How to read a fertilizer grade — and what the label does not tell you
Every fertilizer bag carries three numbers, and every buyer knows roughly what they mean. The trouble starts one level down, where the convention has assumptions built into it that are never printed on the bag and are frequently the reason a shipment is rejected.
The convention
N – P2O5 – K2O, as percentage by weight, in that order.
- N is elemental nitrogen.
- P2O5 is phosphate expressed as the oxide, not as elemental phosphorus.
- K2O is potash expressed as the oxide, not as elemental potassium.
So DAP 18-46-0 is 18% nitrogen, 46% phosphate (as P2O5), and no potassium. TSP 0-46-0 has no nitrogen at all.
Two conversions are worth knowing: P2O5 × 0.436 = elemental P, and K2O × 0.83 = elemental K. A laboratory report may state either form, and a buyer who does not check which one they are reading can be out by more than a factor of two.
Total, available and soluble
This is where disputes come from.
Total P2O5 is everything in the product, including phosphate the plant cannot access.
Available P2O5 is what a standard extraction — usually citrate — shows the plant can use. It is the figure the trade quotes, and the figure the price is built on.
Water-soluble P2O5 is the fraction that dissolves immediately. For MAP and DAP it is typically 87–90% of the available figure; for a purified soluble grade it approaches 100%.
A quotation that says “P2O5 46%” without saying which of the three it means is not a specification. This is the single most common cause of phosphate quality disputes, and it is entirely avoidable: name the basis and the test method, in the contract.
Why grades are not comparable on price
Consider 18-46-0 at one price per tonne and 11-52-0 at another. The second carries more phosphate and less nitrogen per tonne. Whether it is better value depends on whether you want the nitrogen.
The comparison that works is cost per tonne of P2O5 delivered, not cost per tonne of product:
- Take the price per tonne of product.
- Add the freight per tonne to your port.
- Divide by the P2O5 percentage.
- If the product carries nitrogen you would otherwise have bought, credit its value; if not, do not.
Doing that across two offers frequently shows a “cheaper” product to be the more expensive one per nutrient unit — and occasionally the reverse. It takes five minutes and it is the highest-return five minutes in a purchasing cycle.
What the grade does not tell you
The three numbers describe nutrient content and nothing else. They say nothing about:
- Granule size and strength — which decide whether the material blends and spreads
- Moisture content — which drives caking, and which is weight you paid for that is not nutrient
- Water-soluble fraction — which decides immediate availability
- Free acid — which corrodes equipment and bag liners
- Bulk density — which decides blend compatibility
- Impurity limits — which decide whether the grade is legally usable in feed or food
Every one of those belongs in the specification, and every one of them is on the COA if you ask for it.
Compound grades
An NPK compound such as 15-15-15 is a different proposition from a blend of the three straight fertilizers. A compound is chemically granulated, so each granule carries all three nutrients and segregation is not possible. A blend is a physical mixture, so each granule carries one nutrient and segregation is possible if the sizes do not match.
Two products with the same grade number can therefore behave completely differently in the field. If uniformity matters — and for a machine-applied programme it does — the compound is the safer specification.
A short checklist
Before accepting a quotation, confirm:
- The grade, and whether the figures are total, available or water-soluble
- The test method for each nutrient figure
- Moisture, granule size, crushing strength (for granular material)
- Free acid, if it is a superphosphate
- Packaging, liner and loaded quantity
- The documents the shipment will carry
That is a specification. A grade and a price is a starting point for a conversation.
Grade conventions and analytical bases described here are standard industry practice. For a quotation against a specification, ask the export desk.
Questions buyers ask
What do the three numbers on a fertilizer bag mean?
Nitrogen (N), available phosphate (P2O5) and potassium (K2O), as percentages by weight, in that order. A 18-46-0 bag is 18% nitrogen and 46% phosphate with no potassium. The zero is information: it tells you the product supplies none of that nutrient.
Why is phosphate written as P2O5 and not as P?
Because the fertilizer industry expresses phosphate as the oxide, a convention inherited from analytical chemistry. To convert P2O5 to elemental P, multiply by 0.436. The convention is universal in trade, but a laboratory report may state either, so check which one you are reading.
Is a higher grade always better?
No. A higher grade means more nutrient per tonne, which reduces freight and handling cost per nutrient unit. It says nothing about availability, granule quality, storage behaviour or suitability for your soil. Those are separate specifications.
Need a price for a specific cargo?
Give us the product, grade, quantity and discharge port, and the quotation comes back with the specification basis stated.