Silicon (Si) is the second-most abundant element in the Earth’s crust, but most of the Si in soils is in the form of silicate minerals such as quartz, which are unavailable to plants. Plants take up Si primarily in the form of orthosilicic acid (also known as monosilicic acid), but polymeric forms of this silica acid can be taken up as well. This soluble fraction of silica in the soil is dependent on pH and redox conditions. At normal pH conditions, silicic acid concentrations in the soil range from 0.1 to 0.6 millimolar (mM). Above pH 9, silicic acid becomes much less available.
Silicon
A Vital Component for Strengthening and Supporting Plant Tissues.
Silicon in the Soil
Silicon in the Plant
Although Si is considered non-essential for plant growth, it has been shown to provide numerous benefits. Silica supplementation has been shown to boost plant growth and nutrient uptake, and improves tolerance to abiotic stresses such as drought, salinity, and heavy metal toxicity. Plants vary in their Si uptake, but once absorbed by the plant, Si is transported through the xylem and forms complexes with cell wall components including pectins, polyphenols, polysaccharides, and lignins. These Si-organic complexes strengthen plant tissues, reduce water transpiration, and prevent infections by numerous plant pathogens by thickening the leaf cuticle. Some plants like rice can have up to 10% Si content in their shoots.
The majority of Si taken up by plants becomes amorphous silica structures known as phytoliths. These phytoliths can be found in both the cytoplasm and vacuole of plant cells, and they are found everywhere from the root to the shoot. Phytoliths in plant tissues make it difficult for insects to eat and digest the plant matter, reducing the growth rates of pests. Plants with high concentrations of Si also show higher expression of genes related to defensive enzymes. When the plant dies and decomposes, these phytoliths serve as a source of bioavailable Si for other organisms.
Figure from Leroy, N., de Tombeur, F., Walgraffe, Y., Cornelis, J.-T., and Verheggen, F. J. (2019).
Silicon and plant natural defenses against insect pests: impact on plant volatile organic compounds and cascade effects on multitrophic interactions. Plants.
Increasing Silicon Levels
Silica is associated with plant cellular structure, electrical exchanges, and nutrient-to-nutrient ionic bonding. It has also been associated with correcting hollow stem syndrome, in addition to increasing nutrient uptake. Optimum silicon levels can be achieved through soil and plant amendments via foliar or soil application.
Further Reading
Bocharnikova, E. A. and Matichenkov, V. V. (2012). Influence of plant associations on the silicon cycle in the soil-plant ecosystem. Applied Ecology and Environmental Research.
de Tombeur, F., Roux, P., and Cornelis, J.-T. (2021). Silicon dynamics through the lens of soil-plant-animal interactions: perspectives for agricultural practices. Plant and Soil.
Greger, M., Landberg, T., and Vaculik, M. (2018). Silicon influences soil availability and accumulation of mineral nutrients in various plant species. Plants.
Kostik, L., Nikolic, N., Bosnic, D., Samardzic, J., and Nikolic, M. (2017). Silicon increases phosphorus (P) uptake by wheat under low P acid soil conditions. Plant and Soil.
Leroy, N., de Tombeur, F., Walgraffe, Y., Cornelis, J.-T., and Verheggen, F. J. (2019). Silicon and plant natural defenses against insect pests: impact on plant volatile organic compounds and cascade effects on multitrophic interactions. Plants.
Nawaz, M. A., Zakharenko, A. M., Zemchenko, I. V., Haider, M. S., Ali, M. A., Imtiaz, M., Chung, G., Tsatsakis, A., Sun, S., and Golokhvast, K. S. (2019). Phytolith formation in plants: From soil to cell. Plants.
Rajput, V. D., Minkina, T., Feizi, M., Kumari, A., Khan, M., Mandzhieva, S., Sushkova, S., El-Ramady, H., Verma, K. K., Singh, A., van Hullebusch, E. D., Singh, R. K., Jatav, H. S., and Choudhary, R. (2021). Effects of silicon and silicon-based nanoparticles non rhizosphere microbiome, plant stress and growth. Biology.
Zargar, S. M., Mahajan, R., Bhat, J. A., Nazir, M., and Deshmukh, R. (2019). Role of silicon in plant stress tolerance: opportunities to achieve a sustainable cropping system. 3 Biotech.
