Silica and Hydroponics
For myself, I do not see silica as a beneficial additive but, more so, as a must (an essential element) in any balanced/optimized nutrient regime.
Silica (Si) is the second most common element on Earth after oxygen and is abundant in soils.
Silica is abundant in all field grown plants, but it is not present in most hydroponic solutions.
In plants, silica strengthens cell walls, improving plant strength, health, and productivity.
Silica, deposited in cell walls of plants, has been found to improve heat and drought tolerance and increase resistance to insects and fungal infections. Silica can help plants deal with toxic levels of manganese, iron, phosphorus and aluminium as well as zinc deficiency.
Thus, the beneficial effects of silica are threefold: 1) it protects against insect and disease attack (Cherif et al. 1994; Winslow, 1992; Samuels, 1991), 2) it protects against toxicity of metals (Vlamis and Williams, 1967; Baylis et al. 1994), and 3) it benefits quality and yield of agricultural crops (Kathryn E Richmond et al, 2003).
Silica is excluded from hydroponic nutrient formulas because it has a high pH and is unable to remain soluble (hold/remain stable) in concentrated nutrient solutions. Therefore, Si needs to be added to the nutrient tank as a separate element.
Benefits of Si
· Increased disease resistance
· Increased resistance to pathogenic airborne fungi (eg. Botrytis)
· Increased resistance to waterborne pathogens
· Increased resistance to insects/pests
· Increased strength in cell structure
· Increased stress tolerance
· Increased drought tolerance
· Increased salt tolerance
· Increased yields
Silica is not considered to be an essential plant nutrient because most plant species can complete their life cycle without it. However, Si is considered to be a ‘quasi essential’ element for plants because its deficiency can cause various problems with respect to plant growth, development and reproduction. The addition of Si to hydroponic solutions exerts a number of beneficial effects on growth and yield of several plant species, which include improvement of leaf exposure to light, resistance to lodging, decreased susceptibility to pathogens and root parasites, and amelioration of abiotic stresses. Silica can also alleviate imbalances between zinc and phosphorus supply. In general, dicot plants (e.g. tomato, cucumber, peppers) show a tissue accumulation of Si at about 0.5% or less.
A lack of knowledge about the role of silica in horticultural crops became apparent with the change to soilless growing media (hydroponics) in the glasshouse industry in the Netherlands. It was found that in hydroponic systems the Si contents in plant tissue were significantly lower in comparison with crops grown in soil. Research was carried out on the effects of Si application in hydroponic systems. With cucumber, melon, courgette, strawberry, bean, and rose, the Si contents were increased as a result of the addition of Si into the root environment. Results in these trials showed that cucumber, rose, and courgette could benefit from enhanced Si concentration in the root environment, since total yield was increased and powdery mildew was suppressed. 
Si and Fungi Suppression (eg. Botrytis)
Si has been shown in numerous studies to suppress fungal pathogens such as Botrytis. In a study by Adatia et al (1986) conducted on cucumbers grown in recirculating hydroponic systems it was shown that despite regular applications of fungicide, outbreaks of the fungal disease occurred on most of the mature leaves of low Si cucumber plants, while the high Si plants remained almost completely free of fungal pathogens. The conclusion to this study noted:
“The addition of Si could be beneficial to cucumbers grown in areas where the local water supply is low in this element, especially when grown in recirculating solution or in a medium low in Si, e.g. peat.” 
Further research by Shettyet al (2011) demonstrated that Si treatment reduced powdery mildew development by inducing host defense responses in plants.
It is believed that silica deposition at sites of fungal pathogen penetration may be a common component of the host-defense response in a variety of plant families.
Silica is also deposited in the cell walls of roots where it acts as a barrier against invasion by parasites and pathogens. 
For instance, potassium silicate has been shown to act as a preventative against Pythium ultimum. 
Studies have found that soluble Si polymerizes quickly and that disease development is suppressed only if Si is present in soluble form (Samuels et al., 1991b). To minimize disease development, Si must be provided continuously in the nutrient feed in hydroponic systems.
Therefore, a continuous source of soluble silica is very important to combat pathogens. This can be from constant feeding in hydroponics or from retention in the growing medium with soils or soilless mixes.
