Silica Is Everywhere
Silicon is everywhere, in one form or another. The second-most abundant element in the earth’s crust, it bonds readily with oxygen to create some of the most common materials on the planet.
It is the primary component of sand and clay, the basis for granite, amethyst and quartz and the key ingredient in glass, computer chips and fiber-optic cables. It’s even found in toothpaste. It is particularly ubiquitous in the ocean: Plankton and sponges absorb silicon-based compounds and use them to form their microscopic skeletons and spicules.
Given how common silicon is, it may come as a surprise that plants growing in nature do not always have abundant levels of the element available to them. This is because the form in which silicon most often appears in nature, silica, is inert, insoluble and cannot be absorbed. Plants can only take up silicon in the form of silicic acid. (This serves as a catchall term for plant-available silicon, encompassing monosilicic acid, H4SiO4, also known as orthosilicic acid, as well as its oligomers, i.e., disilicic acid and even more complex forms.) Silicic acid is the only water-soluble form of silicon.
The Silicon Cycle
To understand why silicic acid is the only silicon delivery system for plants, it’s important to know how the element is taken up. When silicon compounds meet hydrogen, usually in the form of water, silicic acid forms. Although silicic acid enters plant roots at relatively low concentrations, its concentration increases as it is distributed through the xylem, or vascular tissues, to other parts of the plant. The density of silicon transporters that absorb silicic acid in the roots ultimately determines the amount of silicon taken up by the plant.
Overview of the silicon plant–soil cycle:
- Silicon compounds hydrolyze to produce Si(OH)4
- Si(OH)4 is taken up by the roots
- Si(OH)4 is translocated through the xylem
- SiO2 is polymerized and deposited in plant tissues
- Silicon compounds are deposited in the soil through organic decomposition and other sources
So why not just feed plants silicic acid? Simply put, silicic acid is unstable and undergoes polymerization at high concentrations. Bottled in solution at more than 100 to 200 ppm, it will precipitate, meaning it will surrender its hydrogen atoms and revert to grainy silica. This process works beautifully when silica is deposited in plant tissues, but when it happens prematurely—for example, in a nutrient reservoir—the silicon cannot be taken up by plant roots. This creates a problem for fertilizer manufacturers, who need to supply their customers with a powerful silicon supplement that won’t precipitate in the bottle or after mixing.