Silicon, often referred to as silica, is the second most abundant element in the Earth’s crust, yet it is frequently overlooked in routine plant care. While not classified as an essential nutrient for the majority of plant species, it is widely recognized as a beneficial element. The presence of silicon can significantly improve plant health and resilience, particularly when plants are grown under environmental stress. For plants to utilize this element, the silicon dioxide (SiO2) commonly found in nature is converted into its soluble form, monosilicic acid, which is the only type plants can absorb. Understanding the commercial forms available is necessary to effectively incorporate silica into any growing regimen.
The Role of Silicon in Plant Physiology
Plants take up silicon from the soil solution as monosilicic acid via specialized transporters in their roots. Once absorbed, the plant transports it through the xylem and deposits it primarily in the epidermal cells of leaves and stems. This process involves the polymerization of monosilicic acid, which condenses into a hard, amorphous silica gel, forming a double layer beneath the cuticle.
This physical deposition of silica enhances the structural integrity of the plant by fortifying the cell walls. The resulting mechanical barrier provides increased rigidity to stems and leaves, making the plant tissues stronger and more upright. This stronger architecture helps plants withstand external pressures like wind, lodging, and the weight of heavy fruits.
The protective layer also improves resistance to various abiotic stresses. By reducing water loss through transpiration, silicon helps plants maintain turgor and manage water resources more efficiently during periods of drought or low humidity. Furthermore, silica application can mitigate the negative effects of high salinity and temperature extremes by enhancing antioxidant responses. This physical hardening of the plant surface also forms a defense against piercing or chewing insects and certain fungal pathogens.
Commercial Forms of Plant-Available Silica
The commercial market offers several distinct formulations to deliver plant-available silicon, varying in solubility and speed of action. The most common liquid option is Potassium Silicate, a highly alkaline concentrate formed by dissolving silica sand and potassium carbonate. This form is very soluble and provides both silicon and potassium. However, its high pH, often between 11.3 and 12.3, requires careful adjustment to avoid nutrient lockout when mixed with other fertilizers.
Another liquid option is stabilized Silicic Acid, which is a direct source of the plant-available form, monosilicic acid. Manufacturers use specialized stabilizing agents to prevent the silicic acid molecules from polymerizing into an insoluble form. These products are fast-acting and require much lower application rates compared to silicate solutions, as the plant can immediately absorb the silicon.
For growers preferring a slower-release, solid amendment, Diatomaceous Earth (DE) and calcium silicate products are widely used. Diatomaceous earth is composed of the fossilized remains of diatoms, which are rich in amorphous silica. When applied to soil, DE gradually releases soluble silicon as it breaks down, also helping with soil aeration and moisture retention. Calcium silicate, often derived from steel mill slags, serves a dual purpose by supplying silicon and acting as a liming agent to neutralize acidic soil.
Practical Application Techniques and Dosing
The method and rate of application depend on the product form and the specific growing environment. For liquid concentrates, particularly potassium silicate, it is recommended to mix the silica first in the water before adding any other nutrients. This is important because the high alkalinity will significantly raise the solution’s pH, which must then be lowered to the optimal range of 5.5 to 6.5 for nutrient uptake.
Foliar Spray
Applying silica as a foliar spray allows for rapid, direct absorption through the leaf surface, which is beneficial during times of immediate stress. For stabilized silicic acid products, dilution rates are typically very low, resulting in concentrations of 7 to 45 parts per million (ppm) of silicon. Potassium silicate is usually diluted to a 0.2% to 0.5% concentration for foliar application and applied every 7 to 14 days. A wetting agent or surfactant should be added to the mix to ensure even spreading and adherence of the solution to the leaves.
Soil Drench and Irrigation
When applying liquid silica products as a root drench or through irrigation systems, the goal is to maintain a consistent supply of monosilicic acid in the root zone. For hydroponic systems, a target concentration of 50 to 200 ppm of silicon in the reservoir is recommended. Commercial guidelines often suggest a dilution rate of 1 to 2 milliliters of concentrate per gallon of water. Due to the high pH of potassium silicate, it must be added to the water first and thoroughly mixed before the main nutrient solution is introduced. For soil-based plants, a typical drench application involves a concentrated solution applied directly to the root zone.
Solid Soil Amendment
Granular products, like diatomaceous earth or calcium silicate, are best utilized as a long-term soil amendment rather than a quick fix. For container gardening, powdered silicon can be mixed into the potting soil at a rate of approximately 1/2 to 1 cup per 8 gallons of soil volume before planting. In garden beds, it can be spread over the surface and incorporated into the root zone at rates such as 1 cup per 4 row-feet. These solid forms provide a slow, steady release of plant-available silicon over time and also help improve the soil’s physical structure.