What NPK actually means on a fertiliser bag

The three letters, and what each one does

Every fertiliser bag carries three numbers, and they're the percentage by weight of nitrogen, phosphorus and potassium in the bag. Once you know what each one drives, the whole aisle stops being a wall of packaging and turns into a fairly simple choice.

• N, nitrogen. Leaf and stem. It's what makes things green and pushes soft growth, and it's the one that leaches out of a sandy soil fastest, so it needs replacing most often.
• P, phosphorus. Roots and the early establishment of a plant. It moves very slowly through soil and sticks around, so it's the one most likely to be already present in an established bed.
• K, potassium. Flowers and fruit, plus general hardiness. It's involved in how a plant manages water, so a plant short of potassium tends to wilt and scorch more readily than it should.

Reading the ratio rather than the numbers

What matters is the balance between the three rather than how big they are. A high first number relative to the others is a leaf fertiliser, which is what a lawn or a leafy crop wants. A high last number is a flower and fruit fertiliser, which is what a tomato or a rose wants once it's past making leaves.

Roughly even numbers make an all purpose feed, and for most of a suburban garden that's genuinely all you need. The specialist products aren't a con, they're just answering a narrower question, and if you don't have that narrow question then a balanced feed does the job.

The size of the numbers tells you strength rather than quality. A bag reading 5-5-5 isn't worse than one reading 15-15-15, you just use more of it, and a lot of the organic pelletised products carry low numbers precisely because they're feeding the soil slowly rather than the plant quickly.

The mistake worth avoiding

Piling on a high nitrogen feed because something looks tired is the commonest one, and it's how you end up with a tomato plant that's a metre of magnificent leaf with no fruit on it. Nitrogen tells a plant to grow foliage, and a plant busy growing foliage isn't busy flowering.

The other one is assuming a poor looking plant is hungry at all. Yellow leaves come from waterlogging and from a pH problem at least as often as they come from a nutrient shortage, and feeding a plant that's actually drowning just adds insult. Check what the soil is doing before you reach for a bag.

Where a specialist feed genuinely earns it

Some plants really do want something particular, and it's a short list rather than the whole aisle. Citrus are heavy feeders with a real appetite for nitrogen and trace elements. Roses want potassium once they're budding up. Australian natives in the protea and banksia families are the important exception, because they evolved on phosphorus poor soils and a standard fertiliser can genuinely damage them, so those want a low phosphorus native feed and nothing else.

Everything else in a normal garden will do perfectly well on a balanced all purpose feed applied a couple of times through the growing season. If you want the detail on the all purpose granular we keep, I've written it up properly in Brunnings All Purpose NPK, what it is and when to use it.

Shop related products

• Brunnings Blood and Bone Based Fertiliser 5kg. A gentle general feed that builds the soil as it goes.
• Brunnings Fruit and Citrus Food 2.5kg. For the heavy feeders that genuinely want their own product.
• Brunnings Rose and Flower Food 2.5kg. Weighted towards flowering rather than leaf.
• Brunnings Green Up Lawn Food 2.5kg. High nitrogen, which is the one case where that's the point.

Did you know the Brunnings All Purpose NPK in the 3kg tub is one we keep in the garden centre rather than on the website, so give us a ring on (08) 8536 4333 to check what's on the shelf. There's more of the range in the Brunnings collection and the wider garden centre range.

Read next

• Brunnings All Purpose NPK, what it is and when to use it
• Brunnings Potash Power, and what potassium actually does
• Neutrog Kahoona, what it is and when to use it

I still read the three numbers twice in the aisle, on the grounds that the bags all look identical from a metre away.

Cheers,
Candeece

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