Available phosphorus: why it gets locked up and what pH has to do with it
Phosphorus is the worst-used nutrient, and not because the soil lacks it: because it gets fixed. In calcareous soil it precipitates with calcium; in acid soil, with iron and aluminium. Part of what you apply stays out of the root's reach that same season. This page does not promise to solve that — we have no phosphorus trial of our own and we say so — it publishes what is measured: the P₂O₅, the pH and the chloride of each source.
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laboratory-measured P₂O₅ in PHOSMAX 20, against a nominal 0-20-0 grade
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measured P₂O₅ in PK-FLOW 20, which also supplies 19.43 % K₂O
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sources in the range declaring phosphorus, with their pH and chloride
What happens to the phosphorus you apply
Phosphate is a readily reactive anion. In a calcareous soil, with abundant calcium and high pH, it forms increasingly insoluble calcium phosphates; in an acid soil it does the same with iron and aluminium. Either way the phosphorus is still there, but it stops being available to the root that season. A soil test can therefore report sufficient phosphorus while the crop shows symptoms.
The most favourable solubility window sits roughly between pH 6 and 7. Outside it, in either direction, the fraction the plant can take up narrows. This is soil chemistry, verifiable in any fertility textbook; it is not a property of a brand.
The product's pH is not the soil's pH
A strongly acidic fertilizer does not change a field's pH: the soil's buffering capacity far exceeds the volume you apply. What the product's pH does condition is the spray tank and the irrigation water, where phosphate can precipitate before it even reaches the soil.
Hard irrigation water is the first enemy
In hard water, phosphate meets calcium before it meets the root. That is why loading order and mixing compatibility matter as much for phosphorus as for calcium: they are two sides of the same reaction.
The nominal grade and the measured value do not match
PHOSMAX 20 carries a 0-20-0 grade and its analysis returns 18.9 % P₂O₅. PK-FLOW 20 carries 0-20-20 and measures 14.44 % P₂O₅ and 19.43 % K₂O. The grade is a designation; the measured value is the fact, and it is what we publish.
There is no phosphorus trial of our own here
We have third-party trials on nitrogen, potassium and calcium, and none on phosphorus. What is explained above is soil chemistry, not a result of ours. When a phosphorus trial exists it will be on the trials page, with institution, method and year.
Declared P₂O₅ by product
Values marked as measured come from laboratory analysis; the rest are the declared grade on the datasheet.
| Product | P₂O₅ | Difference |
|---|---|---|
| PHOSMAX 20 | 18,9 % P₂O₅ | — |
| PK-FLOW 20 | 14,44 % P₂O₅ | — |
| BALANCE 7-7-7 | 7 % P₂O₅ | — |
| BALANCE 5-5-5 | 5 % P₂O₅ | — |
| BALANCE 9-4-4 | 4 % P₂O₅ | — |
| BALANCE 6-3-18 | 3 % P₂O₅ | — |
| BALANCE 3-3-10 | 3 % P₂O₅ | — |
The seven sources, with pH and chloride
| Product | Grade | P₂O₅ | pH | Chloride | Form | Datasheet rate |
|---|---|---|---|---|---|---|
| PHOSMAX 20 | 0-20-0 | 18,9 %(measured) | < 1,00 | < 0,05 % | Liquid | Foliar 200–300 cc/100 L · Fertirrigación 5–10 L/ha |
| PK-FLOW 20 | 0-20-20 | 14,44 %(measured) | 7,49 | 2,74 % | Liquid | 2–3 L/ha foliar · 5–10 L/ha fertirrigación |
| BALANCE 7-7-7 | 7-7-7 | 7 % | — | 0,774 % | Soluble powder | 3–4 kg/ha |
| BALANCE 5-5-5 | 5-5-5 | 5 % | 4,50 ± 1,0 | sin ensayo | Liquid | Foliar 150–200 cc/100 L · Fertirrigación 2 L/ha |
| BALANCE 9-4-4 | 9-4-4 | 4 % | — | 1,40 % | Soluble powder | Foliar 150–200 g/100 L · Fertirrigación 2–3 kg/ha |
| BALANCE 6-3-18 | 6-3-18 | 3 % | — | 0,941 % | Soluble powder | 3–4 kg/ha |
| BALANCE 3-3-10 | 3-3-10 | 3 % | 8,00 ± 1,0 | sin ensayo | Liquid | Foliar 150–200 cc/100 L · Fertirrigación 2 L/ha |
No itemised composition and no chloride test: PHOSMAX 20 Suspensión Líquida (pH 6,9 ± 1,00).
Frequently asked questions about available phosphorus
Why does phosphorus get locked up in soil?
Because phosphate reacts with soil cations. In calcareous soil, at high pH and with abundant calcium, it forms progressively insoluble calcium phosphates; in acid soil it does the same with iron and aluminium. The phosphorus remains in the soil but stops being available to the root that season.
At what pH is phosphorus most available?
The most favourable window is roughly between pH 6 and 7. Above it, precipitation with calcium dominates; below it, fixation with iron and aluminium. This is soil chemistry, not a property of any product.
Does an acidic fertilizer lower my field's pH?
No. The soil's buffering capacity far exceeds the volume of product applied. What the fertilizer's pH does condition is the spray tank and the irrigation water: that is where phosphate can precipitate before reaching the soil.
How much phosphorus does PHOSMAX 20 really have?
Its nominal grade is 0-20-0 and its laboratory analysis returns 18.9 % P₂O₅. The grade is a commercial designation; the measured value is the fact. Its chloride is below the detection limit (< 0.05 %) and its pH is under 1.00.
Is PHOSMAX 20 hazardous to handle?
Yes, it is classified. Its safety data sheet carries the signal word «Danger», the GHS05 pictogram and hazard statement H314: causes severe skin burns and eye damage. Gloves, protective clothing and eye and face protection are required. The full sheet is published on the site.
Do you have a phosphorus trial of your own?
No. We have third-party trials on nitrogen (University of Arizona, Amalgamated Sugar), potassium and calcium (Holden Research), but none on phosphorus. We say so because the alternative would be to imply an efficacy we have not measured.