Optimum ppm of Si in Solution
Research on optimum ppm of Si in hydroponic solutions tends to be somewhat variable. However, to generalize somewhat, hydroponic specific research has shown that different types of plants such as wheat, tomatoes and cucumbers react positively to a moderate addition of silicate ions.
Silica as SiO2 (silica dioxide), which is 46.743% Si and 53.25% O, has been shown in various studies to be beneficial to plants in the range of 50 -150 ppm in the nutrient solution. However, what is typically asserted as optimal SiO2 in hydroponic solutions is 100ppm, which equates to 46.7 ppm of Si.
However, there are several important things that you need to be aware of when using silica in your hydroponics system.
Firstly, silica has the tendency to react with other ions and if present at too high levels in the nutrient solution this can cause the precipitation (“drop out”) of other elements from solution. That is, silicates are relatively insoluble and the acidic pH in hydroponics can cause some precipitation of different reaction products of this ion with other ionic species present within the hydroponics solution. The silicate ions can form silicic acid and start to polymerize into complex macromolecular structures. Basically, silicates in hydroponic solutions can act in unpredictable ways. For this reason, lower rates of use versus higher rates of use in hydroponic solution is advised.
Secondly, Si products typically are highly alkaline. Therefore, when added to solution they raise the solutions pH. As pH rises to above 8.0, the form that silica takes in solution changes from non-reactive, non-ionic monosilicic acid, to reactive, ionic polysilicic acids that react with other minerals and precipitate out of solution, giving a cloudy appearance. That is, at high pH of above 8 silica changes to a form that can react with other minerals and precipitate out of solution. The best way to prevent this is to add your Si additive to water (no nutrients – just water) outside of the nutrient tank/reservoir, and then lower the pH to 5.5 – 5.8 before adding it to your hydroponic nutrient solution. Depending on how much ml of silica is required and how concentrated the liquid product is, I tend to recommend prediluting the silica in 5 – 8 litres (1 – 2 US gallons) of water and pH adjusting the solution (water + silica) to 5.5 – 5.8 before adding it to the nutrient tank/reservoir. What I also recommend is to add this solution slowly over several hours. That is, add 20% of the solution and let this circulate into the nutrient. Then add another 20% an hour or so later etc. While no research exists to support this practice as optimal in hydroponics, I have found that by adding the silica to the nutrient tank/reservoir in this way minimizes the risk of nutrient precipitation.
Thirdly, after many years of using silica products in hydroponics, I have found that like all other plant nutrients, too much Si in the root zone antagonizes other nutrients. For example, it has been shown that excessive levels of Si antagonizes iron and zinc. Other than this, it has been shown that silica increases the oxiding power of the roots making Fe and Mn less soluble. Further, some research suggests that while the benefits of Si are seen when used at one level, when its’ use exceeded this ideal level, growth was negatively affected. What this really comes down to is that I tend to use silica at full strength during grow and early bloom, but reduce the strength to about 65 % once the flowers begin to set (I.e. when the bulk of the internodes are formed and flowers/fruit begin swelling). I have found that this offers the best results and that too much Si in solution can negatively impact on optimal fruitset.
Therefore, I recommend Si use in RTW/DTW organic substrates (e.g. coco and sphagnum peat) at the lower end of the scale – this being between 20 (when flowers are setting) to 30ppm (or 42 – 64ppm as SiO2).
In inert medias and water-based systems I recommend Si use at a higher rate of 30 (during flowerset) to 46.7 ppm Si (or 64 – 100ppm as SiO2).
It is important to note that most hydroponic store sold liquid silicon (silicate) products are made using potassium silicate (K2SiO3). Potassium silicate contains 18.204% Si and 50.685% potassium (K). This means that by adding e.g. 30ppm of Si to solution we are adding 83.5 ppm of K to the solution. The addition of potassium through the use of potassium silicate needs to be considered in any optimized nutrient regime.
The Si to K ratio of any potassium silicate product may vary. Therefore, it is advisable that you contact the manufacturer/supplier of the product and ask them how many ppm of Si (or SiO2) and K their recommended dilution rate contributes to the nutrient solution.
