Silicon: The Beneficial Element in Plant Cultivation

By: Due Diligence Horticulture

January 24, 2025

Silicon (Si) is the second most abundant element in earth’s crust and commonly present as silica (SiO₂). Although Si is not classified as an essential nutrient, extensive research has highlighted its crucial role in enhancing plant resilience. In controlled environment agriculture (CEA), where plants are often cultivated  in soilless systems, incorporating silica can lead to stronger plants with improved resistance to diseases  and environmental stresses. This review discusses the benefits of silica in strengthening cell walls,  minimizing disease susceptibility—such as powdery mildew—and enhancing drought resistance. It also  explores methods for incorporating silica into soilless systems and addresses the challenges associated  with continuous liquid feed applications, particularly regarding solubility and pH compatibility.

Benefits of Silica

Strengthening Cell Walls

Silica accumulates in plant tissues, primarily depositing in cell walls as amorphous silica gel (Figure 1). This deposition reinforces cell walls, providing structural support and rigidity. Strengthened cell walls help plants maintain erect growth habits, reduce lodging, and support heavier fruit loads. In many crops, the biostimulant properties of silica has been linked to increased stem strength, improved growth, and resistance to biotic and abiotic stresses[1]. 

Minimizing Disease Susceptibility

Silica contributes to disease resistance through multiple mechanisms. By forming a physical barrier within cell walls, it hinders pathogen penetration, making it more difficult for fungi and other pathogens to invade plant tissues. Additionally, silica canstimulate systemic acquired resistance (SAR) in plants, activating defensive enzymes and phenolic compounds that inhibit pathogen growth[2]. Studies have shown that silica supplementation reduces the incidence and severity of powdery mildew in a wide range of sensitive crops, including cucumbers,  grapes, and strawberries, by limiting spore  germination and hyphal growth[3].

Enhancing Drought Resistance

Silica enhances plants’ ability to withstand drought stress by reducing transpiration rates[4]. Deposits in the  leaf epidermis decrease water loss without significantly affecting gas exchange necessary for  photosynthesis. Silica may also promote the development of deeper and more robust root systems[5],  enhancing the plant’s capacity to access water from deeper soil layers, though this is not typically beneficial  in controlled environments where plants have sufficient access to water. Moreover, Si can maintain turgor  pressure during drought conditions, enabling plants to endure periods of limited water availability.

Methods for Incorporating Silica in Soilless Systems

Continuous Liquid Feed

One method of supplying silica to plants in soilless systems is through continuous liquid feed. Soluble  silicates, such as potassium silicate (AgSil 16H), can be added directly to nutrient solutions. Typical  concentration in continuous liquid feed is about 20 to 100 ppm of silicon, depending on the crop, frequency  of application, and growth stage[6]. This approach provides immediate bio-available silica to plants and  allows for precise control over application rates. However, there are notable challenges that need to be  considered.

The most significant challenge with using continuous liquid feed to supply silica is related to solubility. Silicate compounds tend to polymerize and precipitate, especially at higher concentrations or in the presence of certain ions.  

Precipitated silica can clog irrigation lines, emitters, and filters, leading to maintenance issues and uneven nutrient distribution.High pH (above 12) improves solubility of concentrated silica (Figure 2)[7], but most soilless systems require a nutrient solution pH between 5.5 and 6.5 for optimal  nutrient availability. This mismatch in  pH makes it challenging, but not  impossible, to inject Si in continuous  liquid feed. 

The best practices for injecting Si are:

1. When preparing a concentrated stock tank, use RO water and increase the pH to about 12  using potassium hydroxide (KOH) before adding your potassium silicate product. If possible,  cover the stock tank to avoid exposure to the atmosphere as CO2 will go into solution,  dropping pH and causing silica to precipitate. 
2. Inject silica first to ensure it is sufficiently dilute before coming in contact with other  concentrates, especially calcium and magnesium. After injecting silica, inject the lowest pH  tank (usually the tank with phosphorus), followed by the calcium containing tank, and finally  adjusting pH as needed before delivering to plants.

Media Amendments

Alternatively, silica can be incorporated into the  growing media through amendments. Materials rich in  silica—such as diatomaceous earth, rice hulls, or  slag—can be added to the substrate, providing a slow release source of silica. Granular or powdered silicate  fertilizers, such as wollastonite, can also be mixed into  the substrate before planting. This method offers a  sustained supply of silica and can improve the physical  properties of the growing media by enhancing aeration  and drainage.

Rice hulls added to media at 12% by volume and  Wollastoniate (Calcium Silicate; Vansil 10) added at 1  g per L showed the highest and most sustained release  rate. These amendments therefore offer an alternative  source of silica rather than delivering in continuous  liquid feed. There are still challenges, however. While  rice hulls are relatively inexpensive themselves, there are limited sources and the cost of freight can make implementation a challenge. Wollastonite, on the other  hand, has a much higher bulk density making it easier to ship across the country. Both rice hulls and  wollastonite have the potential to increase media pH by up to 1 pH unit over the course of about a month[8].

Conclusion

Silica, while not essential, plays a beneficial role in enhancing plant structural integrity, disease resistance,  and stress tolerance—attributes particularly valuable in controlled environment agriculture. While  incorporating silica through continuous liquid feed offers immediate benefits, challenges related to solubility  and pH compatibility must be carefully managed. Media amendments provide an alternative approach,  supplying silica over an extended period and potentially improving substrate properties. By understanding  and addressing the challenges associated with silica supplementation, growers can leverage its benefits to  improve crop performance in soilless systems.

References 

1. Savvas, D. & Ntatsi, G. Biostimulant activity of silicon in horticulture. (2015). Scientia Horticulturae 196,  66–81. 
2. Epstein, E. Silicon. (1999). Annu Rev Plant Physiol Plant Mol Biol. 50, 641–664. 
3. Bélanger, R. R., Bowen, P. A., Ehret, D. L., & Menzies, J. G. (1995). Soluble silicon: its role in crop and  disease management of greenhouse crops. Canada Plant Disease, 79(4), 329-336. 
4. Sacala, E. (2009). Role of silicon in plant resistance to water stress. Journal of Elementology, 14(3),  619-630. 
5. Shi, Y., Zhang, Y., Han, W., Feng, R., Hu, Y., Guo, J., & Gong, H. (2016). Silicon enhances water stress  tolerance by improving root hydraulic conductance in Solanum lycopersicum L. Frontiers in plant  science, 7, 196. 
6. Kamenidou, S., Cavins, T. J., & Marek, S. (2008). Silicon supplements affect horticultural traits of  greenhouse-produced ornamental sunflowers. HortScience, 43(1), 236-239. 
7. Fernandez, D., (2023). Common questions about silicone in nutrient solutions.  https://scienceinhydroponics.com/2023/02/common-questions-about-silicon-in-nutrient-solutions.html
8. Dey, M. G., Boldt, J. K., & Bugbee, B. (2023). Dissolution of Silicon from Soilless Substrates and  Additives. HortScience, 58(11), 1282-1290.