Watering, Mowing & Fertilizing
Sound Fertilizing Practices
Fertilizing is an important lawn care practice, as it influences grass color, ability to recover from stress, and helps prevent weed invasions and disease. There are important factors to consider when fertilizing lawns, including choosing the proper fertilizer, how much fertilizer to apply, and when to apply fertilizer.
Too much fertilizer, especially in spring, can be detrimental to lawns.
Nitrogen (N), phosphorus (P), and potassium (K) are the three major nutrients needed by lawns. Nitrogen is the nutrient required most, although too much nitrogen can cause excessive top growth, leading to assorted problems. Percent nitrogen (by weight) is always the first of three numbers on the fertilizer bag, followed by phosphorus and potassium. For example, a 18-6-12 fertilizer contains 18 percent nitrogen. This number is important because it determines how much fertilizer is needed. In most cases, a rate of one pound of nitrogen per 1,000 square feet is suggested for each fertilizer application to the lawn. If high percentage nitrogen fertilizers are used, then less actual fertilizer product is needed to supply that one pound compared to fertilizers with low percent nitrogen. Recommended ratios of N-P-K for lawn fertilizers are 3:1:2 or 4:1:2.
Another important factor in choosing nitrogen fertilizers is what kind of nitrogen is actually in the product. Nitrogen fertilizer may consist of fast-release or controlled-release nitrogen. Advantages and disadvantages are outlined below. For lawns, fertilizers containing controlled-release nitrogen sources are suggested for most applications, primarily because they help assure uniform growth and do not readily burn grass.
Characteristics of Nitrogen Fertilizers for Home Lawns
Nitrogen Type/Fast-Release(e.g., urea, ammonium nitrate, ammonium sulfate)
Advantages
- Quick response (greening)
- Provide nitrogen when soils are cold
- Relatively inexpensive
- May cause undesirable large flush of growth
- Likely to burn grass
- Losses through soil or air more likely
Controlled-Release/Slow-Release
(e.g., ureaform, sulfur-coated urea, milorganite, IBDU)
Advantages
- More uniform grass growth
- Not likely to burn grass
- Losses through soil or air less likely
- May not work on cold soil
- Most are expensive
- May not see quick color change in grass
In addition, a variety of special lawn fertilizers may be found.
- Winterizer fertilizers are typically high in potassium, and although advertised for fall application can be applied in spring as well. Potassium helps increase cold tolerance and disease resistance of grasses.
- Weed and feed products contain a broadleaf weed killer for weeds such as dandelions (fall application); others contain a preemergence herbicide to control crabgrass (spring application).
- Lawn starter products, typically high in phosphorus, are intended for newly seeded lawns.
In addition to the type of fertilizer, application rate and timing need to be considered. About three pounds of nitrogen per 1,000 square feet per growing season is suggested for most full-sun lawns (Kentucky bluegrass; Kentucky bluegrass mixed with perennial ryegrass and/or fine fescue) in northern Illinois (suggested range is 2 to 4 lbs./1,000 sq.ft./season); about half as much suggested for shade lawns. Split into two or three applications, with each single application of nitrogen being about one pound per 1,000 square feet. Percent nitrogen in the bag and size of the lawn (in square feet) are calculation variables as shown below.
To figure how much nitrogen fertilizer to apply to lawns:
- Divide rate of nitrogen desired by percent nitrogen in bag (first number of 3 on bag, use decimal form)
- Multiply answer found by square footage of lawn and divide by 1,000
- 1 lbs. divided by .15 = 6.66, round to 6.7
- 6.7 multiplied by 7,000 = 46,900; divided by 1,000 = 46.9, or 47 pounds
- Therefore, need about 47 pounds of 15-5-10 fertilizer to cover entire lawn.
After calculating how much to apply, when to apply is the final important decision. Early fall (September 1-15) is a key time for fertilizing lawns in northern Illinois, regardless of what type of maintenance program the lawn falls under. Research has also shown benefits of late fall fertilizer applications after grass growth has about stopped but the lawn is not yet dormant. Spring applications are suggested for early May. In most cases, avoid the hot weather months. The schedule found below outlines when to apply fertilizer based on how many applications are to be made annually and the desired lawn quality. Use controlled-release nitrogen fertilizers in May and September and a fast-release source in late fall.
The total amount of nitrogen should be spread over the application schedule. Be careful not to overfertilize in spring.
Lawn Fertilizing Program Schedules for Northern Illinois
| Number of Applications/Year | Suggested Timing Schedule |
| One (low quality lawn) |
Early September |
| Two (moderate quality lawn) |
Early May, Early September |
| Three (high quality lawn) |
Early May, Early
September, Late October/Early November |
| Four (only with
irrigation) (high quality lawn) |
Early May, Mid-June, Early September, Late October/Early November |