It is important to note that people use different terminology when specifying optimal silica levels in solution. For example, it is typically asserted that optimal SiO2 (silicon dioxide) in hydroponic solutions is 100ppm. However, others may specify optimum ppm referring to Si (silicon), K2SiO3 (potassium silicate), or H4SiO4 (silicic acid). The various chemical references used will determine optimal ml per Ltr usage rates to achieve a given ppm of Si or SiO2 in solution. Just be somewhat aware of this when looking at research or talking to hydroponic nutrient suppliers about their recommended usage rates and what this provides to the plants in terms of Si and other elements (e.g. K).
Converting between SiO2, K2SiO3, and H4SiO4
If you want to convert the ppm of Si to SiO2 ppm
Work out the percentage of Si in SiO2 – there is a molecule and mole calculator on the Manic Botanix website that does this for you. You simply enter in the ‘chemical formula’ of SiO2 (be sure to use subscript for the 2), enter 100% in the ‘purity of chemical box’, enter that you want 1 mole in the ‘moles required’ box (any number will actually do but 1 is just fine), hit calculate and the calculator will tell you that the SiO2 molecule consists of 46.743% silicon (Si) and 53.257% oxygen (O).
1) Take the known percentage of Si in SiO2 and calculate that as a percentage of 1 – e.g. 46.743 (% Si in SiO2) x 1% = 0.46743 (be sure to use the percentage -%- button on the calculator)
2) Then take the amount of Si you want in ppm and divide it by the figure less than one. E.g. 30 (ppm Si) divided by 0.46743 = 64.1807 (ppm SiO2)
Converting SiO2 ppm to Si ppm
This one is dead easy. Let’s say you want to convert the ppm of SiO2 to what this is in Si ppm.
1) 100 ppm (SiO2) = 46.743 ppm (Si) – i.e. 100 (ppm SiO2) x 46.743% (% Si in SiO2) = 46.743 (ppm Si)
These sums can be used for conversion between any chemical formulas, whether they be in ppm, grams, or ml. The unit doesn’t matter, just that the answer is in the same units or a conversion thereof (i.e. 100g or 0.1kg)
For example, if a manufacturer tells you that their recommended dilution rate for their potassium silicate (K2SiO3) product gives you 100ppm of potassium silicate in solution. To establish what this is in Si, you use the mole calculator on the Manic Botanix website and it will tell you that there is 18.204% Si in K2SiO3.
Therefore, to establish Si ppm from 100ppm of K2SiO3 – 100 (ppm K2SiO3) x 18.204% (%Si in K2SiO3) = 18. 204 ppm Si.
Knowing this, you would then be able to establish that the manufacturers recommended dilution rate is too low and would then be able to calculate what is required to achieve your sort after ppm in solution.
- “Silicon nutrition in plants” Plant Health Care,Inc.: 1. 12. Retrieved 1 July 2011.
- W. Voogt and C. Sonneveld (2001) Silicon in horticultural crops grown in soilless culture
- M.H. Adatia and R.T. Besford (1986) The Effects of Silicon on Cucumber Plants Grown in Recirculating Nutrient Solution
- R.Shetty, B. Jensen, N. P. Shetty, M. Hansen, C. W. Hansen, K. R. Starkey, H. J. L. Jorgensen (2011) Silicon induced resistance against powdery mildew of roses caused by Podosphaera pannosa
- Pat Brown, Jim Menzies, and David Ehret (1992) Soluble Silicon Sprays Inhibit Powdery Mildew Development on Grape Leaves
- Taiichiro Hattori, Shinobu Inanaga, Eiichi Tanimoto, Alexander Lux, Miroslava Luxová and Yukihiro Sugimoto (2003) Silicon-Induced Changes in Viscoelastic Properties of Sorghum Root Cell Walls
- M. Cherif, N. Benhamous, J. G. Menzies, and R.R. Belanger (1992) Silicon induced resistance in cucumber plants against Pythium ultimum
- Pat Bowen, Jim Menzies, and David Ehret (1992) Soluble Silicon Sprays Inhibit Powdery Mildew Development on Grape Leaves