Last updated 2/25/98
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Purdue Turfgrass Science Program

Fertilizing Established

Lawns

Zac Reicher and Clark Throssell
Purdue University Turfgrass Specialists

AY-22

 

Lawns need to be fertilized to maintain color, density, and vigor. The healthier and more vigorous a lawn is, the better it can withstand stress from heat, drought, traffic, and pets. We often try to achieve a dark green lawn, but the darkest green lawn is not always the healthiest lawn. We should try to achieve a healthy lawn that has moderate growth and good density.

The amount of fertilizer applied annually to a lawn depends on a number of factors. Fertilization programs may need to be adjusted to apply slightly more or slightly less nutrients depending on the following factors:

Fertilizers
All fertilizers will have a series of three numbers displayed prominently on the label. These numbers represent the percentage by weight of nitrogen, phosphorus (as P2O5), and potassium (as K20). For instance, a 24-4-8 fertilizer will have 24% N, 4% P2O5, and 8% K2O. A 46-0-0 fertilizer will have 46% N, 0% P2O5, and 0% K2O.

Though all three elements are important in maintaining a healthy turf stand, N will cause the greatest response. Because of this, most fertilizer recommendations for lawns are listed as lb. N per 1000 ft2. Nitrogen fertilizers come in two basic forms: quick release (soluble) nitrogen and slow release (insoluble) nitrogen. Quick release nitrogen normally causes a response in a week or less, whereas slow release nitrogen will cause a response in three to 10 weeks or more. Quick release nitrogen is inexpensive and may burn leaf blades if applied improperly. Slow release forms tend to be more expensive but will rarely burn leaf blades even when applied at temperatures above 85o. Both N forms can and should be used on lawns. Examples of slow and quick release N forms are listed in Table 1.

 

Table 1. Nitrogen forms found on fertilizer bags.
Quick Release N Slow Release N
Urea Sulfur Coated Urea (SCU)
Ammonical N (NH4) Polymer Coated Urea
Ammonium Nitrate (NH3NO3) Methylene Ureas
. Natural Organics

 

Both forms of N are often blended in one fertilizer bag. This is advantageous because the quick release N gives a response shortly after application and the slow release N results in a more gradual and longer response.

Fertilizing with phosphorus and potassium is also important in maintaining a healthy lawn. The best way to determine how much phosphorus and potassium to apply annually is to follow the recommendations of a soil test (refer to AY-18, Soil Testing for Homelawns). In lieu of a soil test, a general recommendation is to apply 1/4 as much phosphorus and 1/2 as much potassium as nitrogen. For instance, if you apply 4 pounds nitrogen per 1000 ft2 per year, you should apply 1 pound phosphorus and 2 pounds potassium per 1000 ft2 per year.

Fertilization Programs
It is best to fertilize lightly in spring and early summer, little to none in summer, and heavy in fall. A heavy fall fertilization program will produce the healthiest turf throughout the year. Applying high rates of N in spring and summer stimulates excess leaf growth at the expense of root growth. Not only does this force you to mow more often, it reduces turf quality during the summer. High rates of spring and summer N can also stimulate disease, weed, and insect activity.

Table 2 lists the Purdue recommendations for lawn fertilization programs. The maximum program is for homeowners who want the greenest lawn, and are also willing to devote considerable time to mowing and irrigation. The minimum program is for those who are satisfied with a less attractive lawn. Keep in mind that the initial seven points in this publication will affect your fertilization program and the rates may need adjusting. The dates listed are for central Indiana, so adjust 10-15 days sooner in spring and later in the fall for southern Indiana, and later in spring and earlier in fall for northern Indiana.

In addition to the applications listed in Table 2, an April application of 0.75 lb. N/1000 ft2 is recommended where no previous November application was made. Preemergence herbicides applied in April are often combined with nitrogen. If you apply a preemergence herbicide that is combined with fertilizer in April, the May fertilizer application should be skipped. Professional lawn care companies may increase the number of applications while decreasing the rate of nitrogen per application. This can give a more gradual feeding of the grass plants and produce a high quality lawn.

 

Table 2. Purdue recommendations for lawn fertilization programs
. . . . . Nitrogen
. . _________ _Program_ ________ Release
Date lb. N/1000 ft.2 Maximum Standard Minimum Form
September (fall) 1.0 X X X Slow/Quick
November 1-15 (late fall) 1.0-1.5 X X . Quick
May 15-June 1 1.0 X X . Slow
July 15-30 0.75 X . . Slow

 

How Much Fertilizer to Apply?
It is very important to apply the proper amount of fertilizer to your lawn. The following example explains how to calculate the proper amount of fertilizer to apply. Additionally, the fertilizer bag will often list the proper spreader setting for your spreader. If the setting is not listed, refer to the later section on "Calibrating a Fertilizer Spreader." Even if your settings are listed, you should calibrate your spreader to make sure it is still accurate. As spreaders get older, settings gradually change because of wear and tear. Regular cleaning and lubrication of the spreader will help it last longer.

