Si and Value for Your Money


The chemistry of silica in hydroponic solutions is extremely complex but, to put things simply, plants can only uptake monosilicic/orthosilicic acid (H4SiO4). What this means is that only a percentage of any silica additive, when it is applied to solution, is available for plant uptake; i.e. when silica is added to aqueous environments (e.g. hydroponic nutrient solutions) chemical processes/reactions occur whereby non-bioavailable forms of Si become bioavailable as H4SiO4 and vice versa. This peaks in acidic solutions (e.g. hydroponics where pH is below 7), no matter how much Si is added to solution, at no higher than 100 -120 ppm dependent on prevailing conditions (higher concentrations of soluble monomers are only obtained above pH 11-12.). As I said, this is extremely complex chemistry; therefore, for reasons of expediency and to avoid being mugged by chemistry let’s not go there.  Suffice to say, silica in solution is inherently unstable and a high degree of it is non-bioavailable for plant uptake. Additionally, because silica is inherently unstable it has the tendency to undergo changes in solution which results in a large degree of precipitation and/or insolubility over even short periods of time.


In recent times, certain suppliers of silica products have latched onto the inherently unstable nature of silica in hydroponic solutions and released what they claim to be stabilized silica additives, which they also claim means your plants receive more bioavailable mono/orthosilicic acid. However, there are a few significant problems with these claims:


  • While it is possible to stabilize more than 120 ppm of monosilicic acid in a highly acidic aqueous environment with e.g. polyvalent cations, amino acids and components such as PEG 400, only one inventor of a stabilized silica product has attempted to demonstrate that once a stabilized Si product is added to solution it remains more stable than non-stabilized forms of silica. i.e. only a ‘carnitine phosphate’ patent examined stability of diluted solutions; with all others it seems likely that these mixtures decrease in orthosilicic acid once diluted and/or pH adjusted in hydroponic solutions.
  • From a scientific perspective, it doesn’t matter if pure silicic acid or silicate salts are put into solution, only the pH of the solution determines the degree of deprotonation and hence solubility and bioavailability of Si. This means that, theoretically, whether a silica additive is stabilized or otherwise it equates to the same thing once it is applied to the hydroponic solution.
  • In order to stabilize silica additives some manufacturers use almost toxic amounts of polyvalent cations such as boron and molybdenum. This means that when you apply these additives to solution you are not only applying Si but also often high quantities of other elements which may prove detrimental to growth. Additionally, PEG 400, which is used in some of these products, is an artificial, fully synthetic petrochemical which is only biodegradable under certain conditions (e.g. water treatment plants, industrial compost) and is readily assimilated by plants and ends up unaltered in the final, harvested product. This really makes the use of PEG 400 stabilized silica additives highly questionable in the production of medicinal crops.
  • Upon breaking down a couple of these products and looking closely at the monosilicic acid they provide to solution, based on their recommended usage rates, they provide almost no monosilicic acid to solution. For example, when looking at just one of these products – a claimed to be stabilized Si additive that retails for $250.00 a litre – based on its recommended usage rate, it provides 3 ppm of Si to solution. This, due to the molecular weight of silicon versus monosilicic acid (28.08 versus 96.11), translates to 4.4 ppm of monosilicic acid in solution. It’s important to note that most river water contains higher levels of monosilicic acid (to generalize, river water contains 5 – 15 ppm monosilicic acid), so growers might as well save their hard earned money and instead use river water for a source of monosilicic acid in their growing system.


You perhaps take my drift. Basically, these products cost between 50 – 100 times more than far cheaper non-stabilized potassium silicate products (when purchase price against Si supplied to solution is equated) and based on their recommended usage rates they provide very little bioavailable monosilicic acid to solution (no matter what occurs once they are in solution).


Thus, at least for now because new technologies are always emerging, the quality of a silica product for use in hydroponics comes down to the amount (%) of ‘plant available Si’ (PAS) that any given product provides. This can be accurately measured using the 5-day method for determining the soluble silica (Si) concentration.


Currently, as I write this, the Californian Department of Agriculture (CDFA) is working towards standardizing this test for listing Si percentages (as PAS) on silica product labels. This is being done so that consumers are able to evaluate just what they are getting for their money with regards to plant available silica in any given silica additive.


Below is a table showing the plant available Si (PAS) percentages in just some of the various silica products that are available through hydroponic stores and/or agricultural suppliers. All tests conducted using the 5-day method for determining the soluble Si concentration.


Manufacturer Fertilizer Name Total Si % Plant Available Si% (Soluble Si)
Med-Tek Super Silica 30% 14.1%
PQ Corp Agsil 16H (K Sil Powder) 24.6% 8.54%
PQ Corp Agsil 25 (K Sil Liquid) 9.7% 7.6%
Plant Science Concepts OSA 28 (PEG 400 Stabilized Silicic Acid) 2.09% 1.06%
Mills Vitalize 0 – 3 – 1   0.94%
Fertimaster Power Si (PEG 400 Stabilized Silicic Acid) 1.71% 0.88%
Aptus Fasilitor (PEG 400 Stabilized Silicic Acid) 1.61% 0.85%
Loveland Products SST 28% (Stabilized Silica Technology)   0.44%



You’ll note on top of the list is Med-Tek Super Silica. You’ll also note that Med-Tek Super Silica tested at the highest percentage of ‘plant available Si’ at 14.1%. The next highest testing product being Agsil 16H ‘K Sil’ (potassium silicate) powder which tested at 8.54%, or 5.56% less PAS than Med-Tek Super Silica. Actually, so as not to do the hard sell on Med-Tek Super Silica (like, wow it’s better than the rest), Super Silica is simply a highly concentrated K Sil (potassium silicate) product, and other brands may also test this high for PAS. In short, my intent is not to flog Super Silica over all others; these tests were what we had available thanks to an associate running lab analysis on several products; some tests were also accessed through the Journal of AOAC International, Volume 96, Number 2, 2013, pp 251-259, and Med-Tek Super Silica was tested by ourselves in a U.S. based lab in mid-2018. View original lab report


However (this said), you’ll also note that all of the claimed to be stabilized silica products tested at the bottom range for PAS with between 0.44 – 1.06 percent. Basically, the most expensive silica products in this PAS test group tested very poorly.


To summarize, where considering silica additive quality, in the near future the CDFA will have a standard where all silica additives will be required to list a guaranteed minimum of PAS in order for suppliers to sell their product in California. Use this listing to establish just what you are getting for your money where plant available Si is concerned.


Where silica brands are not sold in California they may use another listing standard (e.g. %w/v or w/w KSiO2); therefore, look for the most concentrated brands possible in order to ensure higher levels of PAS than less concentrated brands. Factor in price against concentration also. For example, if a product is only half as concentrated as another, but it is half the price, this represents about the same value for money (put simply).


As for stabilized silica products, to be fair, they may offer other values beyond the Si they provide to solution. For example, some stabilized Si products contain amino acids. Amino acids are biostimulants which are shown to help with plant stress relief. The point really being is that if you are looking for a source of Si I expect cheaper K Sil products offer far better value for money than much more expensive stabilized Si additives.  


Moving on …….


Read about silica science….. 

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