Fertilizing a lawn
Lawn feeding comes down to one number: pounds of nitrogen per 1,000 square feet. The bag analysis, the spreader setting and the calendar are all ways of getting that number right. The labs in this guide use the same math as the Lawn Fertilizer Calculator.
The three numbers on the bag
Every fertilizer bag carries three numbers, such as 24-0-11. They are percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O), always in that order. A 50 lb bag of 24-0-11 holds 12 lb of nitrogen, no phosphate and 5.5 lb of potash. The rest is the other half of each salt, plus filler and coating that make it spread evenly.
Phosphorus and potassium are listed as oxides for historical reasons. As the elements, P is 44% of the P₂O₅ figure and K is 83% of the K₂O figure. Soil test recommendations are usually written in the oxide form too, so the label and the lab already match.
Further down, the guaranteed analysis splits the nitrogen by source. "Slowly available," "water-insoluble" or "polymer-coated" nitrogen releases over weeks to months. The rest is quick-release and greens a lawn within days, and also burns it if overdone.
Try it
Bag analysis to pounds of product
Quick check
A bag of 32-0-4. The target is 1 lb of nitrogen per 1,000 sq ft. How much product goes on each 1,000 sq ft?
Answer: About 3.1 lb. The bag is 32% nitrogen, so 1 ÷ 0.32 = 3.1 lb of product carries 1 lb of nitrogen.
How much nitrogen a year
Annual nitrogen depends mostly on the grass. Faster-growing, higher-traffic grasses need more; fine fescue and the low-input southern grasses need far less, and decline when overfed.
Pounds of nitrogen per 1,000 sq ft per year, low to high maintenance, from extension turf programs. Lawns where clippings are left on the grass sit toward the low end, because the clippings return part of the nitrogen.
Divide the annual total into feedings of 0.5 to 1 lb each. Less than half a pound per pass is hard to spread evenly; more than a pound of quick-release nitrogen at once risks burn. Slow-release products can go a little heavier per pass because they release over weeks.
Phosphorus and potassium follow the soil test, not the calendar. Many established lawns test adequate in phosphorus, and several states restrict phosphorus on established lawns, with an exemption for new seedings.
When to feed
- Cool-season grass (bluegrass, fescues, ryegrass) puts most of its root and tiller growth into fall. That is when most of the year's nitrogen goes on: early September, and again in October or early November while the grass is still green. A light feeding in late spring is enough. Heavy spring feeding pushes top growth at the expense of roots before summer heat.
- Warm-season grass (bermuda, zoysia, St. Augustine, centipede) grows in heat. Feed after it has fully greened up in late spring, through summer, and stop about six weeks before the first expected frost so it can harden off for dormancy.
- Skip frozen or dormant turf and forecasts of heavy rain. Nitrogen that the grass cannot take up washes away. Sweep granules off driveways and sidewalks back onto the lawn.
Setting the spreader
The setting printed on a bag is a starting point for one brand of spreader. Spreaders wear, walking speed varies, and granule size changes between products. A quick catch test shows how close the setting is.
- Mark out a measured area, such as 1,000 sq ft (a 20 by 50 ft rectangle), or 500 sq ft for a small lawn.
- Weigh the spreader with product in it, or weigh out a known amount of product.
- Spread the area at the bag setting, at normal walking pace, with the usual overlap.
- Weigh what is left. The difference is what went down.
Try it
Check the catch test
- Rotary (broadcast) spreaders throw a wide arc that thins out at the edges. Overlap each pass so the edge of the throw reaches the wheel track of the last pass.
- Drop spreaders put product only between the wheels. Overlap the wheel tracks slightly, or every pass leaves a thin stripe.
- Half rate in two directions gives the most even result with either type: half the product walking one way, half crosswise.
- Close the gate at turns and stops. Product keeps flowing while the spreader stands still.
Quick check
Two weeks after feeding, a lawn shows dark green stripes with pale strips between them, all running the same direction. What went wrong?
Answer: Not enough overlap. The pale strips got little or no product. Next time, spread half the rate in each of two directions; for now, a light pass along the pale strips evens it out.
Summary
- The bag numbers are N, P₂O₅ and K₂O as percent by weight. Pounds of product = pounds of N ÷ (N% ÷ 100).
- Annual nitrogen depends on the grass: about 1 to 2 lb per 1,000 sq ft for fine fescue and centipede, 2 to 4 for most cool-season lawns, up to 6 for bermuda.
- Feed in passes of 0.5 to 1 lb of nitrogen.
- Cool-season lawns get most of it in fall; warm-season lawns in late spring and summer.
- Check the spreader with a catch test, and spread in two directions.
Keep reading
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The same bucket reads 1,000, 1,280 or 1,400 ppm depending on the meter. Which scale a meter uses, how to convert, and what temperature does to a reading.
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7 min read IndexAll Bizon guides
Everything published so far, and what is being written next.
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