We all know that there are 6 macro plant nutrients (nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S)) and 10 micro plant nutrients [boron (B), Chlorine (Cl), zinc (Zn), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), nickel (Ni), sodium (Na), and cobalt (Co). Researchers with the USDA-ARS are taking the position that silicon, recently called quasi-essential, is in fact essential for all plants. Research has also shown it provides significant beneficial effects that all gardeners and greenhouse growers should not ignore.
Silicon is the second most common element found in the Earth’s crust. It is the major component of every soil on the planet in the form of sand or quartz crystals. This does not mean that silicon is readily available to the plant. It is absorbed as silicic acid, H4SiO4, by plant roots and is transported through the xylem to stems and leaves.
The way silicon works in plant tissue is not fully understood. We know that silica polymers are deposited in xylem vessels, epidermal cell walls and the base of leaf hairs where water normally exits the plant. This translated into toughening the plants own natural immune system against insects, pathogens and various environmental stresses.
Strengthened cell walls make plants more resistant to insect infestation because it becomes more difficult for
insects to penetrate, puncture, suck, chew, feed and later digest plant material. Stronger plant cells also create a barrier that prevents water loss (transpiration) and fungal penetration. This is why we see plants treated with silicon supplements having less powdery mildew and are less susceptible to drought stress. Treated plants also demonstrate a greater tolerance to cold and heat. To receive the greatest benefit from silicon, it must be applied throughout the plants’ life. Similar to calcium, silicon is absorbed and is set in place in newly growing tissue. It will not translocate like other nutrients if a deficiency were to arise.
Not all plants accumulate significant amounts of silicon. Traditionally it was held that only monocots benefited from the presence of silicon. It is true that grasses such as rice, sugarcane, bamboo and certain perennials accumulate up to 10% of their dry weight as silicon. Current research has shown that even non-accumulating plants with dry mass levels below 0.5% still benefit from silicon because it accumulates in specific tissues. A good understanding of how it accumulates during your crops development is necessary in order to reap the greatest benefit.
It has been reported that silicon has produced increased growth in roses, certain daisy varieties and cucumbers. Increased resistance to pathogens and pests has been reported in greenhouse tomatoes, cucumbers, roses, zinnias and chrysanthemums. It has been found to increase resistance to fungal diseases like powdery mildew in cucumbers and zinnia, blackspot in roses, and Pythium in tomatoes and cucumbers. It has also shown to improve macro-nutrient uptake in non-accumulator plants. An example is reducing poinsettia bract necrosis, a problem linked to calcium deficiency. In chrysanthemums, leaf miner populations were reduced when the concentration of silicon in plant leaves reached 0.4 percent. Silicon-induced resistance to pathogens and insect pests is very important to greenhouse growers if they are practicing integrated pest management and trying to reduce the use or eliminate the use of pesticides.
The USDA-ARS Greenhouse Production Research Group in Toledo, Ohio, has conducted studies on greenhouse crops including begonia, carnation, cucumber, geranium, impatiens, marigold, orchid, pansy, petunia, snapdragon, verbena and zinnia, were grown hydroponically with generous amounts of soluble silicon provided to determine silicon uptake levels by various plant species. Results showed that zinnia, cucumber and New Guinea impatiens are accumulators, while petunia and pansy are not. Another study conducted by the same USDA-ARS group showed that chrysanthemum plants treated with silicon reached a maximum accumulation level in excess of 12%. All greenhouse growers should take data like this into consideration and do their own research on which crops have a positive response to silicon.
Potted plants using media made mostly of peat or coconut coir have no soluble silicon. Some brands of potting mixes advertise that they have added silicon to their mix. Even though this is a good addition, like all added nutrients, it will be leached out or taken up and fixed in plant tissues after a few waterings. More stable forms like rice hulls, for example, are not readily available to the plants until they are broken down by soil borne microorganisms. Since few if any microorganisms are found in typical soilless media, rice hulls will not provide any significant amount of soluble silicon. Silicon from sand and quartz crystals is found in many soils, but are insoluble and therefore unavailable to plants.
The easiest method of adding it to a feeding regimen is to apply a supplement that contains soluble silicate. Potassium silicate should be applied to the growing medium for best results. Potassium silicate provides the benefit of being a source of additional potassium. It is highly alkaline, so one must refer to the manufacturers recommendation as to pH adjustments needed and tank mix compatibility. Foliar applications in greenhouse crops are common but are generally less effective than drenching. There is no phytotoxicity from Si.
Alex Sevilla is the Vice President of Sales & Marketing for Dyna-Gro.