mrburns
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So, we've been hearing a lot about how silicon can do wonders for plants, right? It's pretty awesome because it helps them fight diseases and even strengthens their overall structure. That's why adding silicon to the nutrient mix is a no-brainer. But, like with anything, there are a bunch of myths and questions floating around about how to use Si in plant nutrition and what form of Si is best. No worries, though - I'm here to clear up some of the confusion and answer those common questions about silicon sources and their proper use. Let's dive in!
When it comes to using silicon in nutrient solutions, you'll generally find three main sources to choose from.
First up, we've got basic potassium silicates. These can be solids or solutions that come from the reactions of silica with potassium hydroxide. Popular products in this category include AgSil 16H and liquid concentrates like Growtek Pro-Silicate. Just a heads up – these products typically have a very basic pH.
Next, we have acid-stabilized silicon products, like PowerSi Classic and OSA28. These are always in liquid form and contain monosilicic acid in an acidic environment. They also have stabilizing agents added to prevent the polymerization of the monosilicic acid.
Last but not least, we have non-aqueous products with organosilicon reagents, such as Grow-Genius. These products don't contain water and are derived from reagents like TEOS (tetraethyl ortho-silicate) and other silicon-containing compounds, mainly silicon-containing surfactants. While they aren't in plant-available forms, they will generate these forms when they come into contact with water.
Are you wondering if potassium silicates have "less available" silicon?
Well, let me explain this to you. When you dissolve potassium silicate with a high concentration, it forms silicate oligomers - basically, big silicon chains. These chains stay stable in basic solutions because they have a high negative charge. That's why you can create concentrated potassium silicate solutions with a basic pH. In fact, adding potassium hydroxide to make the solution even more basic can improve the solubility of potassium silicate solids like AgSil16H. But when the concentration decreases, the silicate breaks down into smaller, monomeric silicate anions.
When potassium silicate is introduced into nutrient solutions, a fascinating process occurs. As the concentration decreases through dilution, the silicates undergo hydrolysis, transforming into monomers. By subsequently lowering the pH of the solution, monosilicic acid is produced as the final product. When potassium silicate is appropriately utilized in the preparation of nutrient solutions, the resulting monosilicic acid is readily available for plant absorption.
Contrary to the prevailing belief that potassium silicates are less accessible to plants, they can indeed be perfectly available when employed correctly. Ultimately, potassium silicates generate monosilicic acid, which is highly accessible to plants when prepared and used properly.
How do I correctly utilize potassium silicate?
First, if you're working with a solid, create a stock solution with a concentration not exceeding 45g/L. For AgSil 16H, it's advised to prepare a 15g/gal stock solution and apply it at a rate of 38mL/gal in the final solution (1% injection rate). To enhance the stability of your AgSil 16H concentrate, you may add 1g/gal of KOH. This will result in +9.8ppm of Si as elemental Si and +11.55ppm of K. The KOH addition and the low 15g/gal concentration ensure that the silicate is already predominantly present as monomeric silicate anions.
Next, add this solution to your water first. If you add it after nutrients, the Si will interact with Ca and Mg in a concentrated form, potentially causing stability issues. First, add the solution, then the lowest pH fertilizer concentrate, followed by the Ca-containing concentrate, and finally, if needed, decrease the pH with an acid to achieve the desired level.
Following this process ensures a stable final solution containing monosilicic acid. If you alter the Si concentration in the stock solution, change the injection order, or increase the Si concentration beyond 20ppm of Si as elemental Si in the end solution, you might encounter precipitation and unavailable Si forms.
What's the purpose of using acid-stabilized Si products?
Acid-stabilized silicon sources aren't necessarily more plant-accessible. However, they typically have a low starting pH and minimal mineral content (depending on the preparation process), which can reduce the need for acid additions and help lower the pH of hard water sources. They may also contain stabilizing agents beneficial to plants. Nonetheless, the specific stabilizers and mineral composition will vary significantly by product due to the numerous options available to manufacturers.
Ultimately, at the pH where plants are fed, acid-stabilized Si and potassium silicate sources produce the same monosilicic acid. Thus, plant availability isn't a benefit of using such products.
Why opt for non-aqueous Si products?
Non-aqueous Si products offer higher concentrations than either basic silicon or acid-stabilized liquid silicon products by mass. This is because they're made from Si forms that are highly stable in water-free conditions. Consequently, you can purchase a small quantity and add a minimal amount to your reservoir per gallon of solution prepared. Another advantage is that they're pH neutral and don't affect the pH of nutrient solutions. The silicic acid formation from these products only requires reactions with water, eliminating the need for mineral additions, stabilizer additions, or pH modifications.
An essential point to consider is that the reaction of a product like TEOS with water introduces additional substances into the solution. For instance, adding 10 ppm of Si as elemental Si from TEOS actually results in the addition of approximately 66 ppm of ethanol to your solution. These alcohols can negatively impact root and plant growth, which is why careful consideration is necessary when using non-aqueous Si products. It's crucial to keep in mind that these substances can accumulate in the root zone and potentially cause issues. The specific organics present and their potential effects will depend on the exact formulation. When utilizing organosilicon sources, it's recommended to filter the nutrient solution through a carbon filter to eliminate these organics before they come into contact with plant roots.
