Short answer
You can, but it's a poor choice, and the claim that it's a fine substitute for pH down is a Myth. In a hydroponic trial, vinegar reduced yields of all three crops tested and lime juice reduced two of three, while mineral pH down held pH most stably with the least product [1]. Use phosphoric, nitric or sulfuric acid.
Why
The only controlled comparison we know of grew lettuce, basil and Swiss chard in ebb-and-flow hydro at pH 5.5–6.5. Vinegar cut yields of all three; lime juice cut basil and chard but not lettuce; the phosphoric-acid pH down was the most stable and needed the least product [1]. No cannabis trial exists, but nothing suggests cannabis would react better.
Organic acids are weak and short-lived. The water's bicarbonate neutralises acid as it goes [2], and microbes in the reservoir or medium eat citric and acetic acid, so pH rebounds (grower observation). What's left feeds biofilm.
Alkalinity is the real enemy. If pH keeps climbing, the fix is neutralising bicarbonate with a mineral acid, sized to the water report [2][3].
The numbers
| Adjuster | pH stability | Plant effect (trial [1]) | Adds |
|---|---|---|---|
| Mineral pH down (phosphoric) | Best | No yield loss | P |
| Lime or lemon juice (citric) | Short-lived | Lower yield in 2 of 3 crops | Carbon for microbes |
| White vinegar (acetic) | Short-lived | Lower yield in 3 of 3 crops | Carbon for microbes |
| Nitric or sulfuric acid | Good | Standard practice | N or S |
Verdict: buy a mineral-acid pH down; kitchen acids cost yield and come back to bite.
What to do
- Buy horticultural pH down (phosphoric, nitric or sulfuric).
- Add it after nutrients, a few drops per 10 L.
- If pH won't hold, get a water report and treat alkalinity.
- In an emergency, a small dose of citric acid once is tolerable in soil; replace it at the next feed.
Safety: mineral acids burn. Gloves, goggles, and always add acid to water.
Common mistakes
- Dosing vinegar into a DWC reservoir and growing slime.
- Treating pH rebound with more lemon juice.
- Ignoring the alkalinity that caused the problem.
- Forgetting that acids add nutrients.
Related
- Full article: Water quality: alkalinity and acids
- Answers: how to adjust pH · do I need to pH?
- Articles: pH drift in hydro
- Terms: pH down · phosphoric acid
