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The Grass is Always Greener…

Zac Reicher and Clark Throssell, Purdue University Turfgrass Specialists:

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 85 degrees. Both Nitrogen forms can and should be used on lawns depending on customer desires.
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Driveways and walkways are extremely important to your clients as far as aesthetic appeal. However, if the grass surrounding even the most beautifully designed decorative brick sidewalk is not up to par, the whole project suffers.

Turfgrass needs 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 pests. 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.

Fertilizing Factors

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:

Desires of the homeowner: A beautiful, dark green and dense lawn will require more fertilizer than a thin, lighter colored lawn. Applying more fertilizer annually also necessitates more mowing and irrigation.

Location: The growing season is longer in southern states versus those in the north, requiring more fertilizer to maintain the same turf quality.

Species: Certain species like zoysia, buffalograss, or tall fescue may perform adequately with lower annual fertilizer rates than species such as Kentucky bluegrass or perennial ryegrass.

Weather: A rainy summer will stimulate growth and will usually necessitate more annual fertilizer than a dry summer.

Soil type: Turf grown on a very sandy or a very heavy clay soil will need more fertilizer than turf grown on a silt loam soil. Soil type and pH will have a large effect on the amount of phosphorus and potassium that needs to be applied.

Age and quality of existing lawn: A new lawn will need more fertilizer for the first few years to enhance density. Improving a neglected or thin lawn may also require more annual fertilizer for the first few years.

Clippings: Clippings should always be returned to the lawn; removing clippings for composting or mulch will necessitate more annual applications of fertilizer.

When applying fast-release nitrogen fertilizer or fertilizer with 20% or more nitrogen, make sure to avoid fertilizer burn. Burns like this occur when fertilizer applications overlap, if turf is not watered properly, or if a granular fertilizer is applied to wet grass.
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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.

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.

How Much Fertilizer to Apply?

It is very important to apply the proper amount of fertilizer to your lawn. 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 lbs N/1000 ft2 ÷ % nutrient = Total fertilizer needed/1000 ft2
• Total fertilizer needed/1000 ft2 X Area to be treated in ft2 = lbs fertilizer needed to treat the area.

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.

Apply fertilizer uniformly over the lawn. For drop-type and rotary spreaders, apply the fertilizer so the wheel is at the edge of the pattern from the previous pass.
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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.

Maintenance Details

32: Degrees, fertilizer should not be applied if the soil temperature dips below freezing.

6: Pounds of nitrogen per 1,000 square feet. Many shade trees can tolerate and respond to this high rate of nitrogen.

3: Pounds of nitrogen per 1,000 square feet. Conifers and broadleaf evergreens should not be over fertilized; instead, they should receive a maximum of three pounds per 1,000 square feet.

Source: University of Kentucky


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February 17, 2020, 1:51 pm PDT

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