Is the final Si in the solution more stable with any product type?
No, all three product types, when used correctly, will result in stable monosilicic acid in the solution. The stabilizing agents in acid-stabilized products will be so diluted that any additional stabilizing effect is practically negligible. If Si's concentration is low enough (<20ppm of Si as elemental Si), it will remain stable in the solution indefinitely (I observed no changes in concentration for five weeks). With higher Si concentrations, Si tends to polymerize (regardless of its source), causing stability issues. To maintain a stable Si solution, ensure proper preparation and maintain low concentrations.
If Si availability is mostly the same, why are there differences between products at equivalent Si application rates?
Although all various Si products result in the same Si form in the final solution, acid-stabilized Si products contain numerous additional substances that play active nutritional roles. For instance, Boron and Molybdenum are frequently used as stabilizing agents. Products like PowerSi Bloom also contain "exotic plant extracts" (according to their website). Common stabilizing agents include glycerol, carnitine, choline, and sorbitol, all of which could potentially affect plants at the concentrations added with these products. Often, these stabilizing agents are added at 10-50 times the Si amount by mass, meaning that your Si supplement might add significantly more stabilizing agents than Si itself.
Which product is more cost-effective per delivered mole of monosilicic acid?
Labeling regulations allow fertilizer manufacturers ample room to deceive consumers into thinking a product might be more concentrated or dilute than another. Labeling a product as a "% of monosilicic acid" does not mean that the product contains that percentage of monosilicic acid. Instead, it means that the product contains Si, which, if converted to mono-silicic acid, would yield that percentage. Acid-stabilized Si-containing products are the only ones containing monosilicic acid in its original form, but they are usually limited to low concentrations due to the molecule's reactivity.
Both non-aqueous silicon products and soluble potassium silicate products contain monosilicic acid precursors. One is in the form of organosilicon compounds, and the other is in the form of silicate chains. As mentioned earlier, both precursors can lead to very high conversions to mono-silicic acid when used appropriately.
When comparing the concentrations of various silicon products, it's crucial to convert the quantities to elemental Si percentages. For monosilicic acid % values, multiply by 0.2922, and for SiO2 values, multiply by 0.4674. For instance, 40% Si as monosilicic acid equates to 11.68% Si as elemental Si. Soluble potassium silicates like AgSil 16H typically contain ~24% Si as elemental Si by mass, making them the most concentrated and cost-effective source of bioavailable silicon when used correctly. Generally, higher solubility potassium silicates have lower Si content due to increased K proportions and less need for KOH during stock solution preparation.
Product Name | Price (USD) | Product Type | Si%(as elemental Si) | Amount (g or mL) | Price (USD/g of Si) |
AgSil 16H | 327.18 | Silicate derived | 24.7 | 22678.6 | 0.06 |
Growtek Pro Silicate | 226.99 | Silicate derived | 3.51 | 23000 | 0.28 |
Grow-Genius | 179 | Organosilicon | 11.68 | 500 | 3.07 |
Growtek Gro-Silic | 240 | Silicate derived | 12.85 | 4000 | 0.47 |
Dune Stabilized Monosilicic Acid | 1001.99 | Acid stabilized | 2.2 | 23000 | 1.98 |
OSA 28 | 270 | Acid stabilized | 0.8 | 946 | 35.68 |
The table above displays the price discrepancies per gram of Silicon for different products as of January 2023. When bought in bulk (50 lbs), AgSil16H can cost up to two orders of magnitude lower than other alternatives.
Lab tests that I've conducted, measuring molybdenum-reactive Si, indicate that all Si in AgSil16H can be fully converted to monosilicic acid when adhering to the preparation guidelines mentioned in this article.
What's my take on this?
Having delved into this topic for years, experimented with various products alongside different growers, and conducted my own chemical analyses (preparing stable silicic acids and gauging active Si concentrations), I'd recommend those keen on incorporating Si into their nutrient solutions to opt for a potassium silicate solid product. To ensure optimal pH and stability, prepare an appropriate stock using potassium silicate and potassium hydroxide, then create nutrient solutions from this stock's dilutions. In the absence of a solid product like AgSil 16H, a basic silicate concentrate would be your next best bet. It's usually advised to prepare a more diluted stock from these products, guaranteeing the presence of monomeric silicate in the stock.
In my opinion, acid-stabilized silicon products or non-aqueous Si products aren't worth the extra cost. If you notice better results with a non-potassium silicate product compared to potassium silicate, it's likely due to improper preparation and dilution of the potassium silicate stock, or the presence of an Si-alternative substance in the product. The stabilizing agents themselves are more affordable, so testing these agents' eliciting effects might be a more cost-effective long-term solution.
Non-aqueous silicon reagents might be preferable in situations where mixing stocks and handling basic reagents pose challenges or pH adjustments are limited. Non-aqueous silicon forms also provide more flexibility in terms of mixing errors – incorrect mixing order, variable pH mixing, etc. – since hydrolysis reactions occur easily under diverse conditions. Nonetheless, I recommend using these with carbon filtration to prevent potential issues arising from their organic by-products.
If soluble silicon sources present problems due to your water composition, injector limitations, or costs – and your medium allows for amendments – you might want to consider using solid amendments to supplement Si. This can be a more cost-effective option compared to soluble Si supplementation.
