Silicon is one of the most abundant elements in the Earth’s crust and a common component of nearly all soils. While not classified with primary nutrients like nitrogen or phosphorus, it is a beneficial element for many plants. The presence of silicon provides advantages that improve plant health, strength, and resilience, influencing everything from the cellular level to the plant’s overall structure and defenses.
The Role of Silicon in Plant Structure
Plants absorb silicon from the soil as monosilicic acid, which is taken up by the roots and transported to the stems and leaves. Once inside the plant tissues, it is deposited within and between the cell walls, particularly in the outer layer known as the epidermis. This deposition process hardens the monosilicic acid into amorphous silica, a hydrated glass.
These microscopic silica deposits, sometimes called phytoliths, create a rigid matrix that strengthens the plant’s cells. This internal fortification makes stems and leaves stronger and more erect, which improves their exposure to sunlight for photosynthesis. It also helps prevent “lodging,” where stems bend or break due to wind, rain, or the weight of their own fruit, which is an important factor for a successful harvest.
Silicon’s Protective Functions
Silicon’s benefits extend beyond structural support to directly protect plants from external threats. The hardened epidermal layer created by silica deposition forms a physical barrier against many pests and diseases. For chewing insects, this reinforced tissue is abrasive and difficult to damage, while sucking insects find it harder to penetrate the strengthened cell walls to access sap. This toughness also makes it more difficult for fungal pathogens, like those causing powdery mildew, to breach the plant’s surface.
Silicon also helps plants withstand non-living stressors. The layer of silica in leaves can reduce water loss through the cuticle, improving drought tolerance. In soils with high concentrations of heavy metals like aluminum or manganese, silicon can interfere with the uptake of these toxic elements, sequestering them in the roots and preventing them from reaching the shoots.
Silicon Accumulators in the Plant Kingdom
While most plants absorb some silicon, certain species known as silicon accumulators actively store large quantities, sometimes in amounts equal to or greater than primary macronutrients. This group is dominated by grasses and grass-like plants, with rice, wheat, sugarcane, and bamboo being prominent examples.
For these accumulators, a high silicon concentration is integral to their growth and defense. Rice, for instance, relies on silicon for the strong stalks needed to support its grain heads and for robust resistance against common pests and diseases that threaten yields.
One of the most well-known silicon accumulators outside of agricultural crops is the horsetail plant (Equisetum). This ancient plant is famous for its high silica content, which gives its stems a characteristically rough and bristly texture.
Applying Silicon in Agriculture and Gardening
Growers in both large-scale agriculture and home gardening can provide supplemental silicon to their plants using a variety of available products.
Common forms of silicon amendments include potassium silicate and calcium silicate. Potassium silicate is a soluble product used in liquid feeding or as a foliar spray, particularly in hydroponics. Calcium silicate, sometimes available as industrial slag, is applied as a soil amendment to build up silicon levels over time. Another choice is diatomaceous earth, a powder from fossilized diatoms, which can be mixed into soil.
The application method depends on the product and the grower’s system. Soil amendments are worked into the ground before planting to ensure a continuous supply. Liquid fertilizers or drenches provide silicon directly to the root zone of established plants. While foliar sprays are used, soil or root applications are more effective because they ensure consistent uptake.