Lawn Fertilizer Calculator

The three numbers are the percentage by weight of nitrogen, available phosphate, and soluble potash. A 40 lb bag of 24-0-4 holds 9.6 lb of nitrogen (40 × 0.24), no phosphate, and 1.6 lb of potash; the remaining weight is carrier and filler. That is why you cannot compare bags by price alone — a 40 lb bag of 10-0-5 carries only 4 lb of N, so you need 2.4 times as much product to hit the same rate. Always divide your target nitrogen by the decimal percentage to get pounds of product.

Quick-release nitrogen sources such as urea and ammonium sulphate are salts. Above roughly 1 lb of quick-release N per 1,000 ft² in a single pass, the salt pulls moisture out of the leaf and you get streaks of yellow-brown burn, worst on hot days and on dry turf. Slow-release or coated products can be pushed to about 1.5 lb because the nitrogen meters out over 6–10 weeks. Most cool-season lawns want 2–4 lb of total N per 1,000 ft² per year split across three or four feedings, not dumped at once.

Use the spreader rate this calculator gives you — pounds of product per 1,000 ft² — and find that number on the bag chart for your spreader model. Then verify it: weigh out enough product for a measured 1,000 ft² test strip, apply it at your intended walking pace, and see whether you run out early or late. Adjust the setting and repeat. Apply half the product walking north-south and half east-west; single-pass application is the usual cause of green stripes. Sweep granules off drives and walks back onto the lawn so they do not wash into storm drains.

Usually not, and in many places you legally cannot use it. More than a dozen US states, including Minnesota, Wisconsin, Michigan, New Jersey, New York, Maryland, and Virginia, restrict phosphorus on established turf because runoff feeds algae blooms; typical exceptions are a new lawn from seed or sod, or a soil test showing a documented deficiency. That is why so many bags read 24-0-4 with a zero in the middle. Check your state and county rules before buying, and get a soil test from your county extension office — it is usually $15–$25 and settles the question.

For cool-season grasses like fescue, bluegrass, and rye, the two most valuable feedings are early fall and late fall, with a light spring application; skip midsummer, when heat stress makes nitrogen a liability. Warm-season grasses such as bermuda, zoysia, and St. Augustine want their nitrogen from late spring through summer while they are actively growing, and none within about six weeks of the first expected frost. Water the granules in with roughly a quarter inch of irrigation unless the label says otherwise, and never apply to frozen or dormant ground.