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Fertilization

A properly fertilized lawn will not only look nice, but will be more resistant to diseases like leaf rust and will also fend off weeds by out-competing them. Three basic questions to ask when it comes to lawn fertilization are:

  1.  How much should I apply
  2. When should I apply it and 
  3. What kind of fertilizer should I use?

Remember to consider the desired quality level you want for your lawn when considering these questions. If you want a "low maintenance" lawn, you need to be sure you have a grass species that can tolerate low fertility and minimal irrigation.

It is important to understand the main nutrients that lawn grasses require for adequate growth.

Nitrogen is perhaps the most important nutrient as it helps the grass produce green, healthy leaves. Since leaves are the energy-making factories of the plant, it is important that enough nitrogen be provided to maximize their energy making capabilities without causing an over production of leaves.

Phosphorus is important for strong root growth and is very important during establishment. Since phosphorus does not move readily in the soil and new grass seedlings have limited root systems, providing some phosphorus fertilizer during establishment is very important. Mature lawns have very fibrous root systems and are much more adept at mining phosphorus from the soil. Generally, unless very deficient, fertilizers with low percentages of phosphorus are used for lawns.

Potassium is very important for lawns and helps grasses respond to heat and drought stress. Although deficiencies in potassium are difficult to detect, the importance of potassium in stress management should not be overlooked. Unless a soil test reveals that potassium is very abundant, fertilizers with percentages of potassium similar to nitrogen should be used.

Iron is another nutrient that is important, especially in southern Idaho soils. The high pH soils of southern Idaho tie up iron making it unavailable to the plant causing it to become yellow (iron cholorosis). Iro- containing fertilizers can help alleviate iron chlorosis, but make sure of the type of iron you are buying. Iron in the forms of ferrous sulfate or iron sulfate are absorbed by the leaves. If washed into the soil, the iron quickly changes form and becomes unavailable to the plant. Iron in the form of iron chelate (Fe-DTPA, Fe-EDTA, Fe-EDDHA, and Fe-HEDTA), are more effective as soil applied fertilizers. Iron in this form is available to the plant even when in contact with the soil and the effect is much longer lasting than the foliar-absorbed iron fertilizers.

A general recommendation for cool-season grasses is a range from 0.5 to 5 lbs of nitrogen (N) per 1000 ft2 per season depending on the desired level of quality. Low input lawns with tall fescue or fine fescue will only require about 2 lbs N per 1000 ft2 per season or less, while low input lawns of Kentucky bluegrass and/or perennial ryegrass will require approximately 3 lbs N per 1000 ft2 per season. Medium to high input lawns of Kentucky bluegrass and perennial ryegrass will require 3 – 5 lbs N per 1000 ft2 .

To understand the timing of lawn fertilization, it is important to understand the seasonal growth pattern of a grass plant. In the spring, grasses are coming out of winter dormancy and begin rapid growth using stored energy reserves from last year. Grasses that are over-fertilized with nitrogen in the spring will spend too much of those energy reserves on leaf growth and will not have enough left over to take them through summer’s heat and drought stress. All that is needed in the spring is to supply the grass with just enough nitrogen fertilizer to prevent it from becoming chlorotic (very light green to yellow in color).

As temperatures rise in the summer, leaf and root growth start to slow. Over-fertilization at this time could be very detrimental to the health of the grass and even cause areas to die. Avoid fertilizing during the summer except to prevent chlorosis. Very light applications and use of a slo- release fertilizer will help keep the grass green in the summer without burning or damaging the lawn.

As temperatures cool and hours of light per day diminishes in late summer to early fall, grasses begin preparing for winter by sending their energy reserves to their rhizomes and roots. A fertilizer application at this time will help the plant maximize energy production and most of the engergy will be sent to storage instead of being used for leaf growth.

Table 1 gives recommendations for various grasses at various times of the year. Keep in mind that the March application may be omitted if green-up is satisfactory and a late fall application was made the previous year. In this case, a single application of 1 lb N per 1000 ft2 can be made. Use slow release fertilizers for a late fall application and on sandy soils throughout the year to reduce nitrogen leaching. Additionally, if you are using a mulching mower or otherwise returning clippings to the lawn, you may be able to cut back the nitrogen by about one fourth.

Table 1. Nitrogen fertilization schedule for home lawns.  (Adapted from Colorado State University lawn fertilization extension fact sheet).

