In the past, too much emphasis has been given to spring as the best time to fertilize Kentucky bluegrass. If a lawn is stunted and has a pale to yellowish green appearance, a very moderate feeding at this time would be advisable.

On the other hand, fertilizing a lawn that already had moderate vigor at the time most of us get "spring gardening fever" will stimulate excessive, succulent growth.  Excessive leaf growth usually occurs at the expense of new root growth; this places the plant at a further disadvantage for summer and needlessly increases the amount of mowing required. Turf becomes more susceptible to disease and other stresses that will take their toll during summer.

When to fertilize
All lawns should be fertilized in the fall. Additional late winter or early spring fertilization may be necessary if fall applications were missed. Fertilization at this time will be influenced by desired level of turf appearance, turfgrass species, soil type, irrigation intensity, and fertilizer carrier.

Late spring fertilizer applications may be desirable and even necessary depending on the condition of the turf. When an application is required, do so about mid to late May, after the spring growth surge is over.

Nitrogen fertilizer
These materials fall into two basic groups: soluble and slow-release. Soluble types are available quickly to plants even at low temperatures they stimulate rapid growth and are depleted quickly. Steady, uniform growth requires frequent, light applications. Slow-release types of several different forms release nutrients to plants over a long period of time.

Lawn specialty fertilizers often contain 24 to 50 percent of the total nitrogen in slow-release form and the remainder in quickly soluble forms. This combination gives immediate response in cool weather while the remainder is available over a longer period.

When 35 to 50 percent or more of the nitrogen is a slow-release type, rates may be increased up to 50 percent. With these fertilizers, frequency of application may sometimes be reduced.

A precaution should be observed: Nitrogen sources from urea (quickly soluble) should not be confused with urea-formaldehyde, UF, (slowly available).

Rates and frequency
Recommendations are usually based on amounts required to supply a given amount of nitrogen per 1,000 square feet of lawn. Most lawn fertilizers are "complete" in that they contain the three major nutrients: nitrogen (N), phosphorus (P) and potassium (K) and, therefore, the amount of phosphorus and potassium applied is determined by the ratio of these two elements to nitrogen.

Two fertilizers with label analyses of 20-5-10 and 12-12-12 would contain 20 and 12 percent N, 5 and 12 percent P, and 10 and 12 percent K, respectively. For the first fertilizer, the N:P:K ratio would be 4:1:2, and the second would be 1:1:1. The amount of fertilizer required to apply 1 pound nitrogen to 1,000 square feet can be calculated by dividing 100 by the percentage of nitrogen in the fertilizer.

Suggested annual fertilization schedule For routine maintenance where soil test or experience indicates no major deficiencies, use a lawn fertilizer with an approximate ratio of 3:1:1 or 4:1:1 or 4:1:2 at the recommended rate.

Where soil test indicates low phosphorus or potassium levels or where basic fertility levels are not known, use fertilizer with a ratio that more closely approximates 1:1:1 or 2:1:1 or 3:1:2. If lawn application rates are not given on the container, amounts to apply can be calculated as in the example above.

Lime
Do not apply lime routinely to established lawns unless a soil test indicates a need.  Excess can be as harmful as deficiency. Established lawn soils seldom need to be limed unless a soil test indicates a moderately to severely acid soil of pH 5.5 or lower.

Where lime is needed, apply finely ground or specially pelletized agricultural limestone at rates up to 50 pounds per 1,000 square feet. If more is required, make separate applications about six months apart. Limestone can be applied almost any time, but fall or early winter is the best time.  A very light application of lime, monthly during the season will increase microbial activity in the top of the soil. This will increase the degradation of grass clippings and make available some nutrients found in the soil naturally.