Phosphate is phosphorus combined with oxygen, present in solution as H2PO4- or HPO4 2- depending on pH. Below about pH 7 the H2PO4- form dominates, and that is the form roots take up most readily. Phosphate is also a buffer: it resists pH change somewhat in nutrient solutions.
In practice
Phosphate comes from monopotassium phosphate, monoammonium phosphate or phosphoric acid. It reacts with calcium to form insoluble calcium phosphate when both are concentrated, or when pH rises well above 6.5. That is why phosphate and calcium sit in separate A and B stocks, and why a cloudy reservoir after a pH spike can mean phosphate has dropped out.
Units and formula
Reports may quote PO4, P or P2O5. P = PO4 × 0.326; P = P2O5 × 0.436. Compare like with like.
Don't confuse with
- Phosphite (phosphonate): a related ion sold as a fungicide or "biostimulant"; plants cannot use it as a phosphorus source.
- Phosphorus: the element itself, counted the same whichever ion carries it.
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.Monopotassium phosphate (MKP, KH2PO4) supplies about 22.7% phosphorus and 28.7% potassium with no nitrogen, and is the base of most PK boosters.Monoammonium phosphate (MAP, NH4H2PO4) supplies about 12% nitrogen as ammonium and 26.6% phosphorus, adding P while gently acidifying the root zone.Phosphoric acid (H3PO4) is the most common pH-down acid; it lowers pH while adding phosphorus, about 27% P in the usual 85% grade.An A/B nutrient is a mineral feed split into two concentrates so that calcium (part A) is kept apart from sulfate and phosphate (part B), which would precipitate together.A biostimulant is a product claimed to improve plant growth, uptake or stress tolerance by means other than supplying nutrients, such as seaweed extract, humics, amino acids or microbes.Phosphorus (P) is a mobile macronutrient used in energy transfer (ATP), DNA and cell membranes; cannabis needs moderate amounts and gains nothing from excess.Calcium (Ca) is an immobile secondary nutrient that builds cell walls and membranes; it travels only in the transpiration stream, so deficiency appears in new growth.
