Boric acid is a weak acid of boron, about 17.5% B. In normal nutrient solutions most boron exists as uncharged boric acid, which is the main form roots absorb. It dissolves slowly in cold water; warm water helps when making stock.
In practice
Boron has a narrow window between enough and too much, so boric acid is dosed from a concentrated micronutrient stock, not by the pinch. Final feeds usually aim for about 0.2-0.5 mg/L (ppm) B. Check the water report first: some tap and well waters already supply a meaningful share. Borax (sodium borate, about 11% B) and solubor-type sodium borates are alternatives but add sodium.
Units and formula
1 mg/L of boric acid gives about 0.175 mg/L (ppm) B. To reach 0.3 ppm B, add about 1.7 mg/L.
Don't confuse with
- Borax: sodium tetraborate, a different boron salt with sodium.
- Boric acid as an insecticide: same chemical, different purpose and dose; never dose a feed from pest-bait products.
Boron (B) is a micronutrient needed for cell-wall structure and growing tips; the gap between deficient and toxic levels is narrow.A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.Tap water is municipal or well water used for irrigation; its EC, alkalinity, hardness, sodium and disinfectant vary widely, so it should be tested before use.Sodium (Na) is not essential for cannabis; it enters through water, softeners and some inputs, and at high levels causes salt stress and crowds out potassium and calcium.A stock solution is a concentrated nutrient mix, often 100x or 200x final strength, diluted into the reservoir; calcium and sulfate/phosphate stocks must stay separate.Nutrient toxicity is damage from an excess of an element in the plant, either directly or by blocking uptake of others; boron, manganese, copper and chloride are common culprits.
