Silicon is one of the most abundant elements found in soil, comprising 50-70% of soil mass. Some plants have been shown to accumulate large amounts of silica in their tissues. In fact, silica can often accumulate in plant tissues to higher levels than calcium, magnesium and phosphorus. But because of the relative abundance of silica in quartz-based sand and its lack of obvious deficiency symptoms in plants, agronomists have largely ignored the use of silicon as a fertilizer. Lately, however, some agronomists are taking a second look at it. Although silicon is still not recognized as an “essential” element, its beneficial effects on plant growth are well documented and include improving stress tolerance, disease resistance and yield for a wide range of plants.
Silica additives are especially beneficial in hydroponics applications. Concentrated hydroponic nutrient formulas do not contain silicon. Silicon has a strong negative charge and forms a glue-like compound when added to the concentrated mineral salts used in hydroponics. So silica has to be added to the reservoir separately from other nutrients. In the past, most hydroponics growers used potassium silicate as their silicon source. Potassium silicate is very soluble and it is an excellent source of available silicon. Unfortunately, potassium silicate is also highly alkaline. Even small amounts can cause the pH of the nutrient solution to rise dramatically, potentially locking out other important trace elements such as iron, copper, manganese and zinc. When using unbuffered water sources such as reverse osmosis (RO) water, the pH problems caused by potassium silicate can be amplified.
Fortunately, many growers are now discovering the benefits of flowable silicon as an alternative to potassium silicate. Flowable silicon is pH neutral and it is derived from natural sources of silicon dioxide. The best source of flowable silicon comes from clean, finely-powdered diatomaceous earth. Diatoms are microscopic organisms from the sea and they naturally accumulate high levels of silicon in their cell walls. The natural silicon is known as “biogenic” silica. Water-soluble silicon is removed from the water through a biological process as the diatoms grow and multiply. When the diatoms die, the silicon dioxide dissolves back into the water as part of the natural silicon cycle. So the silica skeletons of fresh-water diatoms are a rich source of silicon for plants.
Flowable silica offers both a quick-release and slow-release form of silicon for plant roots. About 30% of it dissolves within the first few hours of mixing and the rest of the silica remains in suspension and continues to flow over the roots. Some of the silica is trapped by the roots and developing root hairs, where it embeds itself in the cell walls. The embedded silicon aids in the uptake of water, while simultaneously protecting the plant from toxic levels of heavy metals. As the silica continues to dissolve, it is transported to the stems and leaves, where it has a strengthening effect on cell walls. The extra silica helps protect the plant against fungi such as powdery mildew and it plays an important role in the systemic acquired resistance of plants against many pests and diseases. Silica also improves the plant’s tolerance to temperature extremes. Best of all, flowable silica is pH neutral (7.0-7.2), making it friendly to both plants and plant-growth-promoting microorganisms in the root zone.
Harley Smith is a free-lance contributor to Garden & Greenhouse.