Silicon in Agriculture

Silicon

Silicon (Si) is the eighth-most abundant element in the universe and the second-most in the soil. It should be no surprise that this element provides many chemical and physical benefits to plants. However, most Si in the earth is bound to materials such as silicon dioxide (silica, SiO2) and various silicates, all unavailable to plants. The form of silicon determines whether or not it is plant-available. Crystalline silica found in quartz is not easily mineralized. Amorphous silica has a more relaxed structure than crystalline silica and, as a result, possesses higher solubility. Silicon becomes bioavailable to plants once converted to monosilicic acid (also called orthosilicic acid). The concentration of Si in the soil solution ranges from 3.5 and 40 parts per million (ppm, mgᐧL-1) but is generally between 14 to 20 ppm (Marschner, 2012). However, the mineralization of Si is slow and is often not replaced or supplemented, resulting in deficient amounts of Si. Supplying supplemental silicon can help growers reduce their use of other chemical and fertilizer inputs.

Silicon: A Plant Nutrient

All plants require and accumulate silicon. Plant absorption of Si varies from 100 to 100,000 ppm. Silicon is deposited in cell layers in both root and foliar tissue, increasing plant tissue mass. Stronger epidermal layers shield roots from factors such as drought and disease. Silicon also promotes Casparian strip formation (Fleck et al., 2015), improving water use efficiency. Silicon deposited under the cuticle reduces water loss and aids resistance to drought and water-restricted situations. 

Apply silicon during all crop growth stages. Silicon is applied to agronomic and horticultural crops grown in the field and greenhouse production. It can be applied directly to the soil via fertigation or foliarly. Supplying Si from 30 to 60 ppm can encourage uptake and utilization of essential nutrients such as nitrogen (Mohanty et al., 2019), phosphorus (Neu et al., 2017), and calcium (Silva et al., 2021). The addition of silicon can help plants respond to various abiotic stresses too. Silicon induces the reactive oxygen species pathway, part of a plant’s innate defense mechanism. Doing so helps plants respond faster to stress. Silicon also stimulates phenolic compounds, protecting plants from pathogens and environmental stressors. Silicon can also alleviate the effects of heavy metal toxicity, such as copper (Flora et al., 2019). Concerned about toxicity? That is understandable, but rest assured that plants absorb only what they need. Currently, there are no known toxicity symptoms associated with Si for plants within a reasonable silicon range. Silicon can tie up micronutrients at concentrations greater than 1,000 ppm. If micronutrient concentrations are low, high Si can cause nutrient deficiency symptoms to appear. 

Sources of Silicon

Silicon is available in solid and liquid forms as soil amendments, soil conditioners, or fertilizers. Examples of soil-applied silicon include calcium silicate from natural sources such as wollastonite and slag, a by-product of the steel industry. Other natural sources of silicon include diatomaceous earth and rice hulls. Using natural sources of silicon can be enticing. However, the composition and concentration of natural sources vary, and they may not release Si at the same rate to meet the plant’s needs (Frantz et al., 2010). Liquid sources of Si include monosilicic acid, silica nanoparticles, and silicates of calcium, potassium, or sodium. These sources are used in both field and greenhouse production as a drench or foliar spray.

Silica nanoparticles, also called nanostructured silica, are a promising silicon source. Nanostructured silica is an intentionally manufactured silicon dioxide used in food and pharmaceuticals. Its primary particle dimensions are below 100 nm, lending to its nanostructure classification. Nanostructured silica is entirely amorphous, contains no quartz, and is nonhazardous. It has a lot of similarities with some types of biogenic silica, the silica found as phytoliths in plants.

Testing for silicon

Soil testing for Si remains an option rather than a routine practice, partly due to the lack of agreement on which procedures to use (Zellner et al., 2021). Another reason is the lack of knowledge pertaining to the optimal concentration of Si for soils and crops. Few labs test for the concentration of silicon in plant tissue. Some labs claim to test for silicon, yet they use an acid extraction, which is incorrect and provides erroneous results. Instead, an alkaline extraction is used to correctly determine silicon concentration in plant tissue. Be sure to consult with the laboratory before submitting tissue samples. Before implementing silicon, determine a baseline silicon concentration for your soil and crop. After implementing silicon, note of any changes in your plants and retest for Si throughout the season.

Silicon, as ubiquitous as it is in the world and as necessary as it is for healthy plants, is often an unknown limiting factor in crop production. Supplementing with effective forms of silicon can provide several benefits to plants. Testing for silicon is essential and needs to be done correctly. Doing so will help you dial in the correct rate of Si for your crop.

Resources:

• da Silva, D.L., de Mello Prado, R., Tenesaca, L.F.L., da Silva, J.L.F. (2021). Silicon attenuates calcium deficiency by increasing ascorbic acid content, growth and quality of cabbage leaves. Sci Rep 11, 1770.

• Fleck, A., Schulze, S., Becker, M., Specht, A., Waßmann, F., Schreiber, L., Schenk, M. (2015). Silicon Promotes Exodermal Casparian Band Formation in Si-Accumulating and Si-Excluding Species by Forming Phenol Complexes. PloS one. 10. e0138555. 10.1371/journal.pone.0138555.

• Flora, C., Khandekar, S, Boldt, J., and Leisner, S.(2019) Silicon alleviates long-term copper toxicity and influences gene expression in Nicotiana tabacum, Journal of Plant Nutrition, 42:8, 864-878, DOI: 10.1080/01904167.2019.1589508

• Frantz, J.M. & Locke, J.C. & Sturtz, D. & Leisner, S.. (2010). Silicon in ornamental crops: Detection, delivery, and function. Silicio na Agricultura: Anais do V Simposio Brasileiro Sobre Silicio Agricultura. 111-134. 

• Marschner, P. (2012) Marschner’s Mineral Nutrition of Higher Plants. 3rd Edition, Academic Press, Cambridge, 649 p.

• Mohanty, S., Nayak, A.K., Swain, C., Dhal, B., Kumar, A., Tripathi, R., Shahid, M., Lal, B., Gautam, P., Dash, G., & Swain, P. (2020). Silicon enhances yield and nitrogen use efficiency of tropical low land rice. Agronomy Journal. 112. 10.1002/agj2.20087. 

• Neu, S., Schaller, J. & Dudel, E. Silicon availability modifies nutrient use efficiency and content, C:N:P stoichiometry, and productivity of winter wheat (Triticum aestivum L.). Sci Rep 7, 40829 (2017).

• Zellener, W., Tubana, B., Rodrigues, F., and Datnoff, L. (2021). Silicon’s Role in Plant Stress Reduction and Why This Element Is Not Used Routinely for Managing Plant Health. Plant Disease. 105. 10.1094/PDIS-08-20-1797-FE.