Silicon is the second most abundant element on Earth. However, most forms of silicon[1] found in nature are unavailable for uptake by plants. To reap the benefits of silicon, growers must use a fertilizer that contains a highly soluble source of silicon.
That’s easier said than done. While there is no shortage of silicon in the world, not all silicon compounds are equal. Some may not be very soluble or even high in silicon. Others may have significant downsides, such as potential toxicity.
This blog post explains why soluble silicon is important, the different sources of soluble silicon available for fertilizer, and what to consider when selecting a silicon fertilizer.
Plant-available silicon
The first thing to understand about silicon is that plants can take up only one form of this beneficial nutrient: silicic acid. To be precise, silicic acid is actually a range of closely related acids whose base form is monosilicic acid. Also known as orthosilicic acid, monosilicic acid’s chemical formula is Si(OH)4.
It is a soluble, neutral molecule that readily passes through plant-cell membranes alongside water. Once absorbed, it is transported through the plant until it is deposited in leaf epidermal cells, where it condenses into a hard, polymerized silica[2] gel: SiO2·nH2O. This gel imparts strength to plant tissues, delivering benefits such as pest and stress resistance.
Unfortunately, you cannot just feed plants monosilicic acid. That’s because the molecule is highly unstable and precipitates into an insoluble granular form of silica at high concentrations, making it unavailable for absorption. The only way to keep silicon stable for plant uptake is to combine it with other chemical elements.
This is why it’s important to know the source of silicon in a fertilizer. Fertilizers containing sources of soluble silicon break down into silicic acid, feeding plants the only form of silicon they can absorb.
Potassium silicate breaks down into silicic acid and potassium. Source: Inside Silicon Supplements by Emerald Harvest: https://emeraldharvest.co/wp-content/uploads/2019/06/Inside-Silicon-Supplements_White-Paper_Emerald-Harvest.pdf
Sources of silicon
Many raw materials and compounds can serve as sources of silicon fertilizer. However, they vary in their silicon concentrations and solubility, as well as in the other nutrients and elements they come with.
The following are some of the different sources of silicon fertilizer and their pros and cons.[3]
Silicates:
- Calcium silicate is abundant in steel-mill slag byproducts. It also occurs naturally as the mineral wollastonite. However, it is limited in its solubility.
- Magnesium silicate is a compound of magnesium oxide and silicon, also with low solubility.
- Potassium silicate is highly soluble potassium and silicic acid, but it can increase the pH of the nutrient solution.
- Sodium silicate is a highly soluble silicate salt composed of sodium oxide and silicon dioxide. It can lead to toxicity by causing excessive salt to accumulate in plant cells. Sodium also competes with potassium for absorption, potentially contributing to potassium deficiency.
Industrial slags
Slags are non-metallic byproducts of refining and other kinds of industrial production. They consist primarily of various combinations of calcium, magnesium and aluminum silicates. The amount of each depends on the raw materials used, the type of industrial material (e.g., steel) made, furnace conditions and other factors. A risk of using slag in fertilizers is the possible leaching of heavy metals, which can become toxic at high concentrations.
Different types of slag can serve as sources of silicon:
- Blast furnace slag primarily consists of aluminosilicates, calcium-alumina-silicates and other silicates.
- Electric furnace slag contains nutrients such as iron, calcium, silicon and aluminum. Minor nutrient compounds include magnesium and manganese oxides.
- Iron/steel slag is composed primarily of limestone and silica.
Plant-based silicon:
- Raw rice hulls are a rich source of plant-available silicon, containing around 94% silica. But it is slowly released. The remaining 6% includes potassium, calcium, magnesium, manganese oxide and aluminum oxides, as well as phosphorus pentoxide.
- Biochar is made from partially combusted organic waste or biomass. Rice, wheat and sugarcane accumulate silicon, resulting in plant-available silicon in their residues. Biochar is prepared from those residues, but release of silicon from biochar is dependent on temperature, feedstock and the growing medium.
Other silicon sources:
- Diatomite comes from the fossilized remains of diatoms—microscopic organisms related to single-cell aquatic algae that accumulate high concentrations of silicon in their cell walls. Diatomite’s effectiveness is influenced by several factors, including its availability and purity.
- Silica nanoparticles, which are only 20‒40 nanometers in diameter, can penetrate and traverse plant cell membranes. Because they’re foliar applied, there is some concern over unintentional inhalation and how they could impact human health.
- Silicon solubilizing bacteria make silica available to plants by adding hydrogen ions to the growing medium, which promotes hydrolysis, and producing citric, keto and hydroxy carbolic acids, which form complexes with cations. This is not a sustainable source of silicon, as it requires silica bound to a cation to release plant-available silicon.
- Stabilized silicic acid is produced by combing silicic acid with other molecules, thus preventing condensation and polymerization. However, it typically contains a higher concentration of stabilizing agents (2‒5%) than it does silicon, which hovers around 0.7‒0.8%. Thus, stabilized silicic acid is not a rich source of silicon for fertilization. It is also unclear how the silicic acid is released to make it available to the plant.
- Thermo-phosphate is produced by heating a mixture of rock phosphate and magnesium silicate. Its total silicon content is relatively low.
Different sources of silicon used in agriculture. Source: Nagabovanalli, P.B., S. Majumdar, S. Kollalu, et al. 2020. “Meeting Report: 7th International Conference on Silicon in Agriculture.” Silicon 12: 135–138, https://doi.org/10.1007/s12633-019-00107-7
What to look for in a silicon fertilizer
To get silicon into plants, fertilizers must contain a high percentage of soluble silicon in the nutrient solution. Other things to consider when selecting a silicon fertilizer include:
- Potential contamination of the growing medium by unwanted substances in the raw materials
- The cost effectiveness of the product (i.e., price versus silicon concentration)
- Stability during storage and application
And since silicic acid must always be coupled with other chemical compounds or elements to stabilize it enough for mixing, consider agronomic value. That is why Emerald Harvest’s Sturdy Stalk silicon supplement contains potassium silicate. You get the plant-strengthening benefits of silicon, as well as additional potassium for heightening flowering. It is stable in solution, yet it yields silicic acid at the root tips for easy uptake by the plant.
The Emerald Harvest Team
[1] That is, compounds containing silicon.
[2] Silica is another word for silicon dioxide.
[3] Tubana, Brenda S., Tapasya Babu, and Lawrence E. Datnoff. 2016. “A Review of Silicon in Soils and Plants and Its Role in US Agriculture: History and Future Perspectives.” Soil Science 181 (9/10): 393-411, https://doi.org/10.1097/SS.0000000000000179
