We use well water here, which is a little on the hard side. Tomatoes tolerate it, but most other plants start to struggle once past the seedling stage – I would guess it’s because any buffering capacity in the soil has been overcome and the soil is starting to resemble the pH of the water that’s been used.
In the past, I’ve used a few tablespoons of vinegar when watering blueberries that had started to brown, and more recently when I’ve been too lazy to distill water (it takes so long!), I have used 1-2 tablespoons of vinegar in a 4L watering can for spruce (a random guess that has kept them alive thus far).
Fast forward to today, and I figured it was about time to start getting correct pH levels!
Before I get to the numbers (what amounts of citric or vinegar resulted in what pH levels), I’ll mention how I arrived at them.
I grabbed a digital pH meter from Amazon. This is the common cheap yellow one sold under such highly renowned brands as “Dr. Meter”, “Etekcity”, “Dr. Health”, and “Xcellent Global”. They all look pretty much identical, all have mixed reviews, and the biggest difference seems to be how many packets of calibration/buffering powder they come with (if any). This is what it looks like:
Calibration and why accuracy went out the window quickly
There’s a 6.86 and 4.01 pH packet that you’re supposed to use to calibrate. Add the 6.86 to 250mL (a cup) of water, mix it up, stick the meter in a little and adjust the screw. Then do the same with the 4.01 pH packet with a new cup of water.
I did this with distilled water and had the same issue that others have run into: You could calibrate it at 6.86, but then it was a little off at 4.01 (showed 4.10). Calibrate at 4.01 and it’s off at 6.86 (showed 6.65).
Since plants usually tolerate wide pH ranges like 5.5-7.0, this wasn’t a huge deal for me. As long as I’m not at the min/max, it’s not the end of the world if I’m off by 0.1 or 0.2.
- pH of the tap water: 7.5 – 7.7 pH
- pH of water run through the distiller: 6.1 – 6.3 pH
The distilled water’s just listed for reference. It’s expected to be a little acidic since it absorbs CO2 and forms carbonic acid, getting close to 6 was lower than I had expected. I measured a few times (and a few samples) to be sure.
3.5 – 4L of Tap Water (7.5 – 7.7 pH) with Citric Acid added:
- 1/8 tsp citric acid: 6.1 – 6.3 pH
- 1/4 tsp citric acid: 5.3 – 5.5 pH
- 1/2 tsp citric acid: 4.4 – 4.6 pH
- 1 tsp citric acid: ~3.5 pH
- 1 tbsp citric acid: ~2.5 pH
Note that the watering can was filled with tap water to roughly 3.75 L and thoroughly rinsed after each attempt. Measuring out powder is easy too, so I’m fairly happy with these numbers.
Because such small amounts of citric acid have such a large effect, I wouldn’t recommend using citric acid to adjust your pH if operating in “guess mode”. You really need to measure stuff here (and know your original pH) since it doesn’t take much to get into plant-killing territory.
Even with a pH meter, an accidental double-dose or poor measurement could have a drastic effect, so keep that in mind.
3.5 – 4L of Tap Water (7.5 – 7.7 pH) with Vinegar (5% acetic acid) added:
- 1 tbsp vinegar: 5.8 – 6.0 pH
- 2 tbsp vinegar: 5.4 – 5.6 pH
- 3 tbsp vinegar: 5.0 – 5.2 pH
- 4 tbsp vinegar: 4.5 – 4.7 pH
- 5 tbsp vinegar: 4.4 – 4.6 pH
- 6 tbsp vinegar: 4.2 – 4.4 pH
- 7 tbsp vinegar: 4.1 – 4.3 pH
- 8 tbsp vinegar: 4.0 – 4.2 pH
- 9 tbsp vinegar: 4.0 – 4.2 pH (actually 0.07 less)
I was pretty sloppy measuring here since it’s a little cumbersome to pour from the jug of vinegar into a little tablespoon. The difference between 8 and 9 tbsp was really small (0.07), which is why I stopped there. Getting below a pH of 4 wasn’t going to happen without a *lot* of vinegar, and we’re much too acidic for most plants at that point anyway.
The really interesting thing to note here is that things REALLY started to slow down once I hit a pH of about 4.5. I’ve read about spruce being fine at 4.5 and tolerating as low as 4.0. That gives a lot more leeway than I had expected – spruce might be a little tough to over-acidify with vinegar (within reason).
If nothing else, “ballparking” pH with vinegar should be a lot safer than trying to ballpark with a stronger acid, particularly when dealing with more acid-loving plants (spruce/blueberries/etc). I had previously been using 2 tbsp – turns out, they probably would have survived anything up to 9.
Anyway, I’ll leave it at that. This is mainly for my own use, but if you’re reading around and trying to “guestimate” how much you’ll need to acidify your water, hopefully you find something above to be helpful.
However, keep in mind that my tap water is undoubtedly much different from yours. Different starting pH, different dissolved minerals, etc. What brings my water to a plant-happy-place might turn your water into a plant-death-solution, so try to get a hold of some pH testing materials. If that’s out of the question, at least search around to see what results others have come up with ;)