# Preservatives in liquid fertilisers ?



## myxodex (Sep 30, 2014)

I haven't posted for a long time, but have been more or less keeping up with fertilizer discussions in "lurking mode".
I have two fertiliser queries which I will post as two separate threads. So this is the first one. Does anyone here know of any preservatives that can be used with liquid concentrate type fertilisers ?

About 18 months ago I made up my own liquid concentrate fertiliser, a sort of modified Klite as 3 separate stocks, one precipitated slowly, one grew bacteria and the third grew fungi. Some years before that I used Dyna-Grow which I remember was remarkably stable compared to some other liquid ferts I tried. So I'm starting from scratch again and was wondering if anyone here had any ideas on this. 

The only perservative I know of is sodium benzoate which you need to add at least 0.1% and it only works if the pH is 4 or less. At 1 in 5000 dilution (which is what I'm aiming for) would this be harmful to plants ?


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## DavidCampen (Sep 30, 2014)

If you look inside at the bottom of a bottle of DynaGro OrchidPro you will find a lot of insoluble crystals at the bottom. My guess is that this is calcium sulfate (and possibly calcium borate, molybdate and phosphate) precipitating out of solution.

I gave up on trying to formulate a 1 component liquid fertilizer concentrate that contained what I believe to be adequate amounts of calcium and sulfate. I now use a 2 component liquid concentrate system that are only combined in the mixing tank after a significant proportion of the total (RO) water has been added and while the remaining amount of water is being added.

I have had problems with a white (bacterial?) slime growing in some of these liquid concentrate formulations. I have found that for this to happen the solution has to contain nitrogen and especially nitrogen from ammonium ion or amino acids. My current formulations do not seem to exhibit this problem. 

One thing that is helpful in preventing microorganism growth is to make the solutions as concentrated as possible. When I blend my 2 component liquid concentrates I use pre dissolved stock solutions of many of the ingredients. For calcium ammonium nitrate, magnesium nitrate and ammonium nitrate I prepare solutions with a concentration of 400 grams/liter; potassium nitrate solution is 300 grams per liter. I also add a very small amount of a quaternary alkyl ammonium salt such as Physan 20 as a preservative; these high molecular weight quaternary alkyl ammonium salts are hardly soluble in the concentrated salt solutions so for this application I actually use a swimming pool disinfectant that uses quaternary alkyl ammonium salts of slightly lower molecular weight than Physan in the hopes that this will be a bit more soluble in the concentrated salt solutions.


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## PaphMadMan (Sep 30, 2014)

I wouldn't expect sodium benzoate to be a toxicity problem for plants at reasonable concentrations. It occurs naturally in many plant sources, and is an intermediate in some biosynthetic pathways. 

You might also look for a product called PPM (for Plant Preservative Mixture). It is a proprietary product used as a broad spectrum biocide in plant tissue culture - so clearly not harmful to plants in general.

I have no idea if either one would actually be effective or stable for this use. The sodium benzoate is certainly the cheaper and more readily available of the 2.


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## PaphMadMan (Sep 30, 2014)

DavidCampen said:


> I have had problems with a white (bacterial?) slime growing in some of these liquid concentrate formulations. I have found that for this to happen the solution has to contain nitrogen and especially nitrogen from ammonium ion or amino acids. My current formulations do not seem to exhibit this problem.



I have occasionally observed what appears to be Phytophthora or a similar Oomycete (of the "water mold" variety rather than a one of the plant pathogens) growing in laboratory solutions, most notably potassium thiocyanate (KSCN). There are certainly other possibilities, but it could be one candidate for your slime.


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## Stone (Oct 1, 2014)

DavidCampen said:


> > I gave up on trying to formulate a 1 component liquid fertilizer concentrate that contained what I believe to be adequate amounts of calcium and sulfate. I now use a 2 component liquid concentrate system that are only combined in the mixing tank after a significant proportion of the total (RO) water has been added and while the remaining amount of water is being added.
> 
> 
> 
> I agree that a ''2 pack'' system is really the only way to assure a properly balanced fertilizer containing both nitrates and sulphates for mainly inert substrates. Even mixed dry, I have noticed that the slightest humidity will begin to upset the chemistry. But why not keep them as powders (calcium, potassium and magnesium nitrates) seperated from the sulphates etc until needed?


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## DavidCampen (Oct 1, 2014)

Stone said:


> DavidCampen said:
> 
> 
> > I agree that a ''2 pack'' system is really the only way to assure a properly balanced fertilizer containing both nitrates and sulphates for mainly inert substrates. Even mixed dry, I have noticed that the slightest humidity will begin to upset the chemistry. But why not keep them as powders (calcium, potassium and magnesium nitrates) seperated from the sulphates etc until needed?
> ...


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## myxodex (Oct 2, 2014)

Thanks for the useful information. One observation that has made me smile is the proliferation of online hydroponics retailers here in the UK, and the marketing of the various products leaves one in no doubt about the target crop. Anyhow, there has been a distinct move to two component fertilisers in this industry, ... maybe weed is an S hungry crop ?


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