GET CULTURED: Silicon in plant nutrition
by Don Merhaut
Silicon has received attention in the plant community because its use in production may increase crop resistance to disease and insect attack. The possible benefits of silicon in plant growth and development are primarily related to increased disease resistance and tolerance in some plants, especially grasses such as oats and rice. Silicon may also increase the strength of plant cell walls and increase resistance to environmental stress (i.e., reduce water loss).
Silicon is a non-essential element in plant nutrition. For any element to be considered essential, it must be required for a plant to complete its life cycle, required by all plants, and not completely replaceable by another element. (These three criteria are still up for debate in the scientific community.) Even though silicon is not considered a plant nutrient by these criteria, it still has been documented to have beneficial effects on plant growth.
Silicon is the most abundant element in the soil, averaging 54%. However, organic soils have the lowest amount of silicon. The solubility of silicon increases with increasing soil solution pH. Silicon will also interact with other elements such as iron, aluminum, phosphorus and calcium. At lower soil pH, silicon and phosphorus can react and precipitate out of solution.
Plant Uptake and Metabolism
Silicon is taken up passively into plants in the form of silicic acid (H4SiO4), silica anions, or silicon-organic complexes. In rice, silicon has also been documented to be actively taken up into root systems. However, some plants may actually restrict silicon uptake. In the root system, silicon may interact with other nutrients: silicon may increase the uptake of phosphorus and molybdenum, but may inhibit the uptake of boron, manganese and iron. Once in plants, silicon is transported, via the driving force of transpiration, into the leaves.
Environmental Stress Resistance
Silicon is incorporated into cell walls, which will increase cell wall strength and structure. In some plants, silicon may also accumulate within the leaf epidermis, forming silicate “knobs.” These knobs may reduce water loss that occurs from leaf surfaces. In addition, the accumulation of silicate compounds near stomata openings may reduce water loss caused by transpiration. Finally, silicon accumulation in the cell walls of xylem vessels may reduce or limit the compression of vessel elements during periods of high transpiration rates, thus allowing better water flow through the xylem.
Disease resistance from silicon may occur in two ways: physically or biochemically. The physical attributes of silicon are associated with increased cell wall strength, since silicon compounds are laid down in cell walls. The formation of specialized silicon-containing knobs that form on the leaves of some plants may also be a physical deterrent for insect feeding and some pathogens. The biochemical properties associated with silicon are more complex. Silicon may induce certain chemical defense reactions in response to pathogen or insect attack. These responses have been observed in both dicots (broadleaf plants) and monocots (such as grasses).
Sources of Silicon
Silicon can be derived from clay, quartz and other minerals; however, solubility is very low. The most common sources of silicon are calcium silicate and potassium silicate. The high solubility of potassium sulfate renders it suitable for hydroponic systems. However, always check for fertilizer incompatibility to prevent costly precipitation of fertilizer sources.
Use of Silicon in Production
The benefits of silicon in plant production are not well understood. However, if you are considering the use of silicon in crops, do a less-costly, small pilot study. As with other horticultural management programs, always consider stage of crop development and environmental factors. Since silicon is taken up in the transpiration stream, applying silicon during new active vegetative growth will probably maximize silicon accumulation in the developing tissues. Once cell walls have been laid down, there will probably be limited, if any, silicon accumulation in cell walls. Since silicon levels are low in organic soils, and container production is almost 100% organic substrates, there is a possibility of a benefit from silicon fertilization. However, keep in mind that silicon does interact with other nutrients, so a deleterious effect may also occur in certain situations.
Don Merhaut is a UC Cooperative Extension Specialist for Nursery and Floriculture Crops, Department of Botany and Plant Sciences, UC Riverside.