Silica supplements are a popular addition to nutrient regimens, making plants stronger and less susceptible to environmental and pathogenic damage.
Silica (silicon dioxide, SiO2) i s also commonly known as quartz. Large quartz crystals (which can be quite large) are collected for their beauty and for use in jewelry. Diatoms (microscopic hard-shelled single-celled algae) absorb silica to form shells. When diatoms die, they leave the silica shells behind, which is a substance known as diatomaceous earth. Many sands are comprised of mostly silica, which is also a primary ingredient in clay, Portland cement, ceramics, and glass.
Silica is also good for plants, especially for monocotyledons (also known as monocots and are plants with a single seed leaf), but also beneficial to many dicotyledons (also known as dicots, and are plants with two seed leaves). Grasses and grains in particular tend to accumulate silica.
While not generally considered a necessary nutrient for most vegetation, if present in a usable form many plants will absorb silica, and deposit it in their cell walls–forming structures called phytoliths. These phytoliths act in part to reinforce the cell walls in much the same way steel rebar is used to reinforce cement walls. Phytoliths are durable enough to last well beyond the life of the plant, and have been used in plant archeology to identify plants at historical sites.
Some of the other (but possibly related) benefits of silica to plants include improved resistance to damage from fungal pathogens such as powdery mildew. Silica can tie up excess amounts of some elements such as aluminum, manganese and zinc reducing the likelihood of metal toxicity. It mitigates some of the issues from salts, dehydration and heat stress. It has also been shown to improve the reactions of some natural defenses.
In nature, water interacts with the quartz in soil to continuously make small quantities of silicic acid. Wearing down quartz with water can take quite some time, so silicate additives can be used to increase the amount of available silica. Silicic acid on its own is not particularly stable, and plant additives often use potassium silicate (K2SiO3) and/or sodium silicate (Na2SiO3), and/or calcium silicate (CaSiO3) in an alkaline solution. When the solution is added to the slightly acidic environment of a nutrient solution, it breaks the salts and creates silicic acid (and releases sodium and/or potassium and/or calcium). This is why silica additives generally have a liming effect, raising pH (also known as “sweetening”). Plants take up silica in the form of silicic acid (H4O4Si).
Although plants grown in the ground may have sufficient quantities of silicic acid available (depending on the quality of the soil) in soil-less mediums and in hydroponic gardens, often the only silica available is what the gardener adds, and a silica supplement can be helpful in meeting that need.