Lawn fertilizer is dosed by the nitrogen it delivers, not by the weight of the bag. This trips up most people, because two bags of very different products can deliver the same amount of nitrogen. Get the rate right and the lawn thrives. Get it wrong and you either waste product or burn the grass.
The N-P-K numbers
The three numbers on every bag are the percentages, by weight, of nitrogen, phosphorus and potassium. A 20-5-10 bag is 20 percent nitrogen, 5 percent phosphorus and 10 percent potassium. Nitrogen, the first number, drives leaf growth and green up, and it is what sets your application rate. Phosphorus supports roots and potassium supports general hardiness.
Setting a rate
A common target is about 1 pound of actual nitrogen per 1000 square feet per feeding. To find how much product delivers that, divide the nitrogen you want by the nitrogen fraction of the bag.
Worked example and table
To apply 1 pound of nitrogen per 1000 square feet, the product needed depends entirely on the first N-P-K number.
| Product nitrogen | Product per 1000 sq ft |
|---|---|
| 10% | 10 lb |
| 20% | 5 lb |
| 30% | 3.3 lb |
| 46% (urea) | 2.2 lb |
The higher the nitrogen number, the less product you spread for the same result, and the more careful you have to be not to over apply.
Do not exceed the label
More is not better. Over application burns the lawn, wastes money, and runs off into storm drains and waterways. The label rate is a legal maximum as well as a recommendation. Split a season total into several lighter feedings rather than one heavy one, which gives steadier growth and less risk of burn.
Timing through the year
Feed when the grass is actively growing, not when it is dormant in deep heat or cold. Cool season grasses respond best to feeding in spring and autumn, while warm season grasses prefer late spring through summer. A slow release product spreads the nitrogen over weeks and is more forgiving than a fast release one.
Spread it evenly
Stripes of dark and pale grass usually mean uneven application. A broadcast spreader set to the right rate, walked in slightly overlapping passes, applies fertilizer far more evenly than spreading by hand and avoids both burn streaks and missed strips.