What the Three Numbers on Fertiliser Mean

The three numbers are percentages by weight

A bag marked 10-10-10 contains 10% nitrogen, 10% phosphate and 10% potash by weight. The remaining 70% is carrier and filler.

Strictly, the second and third figures are not the elements themselves. They are phosphoric oxide (P₂O₅) and potash (K₂O) — an old convention that survives on every label in the country. It rarely matters in practice, but it explains why the numbers do not match the elemental content if you ever check.

What each one does

Nitrogen (N) drives leafy, vegetative growth. It is the nutrient most often deficient and the one most often overapplied.

Phosphorus (P) supports root development and flowering and fruiting.

Potassium (K) is involved in general hardiness — water regulation, disease resistance, overall vigour.

Those are useful shorthands, not rules. Feeding a flowering plant more phosphorus does not force it to bloom if the real limit is light.

How to compare two products honestly

The percentages let you compare bags of different sizes and strengths, which the marketing rarely does for you.

Multiply the percentage by the bag weight. A 50 lb bag of 10-10-10 contains 5 lb of actual nitrogen. A 25 lb bag of 20-10-10 contains the same 5 lb. The second bag is half the size and identical in nitrogen.

Once you think this way, the price comparison becomes straightforward: cost per pound of actual nutrient, not cost per bag.

Balanced, complete and specialised

A complete fertiliser contains all three nutrients. A balanced one has roughly equal numbers, like 10-10-10.

Balanced is not automatically appropriate. If your soil already holds plenty of phosphorus — common in gardens that have been manured or fertilised for years — a balanced feed keeps adding a nutrient you do not need. Excess phosphorus is an environmental problem in runoff, and Clemson HGIC notes that indiscriminate fertiliser addition can be harmful where levels are already high.

Products with a zero, like 0-20-20 or 21-0-0, exist precisely so you can supply what is short without adding what is not.

Which is why the soil test comes first

A bag tells you what is in it. Only a test tells you what your soil is missing.

Clemson’s list of what testing prevents is worth quoting for its bluntness: under- or over-fertilisation causing “excessive vegetative growth, delayed maturity, salt burn, and wasted money.”

There is one exception. Standard soil tests do not measure nitrogen, because it moves too readily through soil to give a stable reading. Nitrogen recommendations come from what the crop needs rather than from what the soil holds — so for that nutrient, follow crop guidance rather than looking for a number on your report.