The rate on a bag of lawn food is a guide for an average lawn. How much yours needs depends on your grass and the real size of the lawn. Work it out from the nitrogen and you can use any product and get it right.
How much nitrogen does your grass need per feed?
Per feed, per square metre:
| Grass | Nitrogen per feed | With NPK 15-2-10 |
|---|---|---|
| Couch | 4.5 g/m² | 30.0 g/m² |
| Buffalo (soft leaf) | 3.5 g/m² | 23.3 g/m² |
| Ryegrass | 3.5 g/m² | 23.3 g/m² |
| Tall fescue | 3.0 g/m² | 20.0 g/m² |
| Zoysia | 2.5 g/m² | 16.7 g/m² |
Kikuyu is a hungry grass and sits close to couch. In the app, choose Bermuda for couch and St. Augustine for soft leaf buffalo, since they are the same species under their American names.
How do you convert nitrogen to grams of fertiliser?
Divide the nitrogen by the percentage, as a decimal:
grams of fertiliser per m² = grams of nitrogen ÷ (N percentage ÷ 100)
With 15-2-10 on buffalo: 3.5 ÷ 0.15 = 23.3 g per m². With a 25 percent product: 3.5 ÷ 0.25 = 14 g per m². Same nitrogen, much less weight.
Example: a 260 m² buffalo lawn in Sydney
The example lawn in Sydney is soft leaf buffalo, traced on the satellite map in the app at 260 m². With a 15-2-10 buffalo lawn food:
- 3.5 g of nitrogen × 260 m² = 910 g of nitrogen
- 910 g ÷ 0.15 = about 6,070 g, so 6.1 kg of fertiliser
- That is 23.3 g per m²
In the app you trace the lawn on a satellite map and it measures the area. Enter the NPK from the bag and the amount for the whole lawn appears in kilos. Without an NPK it shows grams of pure nitrogen, so you can do the sum for any product.
How do you measure your lawn without an app?
Break it into rectangles, multiply length by width, add them up and take off garden beds and paths. Being 50 m² out on a 260 m² lawn means a fifth too much fertiliser, so it is worth a few minutes with a tape measure.
How do you spread it evenly?
- Weigh out the amount you worked out.
- Spread half walking one way, the other half walking across.
- Sweep granules off paths and driveways back onto the lawn.
- Water it in with about 5-10 mm if no rain is due.
How often should you apply it?
Three or four times a year, six to eight weeks apart during the growing season. When, and how to pick the day, is in When to Fertilise Your Lawn.
What a Green Lawn works out the amounts for the lawn you trace and keeps track of when you last fed it, so you never double up.