I was looking at chelators to solve the calcium sulfate solubility problems for a more concentrated 1-part fertiliser. I’ve found that apparently some organic acids can complex calcium, specifically citric acid, but also others such as malic acid and lactic acid. I’ve already got malic acid that I could try. I would have to change the rest of the composition a bit – I’m specifically using ingredients such as urea phosphate to bring down the pH already. Also, I wonder if organic acids are a good idea for long term pH stability – I wonder if the acids breaking down in the plant’s water reservoir causes pH drift. Lots of things to test. Or I could just order an actual chelator like EDTA.

To be continued.

V8.9 take 1

Mixing order:

• 4.675 g calcium nitrate in about 90 mL water -> dissolves fine
• 6.273 g urea phoshate added -> slightly milky, almost entirely dissolved?
• 8.646 g ammonium sulfate -> partially dissolved, very milky
• Water added to ~200mL -> still very milky (250 mL would have been 500:1)
• Water added to ~400mL -> still milky (500 mL would have been 250:1)
• Water added to ~800mL -> still milky (1000 mL would have been 125:1)

I guess I’m no longer getting calcium phosphate precipitate due to using urea phosphate, but I still get calcium sulfate precipitate.

I guess I should’ve added the other salts first, to prevent calcium sulfate precipitation. With more nitrate in solution, there would be less chance for the calcium to bind with sulfate.

V8.9 take 2

Mixing order:

• 11.13 g potassium nitrate in about 90 mL water -> takes a while to dissolve, but does dissolve ok
• 2.334 g calcium nitrate added -> dissolves ok
• 2.635 g magnesium nitrate added -> dissolves ok
• 3.317 g urea phosphate added -> dissolves ok, slightly milky but no precipitation after a minute
• Water to ~200 mL
• 4.322 g ammonia sulfate -> dissolves ok!! Very slightly milky but no precipitation after a minute
• 0.935 g trace elements -> formed some chunks, but after breaking up dissolved ok

Success! It’s only a 250:1 solution but at least there’s no precipitation any more. This is the most concentrated 1-part hydroponic fertiliser I’ve made so far.

V8.9 take 3

• 22.26 g potassium nitrate, 5 g calcium nitrate, 5.274 g magnesium nitrate in about 200 mL water -> dissolves ok (I dropped half the calcium nitrate, put most back but added half a gram to compensate, ughh)
• 6.273 g urea phosphate added -> dissolves ok, slightly milky but no precipitation after a minute
• 8.64 g ammonia sulfate -> dissolves ok!! Very slightly milky but no precipitation after a minute
• 1.872 g trace elements -> again some small chunks but dissolves ok

However, after a few minutes precipitation does start to form. 500:1 is too much, I guess 250:1 is the best I can do with this amount of sulfate.

What’s next

250:1 is already nice, it’s more concentrated than GT Foliage Focus which is 150-200:1. However there are more concentrated products such as Canna Floragro and Dyna-Gro Foliage-Pro (both around 800:1). Liquid Gold Leaf (500:1) is the only one with significant amounts of sulfur, though – I wonder how they do that. Maybe a chelator.

For V9 I will look at either lowering sulfate concentration to get to 500:1, or using a different form of sulfur. I could get some biosulfur (mostly S8), I wonder how soluble that is.

Edit

Shit, even the 250:1 eventually started precipitating out something. Probably calcium sulfate. Back to the drawing board.

Our lab threw out some surplus potassium phosphate, so I used it to formulate hydroponic fertiliser. That was a mistake. I forgot that potassium phosphate with calcium nitrate gives insoluble calcium phosphate, which happily proceeded to precipitate. I also added some other stuff that may or may not have precipitated out, so I can’t even save the stuff by filtering out the solids and making it into a two-part fertiliser, because I wouldn’t know for sure the composition.

I do have urea phosphate, which does not react with calcium nitrate. Urea-N isn’t as good for growth as nitrate-N on its own, but at least 20% of total N can be replaced by urea without negative effect. My previous formulation using urea phosphate worked great, even though it was more diluted than it needed to be and it had some pre-mixed dry fertiliser in it. It got some algae or something in it, not much of a problem but if it were concentrated enough I think that shouldn’t have happened.

Goals:

• Concentrated 1-part fertiliser, preferably 500:1 or even 1000:1
• Nutrient composition suitable for leafy aroids, taking into account local tap water composition
• Acid enough to bring down tap water pH to ~6 without extra acids
• Ammonia-to-nitrate ratio that keeps pH relatively stable in semi-hydroponics even if all water is taken up, e.g. 3:7 or 4:6 – even if this is not ideal for growth rate
• Preferably formulated from simple salts, no mixes or acids. Mixed trace nutrients are ok

I took nutrient compositions from literature and from commercial products, I downloaded the latest local tap water composition report, and looked up properties of various possible ingredients. I put everything into a big spreadsheet, set up goal values based on requirements, and used a DEPS evolutionary algorithm solver to optimise for the above list. This is also what I did for my earlier formulation, this time I just forgot to check if the resulting ingredients were compatible.

Newest formulation to test: urea phosphate, calcium nitrate, potassium nitrate, magnesium nitrate, ammonium sulfate, trace elements. Starting with 250 mL 500:1, and will try more or less diluted depending on how that goes.

I’m a little worried about forming calcium sulfate, but at low pH (from the urea phosphate) I think it could work? Actually, I should calculate the amount of calcium sulfate that could be formed, and compare it to the solubility. Solubility is about 0.24 wt% at room temperature. At pH=2.5 solubility is only improved by +12%, although high salt level can improve solubility by about 3x. Even in pure HCl you can only get 1.8 wt% calcium sulfate in solution. I guess I could use a chelator like EDTA to keep more in solution if needed (about 4%), but that’s an expensive way to go. Maybe this is why the commercial 1-part formulations don’t have much sulfur and/or calcium, except perhaps Liquid Gold Leaf which I think has a chelator.

Actually EDTA isn’t all that expensive. I should research that.

Or maybe I should just accept a lower ammonium/calcium/sulfate concentration to get a more concentrated 1-part fertiliser, or keep with my low concentration fertiliser.