When, What and How To Feed
If you are going to only fertilize the lawn once, do it in the autumn to prepare the plants for the winter. Otherwise, feed regularly, beginning in spring and continuing through autumn. The best fertilizers for lawns are "complete" ones with lots of nitrogen in them. The percentage of nitrogen should be about double that of the other two main ingredients - phosphorus and potassium. Fortunately you do not have to hunt for fertilizers with such formulas, because various manufacturers prepare and package them especially for use on lawns. Their brand names indicate they are for grass. These are sold wherever garden supplies are handled, and will be prominently displayed at the time of year you should be applying them.
Each sack has instructions for applying the material, and specifies the area of lawn over which it should be spread. The label also tells the percentage of nitrogen and other major elements in the fertilizer. This sometimes affects the price, and the rate at which the material is spread. A fertilizer having 5 to 10% nitrogen is on the weak side, while one with 15 to 20% nitrogen is on the strong side and worth proportionally more in price. Some straight nitrogen plant foods have 20 to 45% nitrogen and are used at correspondingly lighter rates. (You can learn more about the make-up of fertilizers in the section on soils and plant foods—including an explanation of the numerals and percentages.)
It does not make much difference what kind of fertilizer you use. Some are safer because they are "non-burning." But for this convenience you may pay a bit more. As long as the fertilizer is not dusty, and is not used more heavily than about 1 pound of actual nitrogen (10 Ib. of a fertilizer containing 10% nitrogen) per 1000 square feet, there should be no danger of injuring the grass.
Some kinds of grass need more fertilizer than others. Give heavy feeders such as bermuda, Merion bluegrass and bent grasses a total of 8 Ib. or more of actual nitrogen per 1000 sq. ft. per year. (That is, for each 1000 sq. ft. of lawn you would give an annual total feeding of 80 Ib. of a complete fertilizer having a 10-6-4 analysis, or 40 Ib. of one with a 20-10-5 analysis.) Bluegrass lawns, except Merion, get by well with about 4 Ib. of actual nitrogen per 1,000 sq. ft. per year (that is, a total of 40 Ib. of 10-6-4, or 20 Ib. of 20-10-5). Fine fescues can get along on 3 Ib. or less (30 Ib. of 10-6-4 or 15 Ib. of 20-10-5), and this rate is usually adequate also for centipede and bahia grass, though these latter two may look better if fertilized more generously.
The handiest and best way to apply fertilizer is with a mechanical spreader. Often you can rent one at the store where you buy your fertilizer—or at a general rental supply store. When pushing the spreader up and down over your lawn, be sure to overlap the runs slightly so that there are no missed spots. These would show up as conspicuous streaks later—the fertilized grass would be dark green and the missed strips yellowish. Ideally, apply half the fertilizer in one direction and half in the other.
If you suspect you may have overdosed, or if the fertilizer seems dusty, it is a good idea to rinse it off the grass leaves and into the ground, but don't do this with a "Weed 'N Feed" type fertilizer. Most modern fertilizers are pelleted and thus roll off grass foliage easily, at least with erect grasses such as bluegrass. With tight bentgrasses and bermudas, watering-in may be needed more.
Be considerate of your neighbors and the environment. Over-applied fertilizer/herbicide will end up in the storm drain (which flows to the nearest creek) and you may find a citation in your mailbox.