Nitric acid is a strong mineral acid that supplies nitrate when it neutralises alkalinity. Pure HNO3 is about 22% N by weight; commercial strengths are lower. It is the main pH-down acid in commercial greenhouse hydroponics.
In practice
Growers with hard, high-bicarbonate water often use nitric acid so that acidification adds nitrogen rather than too much phosphorus. The nitrogen it adds must be counted in the feed total, which matters during a late-flower N taper. Langenfeld and Bugbee studied nitric acid with ammonium nitrate for pH control in recirculating hydroponics. Concentrated nitric acid is highly corrosive, gives off toxic fumes and is an oxidiser. Most home growers buy it pre-diluted as a pH-down product, and should keep to that. It may be regulated in some places; check local rules.
Don't confuse with
- Phosphoric acid: lowers pH while adding phosphorus.
- Sulfuric acid: lowers pH while adding sulfur.
- Nitrate salts: supply nitrate without lowering pH.
Nitrate (NO3-) is the main form of nitrogen in hydroponic and soilless feeds; its uptake tends to push root-zone pH upward.Alkalinity is water's capacity to neutralise acid, mainly from dissolved bicarbonate and carbonate, reported in mg/L as CaCO3; it decides how much pH Down you need.Gradually cutting nitrogen in late flower, toward zero near the finish, while still feeding phosphorus, potassium, calcium, magnesium, sulfur and micronutrients; GenetixBay's alternative to flushing.Ammonium nitrate (NH4NO3) is a 34-35% nitrogen salt, half ammonium and half nitrate, used in small amounts to steer root-zone pH in hydro.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.A recirculating (closed) system returns drained nutrient solution to the reservoir to be reused, saving water and nutrients but needing more monitoring and sanitation.Phosphoric acid (H3PO4) is the most common pH-down acid; it lowers pH while adding phosphorus, about 27% P in the usual 85% grade.Sulfur (S) is a secondary nutrient taken up as sulfate, used in the amino acids cysteine and methionine; deficiency yellows new growth fairly evenly.Nitrogen (N) is the macronutrient plants use most, built into proteins, chlorophyll and nucleic acids; it drives leaf and stem growth and is fully mobile.