 

Calculating the pounds of fertilizer to apply
Determining the amount of fertilizer to apply when you are given the fertilizer rate in lbs N/1000 ft2:
Desired rate in % nutrient = Total fertilizer
lbs N/1000 ft2 needed/1000 ft2
.
Total fertilizer X Area to be = lbs fertilizer needed to
needed/1000 ft2 treated in ft2 treat the area
For example, how much fertilizer do you need to apply a 16-8-8 fertilizer at 1.25 lbs N/1000 ft2 to a 5000 ft2 lawn?
1.25 lbs N/1000 ft2 .16 = 7.8 lbs 16-8-8/1000 ft2
.
7.8 lbs 16-8-8/1000 ft2 X 5000 ft2 = 39 lbs 16-8-8 to treat a 5000 ft2
lawn at 1.25 lb N/1000 ft2

 

Fertilizer Application
Apply fertilizer uniformly over the lawn. Overlap wheel tracks for drop-type spreaders. For rotary spreaders, apply the fertilizer so the wheel is at the edge of the pattern from the previous pass. Improper spreading of fertilizer will result in "streaking", the alternate dark- and light-green stripes in a lawn. Agricultural type fertilizers (10-10-10, 15-15-15, etc.) that have large particle sizes should not be applied with a drop-type spreader; a rotary spreader should be used. Irrigation or rain following fertilization is important to move nitrogen off the leaf blades and into the soil. Some fertilizer/herbicide combination products cannot be watered-in, so be sure to read the label instructions. Avoid applying fertilizer to drought-stressed or dormant turf, or when temperatures are over 80oF.

Fate of Nitrogen
There is some concern about nitrogen leaching into groundwater or running off into surface water. Research shows that when applied correctly and accurately, nitrogen remains where it is applied. A dense turf canopy prevents water run-off and thus prevents nitrogen from moving across a lawn. The thatch and dense rooting of grass plants absorbs nitrogen and prevents it from moving through the root zone. Be sure to calibrate your spreader, and follow all label instructions on the fertilizer bag to reduce the risk of nitrogen moving away from the target.

 

Rotary Spreader Distribution The Effective Pattern of a Rotary Spreader

A rotary spreader is different than a drop spreader in that the distribution is not uniform across the width of the pattern. More product lands near the spreader and less product lands near the edge of the pattern. To compensate for this, the spreader must be run so the edge of the pattern is just touching the wheel tracks from the previous pattern. This is the "effective pattern." All products and spreaders have a specific effective pattern.

 

Calibrating a Rotary Fertilizer Spreader
1. Measure the width of the effective pattern of the spreader: This is simply the distance from the center of the spreader to the edge of one side of the pattern. See "Effective Pattern of a Rotary Spreader" on the previous page. _________ft.
2. Measure off a convenient distance to run the spreader: the longer the distance, the more accurate the calibration: _________ft.
3. Multiply number in step 1 by the number in step 2 to calculate the area covered by the spreader: _________ft2
4. Weigh a portion of the product and dump into spreader: _________lb.
5. Push the spreader over the area previously measured in step 3, being careful to shut the spreader on and off precisely at the beginning and end of the course.
6. Weigh the amount of product left in the spreader: _________lb.
7. Subtract the number in step 6 from the number in step 4. This is the amount of product applied to the area you have measured: _________lb.
8. Divide the number in step 7 by the number in step 3 to gives lb. product per square foot. _________lb./ft.2
9. Multiply the number in step 8 by 1000 to give lb. product/1000 ft2. _________lb./1000ft2
10. Is this number close to the recommended rate listed on the bag?
Adjust the spreader setting and repeat the process until the spreader
is applying the recommended rate listed on the bag.

 

 

Calibrating a Drop-Type Fertilizer Spreader
1. Measure the distance between the outside holes of the spreader:  _________in.
2. Convert this to feet by dividing by 12:  _________ft.
3. Measure off a convenient distance to run the spreader: the longer the distance, the more accurate the calibration: _________ft.
4. Multiply the number in step 2 by the number in step 3 to calculate the area covered by the spreader: _________ft2
5. Weigh a portion of the product and dump into spreader: _________lb.
6. Run the spreader over the area previously measured in step 3, being careful to shut the spreader on and off precisely at the beginning and end of the course.
7. Weigh the amount of product left in the spreader: _________lb.
8. Subtract the number in step 7 from the number in step 5. This is the amount of product applied to the area you have measured: _________lb.
9. Divide the number in step 8 by the number in step 4 to give lb. product per square foot. _________lb./ft2
10.Multiply the number in step 9 by 1000 to give lb. product/1000 ft.2. _________lb./1000ft2
11. Is this number close to the recommended rate listed on the bag? Adjust the spreader setting and repeat the process until the spreader is applying the recommended rate listed on the bag.

Purdue University

Cooperative

Extension Service

West Lafayette

Indiana 47907

3/98

Cooperative Extension work in Agriculture and Home Economics, state of Indiana, Purdue University, and U.S. Department of Agriculture cooperating;
H. A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the acts of May 8 and June 30, 1914.
The Purdue University Cooperative Extension Service is an equal opportunity/equal access institution

 

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