The number on the spreader setting chart is a starting point, not a prescription. How much fertilizer your lawn needs depends on two things the bag cannot know: what grass you have and how big the yard really is. Get those right and you can use any brand and still hit the target.
How much nitrogen does your grass need per feeding?
Nitrogen is what you are really buying. Most of the rest of the bag is carrier. Per application, per 1,000 square feet:
| Grass | Nitrogen per feeding | With 30-0-4 |
|---|---|---|
| Bermuda | 0.92 lb | 3.07 lb |
| Kentucky bluegrass | 0.72 lb | 2.39 lb |
| Perennial ryegrass | 0.72 lb | 2.39 lb |
| St. Augustine | 0.72 lb | 2.39 lb |
| Tall fescue | 0.61 lb | 2.05 lb |
| Zoysia | 0.51 lb | 1.71 lb |
| Fine fescue | 0.41 lb | 1.37 lb |
| Centipede, buffalograss | 0.31 lb | 1.02 lb |
Not sure what you have? Use 0.6 lb. It is a safe middle for mixed lawns.
How do you convert nitrogen to pounds of product?
Divide the nitrogen rate by the first number on the bag, as a decimal:
pounds of product per 1,000 sq ft = pounds of nitrogen ÷ (N percentage ÷ 100)
With 30-0-4 and bluegrass: 0.72 ÷ 0.30 = 2.39 lb per 1,000 sq ft. With a 16-4-8 bag for the same lawn: 0.72 ÷ 0.16 = 4.5 lb. Nearly twice the weight for the same nitrogen, which is why “one bag per yard” does not work across brands.
Example: a 5,849 sq ft lawn in Chicago
The example lawn in Chicago is Kentucky bluegrass, traced on the satellite map in the app at 5,849 sq ft. With a 30-0-4 fertilizer:
- 0.72 lb of nitrogen × 5.849 = 4.2 lb of nitrogen total
- 4.2 ÷ 0.30 = 14.0 lb of product
- That is 2.39 lb per 1,000 sq ft
In the app you trace the lawn on a satellite map and it measures the area. Enter the N-P-K from your bag and the amount for the whole lawn appears in pounds. Without N-P-K, it shows the ounces of actual nitrogen, so you can do the conversion for any product.
How do you measure your lawn without an app?
Break it into rectangles, multiply length by width for each, and add them up. Treat curved beds as triangles or half circles and subtract the house footprint, driveway and large beds. Being off by 1,000 sq ft on a 4,000 sq ft yard means 25 percent too much fertilizer, so it is worth ten minutes with a tape measure.
How do you spread it evenly?
- Weigh out the amount you calculated.
- Set the broadcast spreader to half the rate on the bag.
- Walk the lawn in one direction, then again at a right angle.
- Blow or sweep granules off the driveway and sidewalk back onto the grass.
If your spreader is not on the bag’s chart, calibrate it: spread over 1,000 sq ft of driveway, sweep it up and weigh it. For 30-0-4 on bluegrass it should come to about 2.4 lb.
How often should you apply that amount?
Three or four times a year for most lawns, six to eight weeks apart. That puts a cool-season lawn at roughly 2 to 3 lb of nitrogen per 1,000 sq ft per year. Which months, and how to pick the day, is in When to Fertilize Your Lawn.
What a Green Lawn calculates product amounts for the lawn you trace, and tracks when you last fertilized so you never double up.