This.
I use powdered gypsum (Calcium Sulfate) as its relatively neutral and adds sulfur to my nutes which increases flavor profile. And as stated, its dirt cheap they’re basically giving it away.
- BD
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CaCO3 solubility is pretty ass in DI H2O. As you posted, if he has dissolved CO2 in his res, it’ll increase solubility as well as if he were to decrease the overall temp of his res (CaCO3 is weird like that). But, with those side reactions to make the CaCO3 more soluble, there are going to be pH swings. I wouldn’t recommend it as I would not want side reactions to occur within my res potentially throwing off my ratios and dealing with pH swings. A much more simple solution is just to add gypsum (not amazing soluble, but enough to get the job done, and its relatively neutral).
- BD
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Everyone who’s read a wikipedia article is a chemist, I guess.
Calcium carbonate powder is not readily soluble in water for nutrient solutions. That’s why people use things like cal-nit, calcium chloride, cal-phos, etc.
Thanks for the contribution everybody.
@BDGrows You use hydro and no problems dissolving gypsum? I will check that out. 
@Pursuer You should be open to change your mind on topics or you will miss out on learning. Also it’s dangerous to give out misinformation confidently as newbies might pick up on it and have unneccessary problems. As @vernal and @bdgrows explained, carbonates are the stuff that makes water hard. Yes, they do act as a pH buffer but they are also mostly unusable for the plants since they are way less bioavailable than the other compounds mentioned in this thread.
I don’t use gypsum for hydroponics, my tapwater is hard enough where I have the opposite problem you do. I do use it for my outdoor plants/lawn and it works gloriously, I can rapidly see results which indicates to me it’s fairly soluble, unlike limestone which takes years to break down, but I add limestone mostly just to adjust pH.
It’s definitely not as soluble as calcium chloride or calcium phosphate, dissolves better in cold water than hot water weirdly enough. IDK how much calcium you need to supplement, if gypsum isn’t working I’d try calcium chloride/phosphate/EDTA or similar chelated form like others suggested.
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So im working through using gypsum as a calcium supplement and using epsom as a seperate.
Tell me if my logic/chemistry makes sense…
So, I’m going to base it off my vinegar solution, if its approx 5%, then I have 5 ml in 100, I already rinsed some of my gypsum, leaving a gray powder, now I’m soaking it in white distilled vinegar until it stops bubbling (holy crap does it smell like sulphur btw). So my theory works. Now to clean it up and figure i out on paper.
Molarity of pure acetic acid is 17.416 (I’m assuming 5% concentration), molarity of caso4 is 136.14. So for 1g clean and dry caso4 I’ll need about 156 times the weight of vinegar, about 160 ml, to have a slight excess.
Caso4 is 29% calcium. So every gram of caso4 i dissolve into 160 ml vinegar will give me about .29g of dissolved calcium
Cal mag is 3.2% ca , this will be a .2% solution. So use 15ml for 1 ml calmag bottled, right?
I apologise. I’m an idiot (and an asshole). Went storming down stairs thinking what are these guys going on about with it not being soluble? I use it all the time! Turns out I was mixing it up in my head with something else.
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It’s not very scientific, but you could boil a pan full of eggs. Save the water there will be lots of dissolved calcium in it from the egg shells.
A 5% by volume solution would be 5mL into 95mL to form 100mL. Therefor, 5% of that solution is vinegar. Also you have to take into consideration the dilution factor of the vinegar youre using (most vinegar in stores is not pure acetic acid). So you’ve diluted it further and that needs to be again calculated to determine your molarity. Also, gypsum is a solid, and therefor would be listed as a g/Mol rather than a molarity. Lemme sober up and go through the balanced equation and dilutions and see what I come up with… If you get a fine enough powdered gypsum, it should be water soluble. I figure itd be easier just to add epsom salt along with gypsum and then water with that as youre also getting a good sulfur boost.
Interested,following
Im an organic grower but Im surprised no one has mentioned solution grade gypsum from places like Kelp For Less. Im not a hydro grower so I dont pretend to know how it would work. My soil mixes I add solution grade gypsum and lime. Plus a little Biomin liquid calcium and epsom salts mid flower if I think it needs it but it doesnt most of the time, I just use it once in a water feeding mid flower at most. I put plenty of solution grade in the soil mix, on the order of two red solo cups per 3 cubic feet of soil and one red cup of lime.
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