First of all, here's my approach to acidification (more info than you probably want! ):
When you read my presentation, this will be the first response you see:
Not the only way, but contrary to what Aloinopsis said, a pH meter like the Milwaukee Instruments pH 600 is the most accurate.Aloinopsis wrote: ↑Tue Sep 10, 2019 11:09 amYou can also use pH test strips designed for aquarium use, culinary use, or medical use (such as for measuring urine pH, common in urologists' offices). I've used the aquarium and medical ones over the years with excellent results. They're just as accurate as a pH meter, easier to handle, and biodegradable so you can just put them in the compost. There's nothing wrong with a pH meter, but it's not the only or best way to measure water's pH.
Okay, back to your question -- there were years when I would have to use 2 or 2.5 tsp. of white vinegar to get a pH of about 5.5 per gallon of my watering solution. The tap water here in L.A. isn't as hard as it used to be, so I'm finding that 1 tsp. vinegar per gallon of water plus fert gives me a final pH of about 5.8. (I could add a little bit more to hit the target of 5.5, but I don't think we need to be all that strict about it.) If you have confidence in using pH test strips, that's fine -- the point is to make sure that you get an accurate reading of the pH in your tap water before you determine how much acidifier you'll need. Unless the tap water in Antwerp is incredibly hard, I can't imagine that you would need more than 2 or 3 tsp. of white vinegar per gallon. If that's not sufficient, citric acid is easily available online. Always, always, always test the final pH of your acidified watering solution -- if it's below your target range, dump it out and reduce the amount of acidifier so that you're not over-acidifying your plants.
That's right -- US gallons. Just to give you some perspective here -- being an apartment-dweller myself, all I have is 12 square feet of outdoor growing space with a total of 68 cacti (mostly smaller species, although some of them aren't so small anymore!). 1 gallon of Potassium sulfate stock solution lasts me for the better part of a growing season, adding 5 fluid ounces of stock solution per gallon of watering solution. Remember that I grow the vast majority of my cacti in a soil-less 60% pumice/40% granite gravel mix, so it's an essentially hydroponic approach which requires fertilizing every time I water. If you're growing your cacti in a mix containing soil, you won't need to fertilize as often. Should that be the case, also remember that ferts lower the pH -- for the times when you water your cacti without fert, you'll need to adjust the amount of acidifier accordingly.ohugal wrote: ↑Sat Aug 13, 2022 8:14 am I can store a stock solution of potassium sulfate in the basement. It's dark most of the time and the temperature doesn't fluctuate as hard as in the rest of the house. I have a scale with is 0,1 gram accurate. In this case I could use distilled water. How much of the stock solution do you add to the nutrient solution? Just to be sure I calculate correctly, you are working with US gallons, correct?
One more thing about the Potassium sulfate...
My calculations for creating a 1-gallon stock solution and amount of stock solution going into a gallon of watering solution are specific to NPK 7-7-7 diluted at the rate of 1/2 tsp. per gallon for watering. If members use different NPK values and/or dilution rates, they'll need to come up with different calculations. In case anyone needs help with that, let me know.