Silicon is the second most abundant element in the Earth’s crust, but it is often deficient in plant-available forms in highly weathered soils. Although not metabolically necessary for most plant species, silicon is recognized as a beneficial element that improves plant health and resilience. Plants absorb this element only in the form of monosilicic acid, the sole soluble form taken up by roots. Understanding the difference between the total silicon content in a soil amendment and the fraction that converts into this plant-available form is the first step toward effective application.
Specific Sources of Silicon for Soil
Silicon is available to growers in both solid, slow-release materials and concentrated liquid formulas. Solid amendments are typically applied to the soil and rely on natural breakdown to release monosilicic acid over time. Diatomaceous Earth (DE) is a common solid source, composed of the fossilized remains of diatoms that accumulate high concentrations of amorphous silica. The fine particle size of DE promotes faster conversion into the absorbable acid form.
Calcium Silicate, often sold as slag, is another long-lasting solid amendment that can also provide a liming effect to raise soil pH. For those seeking an organic, naturally occurring source, rice hull ash (RHA) is a byproduct of rice milling that contains a very high percentage of silica, ranging from 85% to 98%. The rate of silicon release from RHA is slow, making it an excellent material for building up long-term silicon reserves in the soil.
Liquid concentrates, such as Potassium Silicate (K₂SiO₃), offer a highly soluble source of silicon that is immediately available for plant uptake. When dissolved in water, potassium silicate hydrolyzes to release monosilicic acid and potassium, making it suitable for modern irrigation systems or foliar feeding. Stabilized monosilicic acid products are also available; these are the most bioavailable option because the silicon is already in the plant-ready form, bypassing the need for chemical conversion.
Techniques for Applying Silica Amendments
Application method depends on the product form, with dry materials requiring soil blending and liquids allowing for drenching or foliar application. For dry amendments like Diatomaceous Earth or calcium silicate, the material must be thoroughly incorporated into the root zone before planting. A typical recommendation for bulk soil is to blend the dry silicate at a rate of approximately 5 pounds per 10 square feet, mixing it into the top 6 to 12 inches of soil. For container growing, incorporating the amendment at a rate of 10% by volume into the potting mix provides a substantial and long-lasting silicon reserve.
Liquid potassium silicate products are highly alkaline (often above pH 11), requiring careful handling and application. When preparing a tank mix, the silicate should be added to the water first and mixed thoroughly before introducing any other nutrients or pH adjusters. This initial mixing prevents the silicate from reacting prematurely with other concentrated fertilizers, which could cause precipitation or “gelling”. For a general soil drench, a common rate is about 1 to 2 milliliters of concentrate per liter of water, applied weekly or with every watering.
Foliar application delivers silicon directly to the leaf surface, offering quick protection against pests and diseases. The recommended dilution for foliar spraying is often lower than a drench, typically around 2 to 3 milliliters of concentrate per liter of water. Because silicates are highly basic, the mixed solution’s pH should be adjusted down to a range of 5.5 to 6.5 using a pH-down product, like phosphoric acid, to optimize plant absorption and prevent leaf burn. Foliar sprays are most effective when applied to all plant surfaces, especially the undersides of the leaves, and are best performed in the early morning or late evening to prevent the solution from drying too quickly.
How Silicon Enhances Plant Resilience
Once absorbed as monosilicic acid, the element travels through the plant’s vascular system and is deposited in the epidermal cell layers. This process, known as silification, creates a hardened, amorphous layer of silica directly beneath the cuticle, forming a mechanical barrier. This physical layer strengthens the plant’s architecture, providing structural support that reduces the risk of lodging (the bending or breaking of stems under wind or heavy fruit load).
The silica layer also mitigates various environmental challenges, collectively known as abiotic stress. By depositing in the leaf tissues, silicon can help to reduce excessive transpiration, which is the loss of water vapor through the leaves. This improved water retention enhances the plant’s tolerance to heat, drought, and salinity stress. For example, silicon helps to counteract salt stress by reducing the uptake and translocation of toxic sodium ions into the shoot.
The hardened epidermal layer is particularly effective in providing a mechanical defense against various biotic stressors. The tough silica cuticle acts as a physical deterrent, making it more difficult for piercing-sucking insects to penetrate the plant tissue. Similarly, the layer serves as a barrier against fungal pathogens, hindering the ability of fungal hyphae to successfully enter and colonize the plant cells.