Phosphoric acid is a mineral acid of phosphorus. The 85% technical grade contains roughly 27% P by weight. Diluted retail pH-down products are usually phosphoric acid at a much lower strength. Adding it lowers pH and supplies phosphate at the same time.
In practice
For most growers with soft or RO water, the phosphorus from pH-down is small. With hard, high-alkalinity water, the acid needed to neutralise bicarbonate can add a meaningful amount of P, sometimes tens of mg/L (ppm). Count it towards the feed's P total, or split acidification with nitric or sulfuric acid. Concentrated acid is corrosive: wear eye and skin protection, add acid to water and never the reverse.
Don't confuse with
- Nitric acid: lowers pH and adds nitrogen instead.
- Citric acid: an organic acid that microbes break down, so pH drifts back up.
- Phosphorous acid (phosphite): a fungicidal compound, not a P nutrient.
pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.Phosphate is the ionic form in which plants absorb phosphorus, mainly H2PO4- at the normal root-zone pH of 5.5-6.5.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Nitric acid (HNO3) lowers pH while adding nitrate nitrogen; growers use it to neutralise hard-water alkalinity, often alongside or instead of phosphoric acid.Phosphorus (P) is a mobile macronutrient used in energy transfer (ATP), DNA and cell membranes; cannabis needs moderate amounts and gains nothing from excess.pH Down is any acid used to lower solution pH, commonly phosphoric, nitric or sulfuric acid; each adds a nutrient, and all need gloves and eye protection.
