Si uptake have been proposed for plants having different degrees of Si accumulation, namely active and passive.
Plants with an active mode of uptake take up Si faster than water, resulting in a depletion of Si in the uptake solution.
Plants with a passive mode of uptake take up Si at a rate that is like the uptake rate of water and no significant changes in the concentration of Si in the uptake solution are observed.
Once taken up by the plant, silicon is translocated in the xylem from the roots to the shoots. Silicic acid is concentrated and polymerised to silica [SiO2] or silica dioxide and finally deposited on the different root, leaf, stem and even fruit tissue. This then allows the plant to have great natural resilience to abiotic (drought, frost, heat) and biotic (fungus, bacteria, insects) stresses. The role of silica in the soil needs to be understood as it directly affects the amount of silica that is available to be taken up and used by the plant. Soils that are prone to locking up phosphorus will likewise have less available silica available to the plant. This is best explained by looking at silica once it becomes mobile.
Once mobile, silica will absorb into either existing free iron, calcium or aluminium, or into existing insoluble phosphorus compounds, such as calcium [CaHPo4], Aluminium [2Al (H2Po4] and Iron [2FePo4].
This has the effect of:
1. Transforming slightly soluble phosphates into mobile forms that are more plant available.
2. The binding of elements that are hostile to phosphorus allowing free phosphorus to be plant available for longer.
In summary, the whole system of how silica works in the soil and the plant needs to be understood so, if the agricultural goal is to increase the plant’s resilience via silica applications, then adequate silica is applied at the right time, in the right form (granular or liquid) in the correct amounts. ☐