Silicon tops the list of beneficial elements most often used by gardeners. Silicon is not an essential element—it is not required for the normal growth and development of most plants, and it is not one of the 14 mineral elements found in concentrated, full-spectrum hydroponic nutrient formulas. In fact, it would be difficult, if not impossible, to include silica directly in a concentrated nutrient formula.
How do I know? One time I tried mixing potassium-silicate concentrates with fertilizer concentrates just to see what would happen, and it turned into glue right before my eyes. I ended up throwing the whole measuring cup away since I couldn’t even clean out the sticky goo from the mixing container.
So, silica should be considered an additive, not a primary nutrient. Although silica is not essential for plant growth and development, it can provide many benefits. It helps strengthen cell walls, guards against environmental stress, improves the uptake of water and minerals and adds an extra level of protection against fungal diseases such as powdery mildew.
Commercial cucumber growers often add 50 to 100 ppm of silica to their water before adding fertilizer, and they continue to provide silica throughout the crop’s growing cycle. If the plant comes under attack by a fungus, silica is mobilized to the infection site to harden the surrounding tissues and help prevent the spread of the disease.
The most common source of silicon in the gardening industry is potassium silicate. It is water soluble and completely available to the plant; the downside is that it is also alkaline, and often significantly raises pH. In fact, I often use potassium silicate specifically for pH adjustment. If I happen to add too much acid when adjusting the pH of my hydroponic nutrient formula, I add a little potassium silicate to bring the pH back up. At least, I tell myself, the silica is helping my plants, not just correcting my mistake, but most of the time I can’t justify using potassium silicate in my reservoir since my well water is highly alkaline to begin with. If I add potassium silicate and the pH rises much above 7, there is a risk of locking out important trace elements such as iron, copper, manganese and zinc.
Fortunately, there is now an alternative to potassium silicate—biogenic silica. Biogenic silica is derived from a natural source of silica and is pH neutral. With a pH of about 7 to 7.2, it is plant friendly and doesn’t lock out essential elements. Biogenic silica originates from living, fresh-water diatoms (a type of algae).
During their life cycles, microscopic diatoms pull silicic acid out of the water and store large amounts of crystallized silica in their outer cell walls. When the diatoms die, much of the silica dissolves back into the water and the rest of it collects in sediments to be recycled by geologic forces.
Eventually, some of the silica returns to land in pockets of pure, biogenic silica. The silicates mined from these clean, rich deposits are a gardener’s dream, providing both water-soluble and flowable biogenic silica in a form that is beneficial to plants and plant growth-promoting micro-organisms.
Although biogenic silica is derived from diatoms, it should not be confused with the diatomaceous earth used in insect control. Diatomaceous earth is the fossilized remains of diatoms, which have sharp edges that tear up insects crawling over them.
The more highly transformed biogenic silica from volcanic sources does not have sharp edges. In fact, the biogenic silica particles are smooth and porous, with a fine matrix of tiny holes that can filter water as it passes through them.
As the water flows through the particles, mineral ions adhere to silica surfaces, releasing the minerals to plant roots on demand. So, biogenic silica is not just a great source of plant-available silica, it is a reservoir for other important nutrients, making trace elements more available to the plant instead of locking them up.
Since biogenic silica comes from a natural source, it is not 100% water soluble. It is usually provided as a dry powder with the consistency of talcum powder. When added to water, about 25 to 30% of the silica dissolves into various forms of silicic acid and becomes immediately available to the plant.
The rest of the silica is flowable—becoming trapped in the root zone to provide a slow-release form of silica and other beneficial minerals. The flowable particles are small enough that they won’t clog emitters, but they must be stirred up to keep them suspended in water. If they are not stirred, some of the fine particles will settle out of the solution in a few hours.
Silica is great for plants, but the greatest benefit of biogenic silica is its effect on plant growth-promoting bacteria in the root zone. Beneficial bacteria thrive at a neutral pH, so biogenic silica makes a perfect home for bacteria that colonize the root zone. In fact, biogenic silica is actually one of the best carriers for microbial blends used to inoculate plant roots.
As the fine particles of silica embed themselves in the root zone, the microbes use the particles as launching stations to the developing roots. In addition, the trace elements that adsorb to the surfaces of the silica particles are readily available to microbes, speeding up their metabolism and providing a biological bridge to the roots.
Along with the microbial-enhanced silicon and trace elements, micro-organisms provide vitamins, enzymes and growth hormones that stimulate root development and provide a nutritious soup of natural plant protection agents. Therefore, plant growth-promoting bacteria in the root zone, in conjunction with the biogenic silica, greatly enhance the plant’s natural resistance to stress and disease.
Silicon is most beneficial to plants that accumulate high levels of extra silica in their tissues. In rice, for example, silica is already considered an essential element, and many grasses and other monocots are significant silicon accumulators.
In contrast, most vegetables and other dicots accumulate only trace amounts of silica, so the benefits of synthetic, water-soluble silica are often limited. Biogenic silica, on the other hand, provides water-soluble silica when plants need it the most, and it provides a host of additional benefits during the rest of the growing cycle.