The very best time to fertilize cool-season lawn grasses such as bluegrass, perennial rye and fescue is in the fall.An application in early October, another in early November and a third in early December is recommended.

Many home owners fertilize their lawns only in the spring! Almost all authorities say that is the wrong time to do it. Spring applications of fertilizer result only in additional top growth at a time when it is not needed and the grass has to be cut more often.

Warm-season grasses such as Bermuda and zoysia should be fertilized only during May, June, July and August.

Research has shown that fall fertilization causes the grass to retain color longer in the winter and regain color earlier in the spring without stimulation of excessive spring growth. The real bonus is the root growth that occurs. Existing roots thicken and new rhizomes develop. Roots continue to develop during the winter when temperature are above freezing.

A strong root system permits the grass to resist the stress of summer and drought and, even if hit by a disease, it then has a much better chance of recovery.

Fertilizing the grass even in late December should not cause winter injury, specalists say.

There are all kinds of formulas and programs for fertilizing lawn grasses. Almost every turfgrass specialist has his own. A particular program will work best in a particular situation, but may not do well somewhere else.

For the Washington, D.C., area specialists generally recommend application of four pounds of nitrogen per 1,000 square feet per year for Merion, Fylking, Adelphi, Baron and Sydsport and other improved varieties of Kentucky bluegrass. The common Kentucky bluegrasses such as Kenblue and South Dakota certified do better with about 3 pounds of nitrogen per year.

Tall fescue, such as Kentucky 31, should receive about three pounds of nitrogen per year, and creeping red foscue about two pounds.

To apply the four pounds of nitrogen, specialists suggest putting down one pound in early October one and one-half in early November and one and one-half in early December. To apply three pounds, apply one pound in early October, one in early November and one in early December.

A 100 pound bag of fertilizer such as 106-4 contains 10 pounds of nitrogen, 6 pounds of phosphorus and 4 pounds of potash. To apply four pounds of nitrogen would call for 40 pounds of 106-4.

Don't fertilize the lawn when the grass is wet. It may seriously burn the grass. If too much nitrogen is applied at one time the amount will depend on the temperature as burn is more likely when the weather is hot, the grass can be burned. Sometimes burn is due to faulty application.

A clogged or poorly adjusted spreader that doesn't spread evenly can put down too much in some places and not enough in others. Starting and stopping the spreader with the trap open can cause uneven distribution and perhaps burns.