Grass Type¹ & Maintenance Level² Mid March – Mid April Early May – Early June July – Early August Mid August – Mid September Early October – Early November
Rates are in lbs of N per 1000 ft2
KBG – Low 1/2 1/2 none 1 1 (optional)
KBG – Med – High 1/2 – 1 1 none 1 1 – 2 (2 is optional)
Tall fescue – Low 1/2 1/2 none 1 1 (optional)
Tall fescue – Med – High 1/2 1 none 1 1 (optional)
Fine fescue – Low 1/2 1/2 none 1/2 none
Fine fescue – Med 1/2 1 none 1 none
Buffalograss none 1/2 – 1 1/2 – 1 none none
¹Grass Type: KBG = Kentucky bluegrass
²Maintenance Level: Low = low maintenance, Med = medium, High = high maintenance

There are many types of fertilizer available for purchase, and it can be quite confusing trying to choose the correct one. Understanding the nutrient requirements of the grass as described above will help narrow the choices. Understanding the different types of fertilizers, will help narrow the choices even further.

Fertilizers can be divided into two major groups, fast release and slow release. This refers to how quickly the nitrogen is released and made available to the plant. You may also have heard about organic vs. inorganic fertilizers. This refers to the chemical composition of the fertilizer. It is important to understand that the term “organic” means the fertilizer contains carbon in the chemical structure. Organic fertilizers include natural materials such as sewage-based products like Milorganite®, animal by-products like manures and bone meals, and plant by-products like corn gluten meal. There are also synthetic organics like urea which is very common in the agricultural industry, but less so in the turf industry because of its very fast release and high burn properties. There are, however, also many forms of urea that have been developed to slow its release and lower its burn potential.

Fast-Release Fertilizers

Fast-release fertilizers are quickly released into the soil and available for uptake by the plant. The advantages of these materials are that they are relatively inexpensive, are not dependent on temperature for release and give the grass a quick response. The disadvantages of fast-release fertilizers are that they are more likely to burn the turf if applied incorrectly (during hot periods or at too high of a rate), the response is generally short lived, and because of their water solubility they are more likely to leach through sandy soils or runoff of compacted soils. Several light applications can compensate for the quick, short-lived response, but this requires more labor.

Slow-Release Fertilizers

Slow-release fertilizers cause a more uniform color and growth response over a longer period of time than the fast-release fertilizers. They are also less likely burn the grass. Some products will release nitrogen into the soil slowly over several weeks or even months. All the natural products like manures, bone meals, etc. are slo- release fertilizers and require soil microbes to release the nitrogen to make it available to the plant. When applied to cool soil temperatures, below 50° F, the grass may not respond at all to the application. Most of the natural slow-release fertilizers are quite low in nitrogen content so it will take more product to apply the same rate of actual nitrogen as it would with a synthetic fertilizer with a higher nitrogen content. Other types of slow-release fertilizers include sulfur-coated urea, polycoated urea and other urea based products. These products tend to be more expensive, but provide a good uniform turf response.

Fertilizer stains on sidewalk

Stains on sidewalk caused by mis-application of fertilizer particles. (Photo courtesy: A.J. Koski, Colorado State University)

Drop spreaders and rotary spreaders are available for fertilizer applications on home lawns. Both can do an effective job if used correctly. Drop spreaders, as the name implies, drops the fertilizer material directly from the spreader onto the lawn placing the fertilizer precisely in the location desired. Drop spreaders take longer because you must cover every square inch with the spreader.

With rotary spreaders, the fertilizer material is spun out over a large area.  A lawn can be fertilized more quickly with a rotary spreader because of the area covered in one pass, but some fertilizer invariably gets thrown where it is not wanted such as sidewalks and streets. Make sure to sweep up fertilizer off sidewalks as the prills can cause stains and can wash into storm drains.

Misapplication of fertilizer

Misapplication of fertilizer using a drop spreader. (Photo courtesy: Iowa State University)

Regardless of the type of spreader used, it is a good practice to cut the application rate in half and apply the fertilizer in two directions to avoid stripping patterns.

Low maintenance can have several meanings. To some people, low maintenance means no maintenance at all. In this case the grass is left to grow much like a meadow or grass along a highway, etc. To others, low maintenance means little if any fertilization, minimal irrigation, infrequent mowing, and pest control only if absolutely necessary. The latter may be what most people would prefer in a home lawn as long as it results in acceptable quality. It is important to understand that the quality expectations of lo- maintenance lawns should not be very high since minimal inputs will result in a turf of minimal quality.

University of Idaho Extension

Physical Address:
E. J. Iddings Agricultural Science Laboratory, Room 52
606 S Rayburn St.
Moscow, ID

Mailing Address:
University of Idaho Extension
875 Perimeter Drive MS 2338
Moscow, ID 83844-2338

Phone: 208-885-5883

Fax: 208-885-6654

Email: extension@uidaho.edu

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Barbara Petty