I used to use GH calimagic, or Botanicare CalMg with my reverse osmosis water, for deficiencies but it was getting real expensive, so I searched for alternatives. I can't vouch exactly for the science behind what I did, but I can verify that my plants didn't die, and they also seemed to not have Cal or MG deficiency.
Basically I took information from various people who have already done similar, and somewhat modified it in different ways. Basically the major brands of nutrients based on their bottle labeling got their calcium and magnesium from things I had trouble finding, or seemed too dangerous for me to want to have mailed to my house. The major brands of Cal/Mag supplements have Calcium/Magnesium/iron chelate. So I'll tell you whats in mine, how much to use, and why I chose the amounts. I mixed the chemicals with my own RO water. The qtys that I purchased could make like 100+ gallons I think.
Magnesium Sulfate (Epsom Salts) - This can be purchased online, but I bought it from CVS for like $2. You can find it in many drug stores and stuff, it's considered a plant nutrient on its own, and it's used as bath salts and other things.
Calcium Nitrate - I purchased this on eBay. I got 5lbs of it for $14.00, and turns out I bought way too much. Primarily a fertilizer additive though it has some other uses. Seemed pretty harmless to buy online, and was easy to find.
Chelated Iron - Iron EDTA - This guy I don't know much about. I found some on eBay for like $15, though I've seen it in many forms. The one I used was actually a powder. I think the powder is for commercial sized fields of vegetable crops, but someone had made tiny packages of it to sell on eBay. I think the nutrient companies are selling water soluble iron chelate with nitrogen now, which would probably be what I buy next time.
Now to figure out the proportions I used an old formula by "FATMAN" from a different message board and plugged it into Hydrobuddy a nutrient calculator to figure out the weights. The one problem is that the PPM vs the Weights FATMAN recommended didn't match. So I basically just plugged his PPMs into HydroBuddy, and used those weights to make my mixture.
fatmans Cal-Mag
PPM
Nitrogen 200
Magnesium 120
Calcium 259
Sulfur 160
Iron 10.00
I plug these PPMs into HydroBuddy and the results it gives me are so:
Magnesium 120ppm
Calcium 259ppm
Sulfur 160ppm
Iron 10.00ppm
Per 1 Gallon of water:
IRON EDTA : .291 grams
Calcium Nitrate: 5.8 grams
Magnesium Sulfate: 4.607
Please note that these amounts are pretty tiny. I used a pretty accurate scale for this and made a gallon. Next time I plan to make a 5 gallon batch. I feel like it could be more accurate if I could work with larger qtys of these chemicals. Another side note, mixing Calcium nitrate and Magnesium Sulfate is supposed to create Gypsum. So I made sure to completely dissolve my Calcium nitrate in my gallon of water before adding the Magnesium Sulfate or Iron Chelate.
I am not a scientist, and I honestly don't know how to work Hydrobuddy that well. I have been growing for some years now, though I don't consider myself a grand master or anything. I did go through 1 gallon of this mix with a fairly large crop. I used approx 1 teaspoon per gallon of water with my regular nutrient schedule. (I use General Hydro 3 part + some supplements). I added this CalMag mixture at least 70% of the time that I watered. I would normally just add it every time, especially since its so cheap, but sometimes I was in a real hurry or something or paranoid about my experimental mixture.
I can't give you a guaranteed analysis or anything, but I can tell you this. My plants grew pretty much the same as if I had been still feeding the name brand CalMag. And my plants didn't seem to have deficiency in Calcium or Magnesium. I'm making some more, and I'm going to keep using it. I'll post if something changes, but I would love to hear input from people smarter than me.
