25 June 2026 · 6 min read
Lawn fertiliser numbers explained: what NPK actually means
Confused by 22-3-12 on the bag? Here's what each number is and how to pick the right ratio for your lawn and season.
Every bag of lawn fertiliser has three big numbers on the front. Something like 22-3-12 or 10-10-10. Those numbers are the percentage of nitrogen, phosphorus, and potassium in the bag — in that order. Pick the right ratio for the season and your lawn does the rest of the work.
What do the three numbers mean?
They are always in the same order: N - P - K.
- N is nitrogen. Makes the lawn green and helps it grow.
- P is phosphorus. Builds strong roots.
- K is potassium. Helps the lawn handle stress, cold, and dry spells.
So a 22-3-12 bag is 22% nitrogen, 3% phosphorus, 12% potassium. The rest is filler that helps spread it evenly.
The higher the first number, the faster you'll see green-up. The higher the third number, the tougher the lawn becomes.
What does each nutrient actually do?
Nitrogen (N)
The big one. Nitrogen drives leaf growth and gives the lawn its colour. A nitrogen-starved lawn looks pale yellow-green and grows slowly.
Too much nitrogen is a real thing. Push too hard and you get fast soft growth that's weak to disease, weeds, and heat stress. You'll also mow twice a week and burn the grass if you overdo it.
Phosphorus (P)
Builds roots. New lawns and freshly seeded patches need phosphorus to get established. Once the lawn is well-rooted, you barely need to add any.
Many regions now restrict phosphorus fertiliser because it runs off into waterways and causes algae blooms. Most maintenance fertilisers have only 2-3% P for this reason. That's plenty for an existing lawn.
Potassium (K)
The toughener. Potassium helps the grass handle cold, heat, drought, and disease. It doesn't make the lawn greener but it makes everything else the lawn does work better.
Most home lawns are short on potassium because rain washes it out. A winterising fertiliser is usually high in K.
What ratio should I pick?
Match the ratio to what your lawn needs right now.
| Season / situation | What to look for | Why |
|---|---|---|
| Early spring | High N, low P, medium K (eg 22-3-12) | Wake up the lawn, push green growth |
| Late spring / early summer | Balanced (eg 16-4-8 or 10-10-10) | Steady growth, not too much push |
| Mid summer (hot) | Low N, medium K (eg 8-2-12) | Avoid pushing growth in heat stress |
| Autumn | Balanced or higher K (eg 13-2-13) | Build root strength before winter |
| Late autumn / winterising | Low N, high K (eg 6-1-18) | Toughen up for cold |
| New lawn (seed or sod) | High P starter (eg 18-24-12) | Help roots get going |
Common fertiliser brands and ratios
Here's what's commonly on shelves around the world:
| Region | Brand | Typical ratio |
|---|---|---|
| AU | Scotts Lawn Builder | 22-1-8 |
| AU | Yates Dynamic Lifter Lawn | 5-1-5 (organic) |
| AU | Lawn Solutions Premium | 21-1-11 |
| NZ | Tui LawnForce All Purpose | 10-2-5 |
| US | Scotts Turf Builder | 32-0-4 |
| US | Milorganite | 6-4-0 (organic) |
| US | Espoma Organic Lawn | 9-0-0 |
| UK | Miracle-Gro Evergreen | 10-3-3 |
| UK | Westland Aftercut | 13-0-5 |
| CA | Scotts Turf Builder | 30-0-4 |
| ZA | Wonder Lawn & Leaf | 7-1-3 |
| ZA | Atlas Lawn Fertilizer | 5-1-5 |
If the bag has a fourth number (like 22-3-12-2), that's usually sulphur or iron. Both help with deep green colour.
How much do I spread?
The bag will say, but the rough rule is:
- For a 200m² backyard (typical house lawn): half to one full bag of a 5kg fertiliser, depending on the strength of the N number.
- For a 50m² front lawn: about a quarter of a bag.
Use a hand spreader to keep it even. Walk in one direction first, then walk across in the other. Two passes at half rate is better than one pass at full rate. Water it in well right after you spread.
If you spread too much, expect to see streaks of dark green and burnt brown where you went heavy. Not the look you want.
What's the difference between organic and synthetic NPK?
The numbers mean the same thing. The difference is how fast the nutrients release.
- Synthetic (Scotts, Miracle-Gro) — works fast, lasts 4-6 weeks, easier to burn the lawn.
- Organic (Milorganite, Yates Dynamic Lifter) — works slowly, lasts 8-12 weeks, almost impossible to burn.
Most home lawns do well on a mix — synthetic in spring for a fast wake-up, organic through the rest of the year for steady feeding.
When should I fertilise?
Roughly every 6-8 weeks during the growing season. So 4-5 times a year for most lawns.
Don't fertilise:
- In the middle of a heatwave (you'll burn the lawn)
- In winter dormancy (the lawn isn't growing so it can't use it)
- Right before heavy rain (it all washes away)
- Within 2 weeks of new seed (use a proper starter fertiliser instead)
Frequently asked questions
Is 10-10-10 a good all-round fertiliser?
It's fine but not ideal. Most lawns don't need that much phosphorus. A ratio closer to 3-1-2 (so 24-8-16 or 12-4-8) suits established lawns better.
Why is the phosphorus number so low on most lawn fertilisers?
Phosphorus runoff pollutes waterways. Many regions limit it by law. Existing lawns don't need much P anyway. Save the high-P bags for new seed or sod.
Can I just use a high-nitrogen bag year-round?
You can but you shouldn't. Too much nitrogen makes soft weak growth that gets hammered by disease. Mix in a higher-K bag in autumn to balance it out.
What about iron supplements?
Iron deepens the green without pushing growth. Great in summer when you want colour but don't want to mow more often. Look for "lawn greener" or "iron sulphate" products.
Does it matter if the lawn is wet when I fertilise?
Yes. Spread on a dry lawn, then water it in. Wet leaves grab the granules and burn.
How Lawnova helps
Lawnova picks the right NPK ratio for your specific grass type, soil, and the time of year. We tell you what to buy, when to spread it, and roughly how much. No more standing in the garden centre staring at numbers. Sign up here.