Silicon, the second most abundant element in the Earth’s crust after oxygen, is a common component of mineral soils. While not universally classified as an essential nutrient for all plants, its widespread beneficial effects on plant vitality and resilience are increasingly recognized across many plant species. Plants can accumulate silicon in amounts comparable to macronutrients like phosphorus, nitrogen, and calcium. This element plays a substantial role in enhancing overall plant health, even for species that can complete their life cycle without it.
Role of Silicon in Plant Health
Silicon contributes to plant structural integrity by strengthening cell walls, leading to increased rigidity and reduced lodging, which is the bending or breaking of stems. It is deposited in various plant parts, including roots, leaves, stems, fruits, and grains, where it reinforces tissues and enhances natural defense properties. This reinforcement helps plants maintain an upright posture, which improves light capture for photosynthesis and overall growth.
Beyond structural benefits, silicon bolsters plant defense mechanisms against pests and diseases. It forms a physical barrier within cell walls, making it harder for piercing-sucking insects to penetrate and restricting the growth of fungal hyphae. Silicon also stimulates biochemical defenses, such as the accumulation of lignin, phenolic compounds, and defense phytoalexins, further enhancing resistance to pathogens.
The element also improves plant tolerance to various abiotic stresses, including drought, heat, cold, strong winds, salinity, and heavy metal toxicity. For instance, silicon can reduce water loss by decreasing transpiration from the cuticle and improving water uptake efficiency. It aids in detoxifying heavy metals by reducing their uptake and translocation, and by stimulating antioxidant systems. Silicon can also enhance the absorption of beneficial elements such as zinc, calcium, and nitrogen, while regulating phosphorus uptake.
Forms and Uptake of Silicon by Plants
Plants primarily absorb silicon from the soil solution as monosilicic acid, also known as orthosilicic acid (Si(OH)4). This is the soluble form of silicon that plant roots can readily take up. Silicon exists in various forms in nature, including silicon dioxide (silica, SiO2) and various silicates, but most of these forms are unavailable for plant uptake until they are converted into monosilicic acid.
Commercial silicon products for plant application often contain forms like potassium silicate or calcium silicate, which break down into monosilicic acid when applied. Diatomaceous earth, composed almost entirely of silicon dioxide, is another source that provides soluble silicon to plants. The solubility of these different forms dictates their availability for plant absorption. While passive uptake through mass flow and diffusion occurs, some plants, like rice, exhibit an active ability to concentrate silicon from the external solution. Once absorbed, monosilicic acid is transported in the xylem and deposited in plant tissues, often as amorphous silica, also known as phytoliths, which are microscopic particles of hydrated silicon dioxide.
Applying Silicon for Plant Growth
Silicon can be administered to plants through various methods, including soil drenching, foliar spraying, and incorporation into hydroponic nutrient solutions. Soil application is considered effective, with products like calcium silicate or potassium silicate being common choices. For instance, calcium silicate can be applied to soil as a liming material, which also provides silicon.
Foliar application, while used, may be less effective for some plants compared to soil application, though it has shown benefits against powdery mildew in certain crops. When using silicon fertilizers, it is important to follow product-specific instructions regarding application rates and frequency, as these can vary depending on the plant species and cultivar. For a continuous supply, consistent application is suggested; however, exceeding high concentrations may cause issues in some non-accumulator plants. Silicon fertilizers can be alkaline, increasing the pH of stock solutions, which may reduce the solubility of micronutrients and cause precipitation, so using separate stock tanks for silicon and standard fertilizers is an advisable practice.
Signs of Silicon Imbalance
While true silicon toxicity from over-application is rare in plants, insufficient silicon can lead to noticeable symptoms. A lack of silicon can result in weakened stems and increased susceptibility to lodging, where plants fall over. Plants may also exhibit soft, droopy leaves, which can reduce photosynthetic activity and ultimately lower yields.
Silicon-deficient plants show heightened vulnerability to pests, such as spider mites, aphids, and whiteflies, and diseases like powdery mildew, blast, and brown spot. This increased susceptibility is due to the lack of a strong physical barrier within cell walls and reduced stimulation of defense mechanisms. Silicon deficiency can also lead to malformed leaves, retarded growth, and issues with fruit set or malformed fruit.