The Homeowners Column

The Homeowners Column

Understanding Lawn Fertilizers

March 17, 2005

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

People have a love/hate relationship with their lawn. We want the lawn to look nice to some degree, yet we usually consider the mowing and maintenance a chore. Fertilizing is an important lawn care practice which gives grass its deep green color, enables grass to recover from stress and helps prevent weed and disease invasions. According to the U of I Extension lawn challenge website here are some considerations when fertilizing a lawn.

Nitrogen (N), phosphorus (P), and potassium (K) are the three major nutrients needed by plants. Nitrogen is the nutrient required most in lawns, although too much nitrogen can cause problems. Percent nitrogen (by weight) is always the first of three numbers on the fertilizer bag, followed by phosphorus and potassium. For example, an 18-6-12 fertilizer contains 18 percent nitrogen. This number determines how much product is needed. Typically a rate of one pound of nitrogen per 1,000 square feet is suggested for each lawn fertilizer application. If high percentage nitrogen fertilizers are used, then less actual product is needed. Recommended ratios of N-P-K for lawn fertilizers are 3:1:2 or 4:1:2.

Another important consideration is the type of nitrogen in the product. Nitrogen fertilizer may consist of fast-release or controlled-release nitrogen.

Examples of fast-release nitrogen include urea, ammonium nitrate and ammonium sulfate. Advantages of fast release include quick greenup, nitrogen availability in cold soils and less expense. Disadvantages may include undesirable fast growth, grass burn potential and loss through soil or air.

Controlled-release/slow-release types include ureaform, sulfur-coated urea, milorganite, and IBDU. Advantages include more uniform grass growth, less burn potential and less loss through soil or air. Disadvantages include reluctance to work in cold soil, more expense and slower green up.

Lawns fertilizers containing controlled-release nitrogen sources are suggested for most applications. Fast-release source can be used in late fall.

A variety of special lawn fertilizers are also available.

  • Winterizer fertilizers are high in potassium. Although advertised for fall application, they can be applied in spring.
  • Weed and feed products contain a broadleaf weed killer for dandelions and other weeds. Others contain a preemergence herbicide to control crabgrass in spring. These products are convenient but timing may be compromised. For instance crabgrass control would typically be put down in early April whereas spring fertilization is recommended for early May.

Fertilizer application rate is determined next. 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) 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.

To figure how much nitrogen fertilizer to apply to lawns:

  • Divide rate of nitrogen desired by percent nitrogen in bag (first number on bag, use decimals)
  • Multiply answer found by square footage of lawn and divide by 1,000

For example, to apply 15-5-10 fertilizer to 7,000 square feet of lawn at a rate of one pound nitrogen per 1,000 square feet, make these calculations:

  • 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.

When to apply is the final important decision. Use the following chart to plan your fertilizer applications. The total amount of nitrogen should be spread over the application schedule. Be careful not to overfertilize in spring.



Number of fertilizer Applications/Year

Suggested Timing Schedule

One

Early September

Two

Early May, Early September

Three

Early May, Early September,
Late October/Early November

Four (only with irrigation)

Early May, Mid-June, Early September, Late October/Early November

For more info on lawn care http://www.urbanext.uiuc.edu/lawnchallenge/

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