Bob Mugaas, UMN Extension Educator
This is the second article of a three part series on Understanding and Using Home Lawn Fertilizers. This article will focus attention on the nutrient nitrogen, why it gets so much emphasis in lawn care and, the roles that it plays in maintaining a healthy, vigorous lawn. As we begin this article, it is important to remember that while other nutrients such as phosphorus and potassium are important for healthy lawns, it is the nutrient nitrogen upon which nearly all general recommendations for home lawn fertilizer applications are based.
Photo 1: Healthy, dense lawn promoted by proper use of N fertilizers. Bob Mugaas.Grass plant absorption of nitrogen and its impacts on grass growth
Nitrogen (N) is the mineral element used in the largest quantity by our grass plants. It is absorbed primarily from the soil by the plant's root system. However, nitrogen is not absorbed by the plant in its elemental form (N). The two most commonly absorbed forms are the nitrate ion (NO3
-) in which N is combined with oxygen(O) and contains a single negative charge on the molecule and the ammonium ion (NH4
+) in which N is combined with hydrogen(H) and contains a single positive charge on the molecule. Of these two forms, the nitrate form is absorbed in the largest quantity by the plant root system.
Once N is inside the plant it becomes an integral part of many other plant molecules and processes. Among those various roles, one of its most important is in stimulating shoot growth which aids in spring green up and helps promote recovery from injury and environmental stresses. Interestingly, shoot growth is often stimulated at the expense of root growth. Thus, lower, more modest use of nitrogen fertilizer, especially in the spring of the year, is actually healthier for the plant as shoot growth will be more in balance with root growth. In turn, the plant will be in a healthier condition and better able to survive the more stressful summer growing conditions. See Picture 1.
Nitrogen also plays an important role in the production of the green pigment known as chlorophyll which in turn is responsible for producing the green color in grass. This is why you often find yourself mowing more frequently and observing a deeper green color to your grass following a nitrogen fertilizer application. After a few days following a nitrogen fertilizer application, any lawn areas that were skipped or missed will also be very evident. Non-fertilized areas or skips will be distinctly lighter green compared to those areas receiving fertilizer. Depending on the degree of misapplication, it may be quite embarrassing as well.
Photo 2, Bob Mugaas.Nitrogen needs can vary by grass species
While nitrogen is used by grass plants in larger quantities than other nutrients, there is some variability as to how much is needed among the different lawn grass species. For example, Kentucky bluegrass and perennial ryegrass require larger amounts of N than the fine fescues to remain healthy and vigorous. This is one of the main reasons why the fine fescues are the plant materials of choice when it comes to implementing low input lawn care programs or where no-mow plantings are desired. Under otherwise favorable growing conditions, fine fescues can get by with as little as 1# or even less of actual N per 1000 ft2
Picture 2 illustrates the difference in texture between Kentucky bluegrass and the fine fescues. If your lawn is dominated by the finer textured fine fescue grasses, you can likely reduce the amount of nitrogen applied as noted above and still expect to maintain acceptable or better lawn quality.
Why the need for supplemental nitrogen
While grass plants are moderate to high users of nitrogen depending on species, it is not the only fate of nitrogen in our soils. There are two other soil related processes that play an important role in how much nitrogen is available for plant growth during the growing season. First, the nitrate form of nitrogen can easily be carried along with the vertical movement of water down through the soil. This is the process known as leaching. It effectively moves nitrate nitrogen beyond the root systems of our grass plants and now has the potential to continue downward through the soil and potentially pollute ground water supplies. Second, nitrate nitrogen can also be converted to nitrogen gas (N2) through a process known as denitrification. This most commonly occurs during warm, wet soil conditions.
The end result of these three major soil nitrogen losses along with losses from a few other minor soil processes is that our soils often lack the necessary amount of nitrogen needed to support healthy, vigorous grass growth at all times during the growing season. Hence, there is often a need to supplement our soils with some additional nitrogen containing fertilizer in order to sustain a healthy turfgrass system throughout the year.
Natural nitrogen inputs to a lawn
Before delving into nitrogen fertilizers, it is important to remember that fertilizer is not the only means by which our lawns receive supplemental N inputs. For example, as organic matter is continually being decomposed by the soil microbes, nutrients, including plant available nitrogen, are released back to the soil for use by the grass plants or perhaps reused (and rereleased) by other microorganisms. When soil test results indicate organic matter levels of 3.1% or higher, about 0.5 pounds (or slightly more) of actual nitrogen per 1000 ft2 can be returned to the lawn annually.
Grass clippings generated during regular mowing and left on the lawn over the entire season will supplement the lawn with the equivalent of about one complete fertilizer application annually or about 1 pound of actual N per 1000 ft2 per year. Under very low maintenance situations, including no irrigation, these natural supplies of N may be sufficient to sustain the turfgrass. However, some (0.5 - 1.0 #N per 1000 ft2) supplemental nitrogen will usually be necessary to sustain an acceptable turfgrass stand having at least medium green color and sufficient density to completely cover the soil surface.
Photo 3: Necrotic ringspot disease on Kentucky bluegrass. Bob Mugaas.When there is too much nitrogen
Over abundance or deficiency of available nitrogen are both problematic for grass plants. Excessive levels of nitrogen:
1) stimulate rapid shoot growth while slowing down root growth and increasing the need for more frequent mowing; 2) deplete the plant's carbohydrate reserves more rapidly, which in turn can result in less stress tolerance and slower recovery from any injury to the plant; 3) result in thinner, more succulent leaf tissue, which increases moisture loss and therefore creates a greater need for water; 4) can predispose the plant to greater insect and disease problems; (Picture 3 illustrates Necrotic Ring Spot disease on Kentucky bluegrass; a disease often associated with excessive nitrogen fertilizer applications.); 5) contribute to more rapid and excessive thatch development; 6) leach through the soil beyond the root system potentially polluting ground water resources when not used by the grass plant.
Photo 4: Crown rust on Kentucky bluegrass. Bob Mugaas.When there isn't enough nitrogen
On the other hand, a deficiency of available nitrogen results in: 1) slower grass plant growth; 2) paler green color thus decreasing photosynthetic capacity and therefore food production; 3) slower recovery from injury thus decreasing traffic and wear tolerance; 4) increased susceptibility to crown rust, red thread and dollar spot diseases; (Picture 4 illustrates crown rust on Kentucky bluegrass.); 5) decreased tolerance to environmental stresses such as drought conditions and/or high temperatures; 6) decreased lawn density thus opening the door for increased weed invasion and decreased ability to both reduce the amount of runoff as well as slow the velocity of runoff water from the site.
A brief overview of forms and sources of N fertilizer:
While a University of Minnesota soil test report can be very helpful in determining what your lawn needs, it doesn't measure the amount of nitrogen actually in the soil as part of a standard soil test. Because of the various ongoing and ever changing nitrogen losses described above, a nitrogen value won't be very helpful. In other words, a measured N level today will not likely be the same a week from now or even in a few days from now depending on weather conditions, grass growth and cultural practices being carried out. Nonetheless, the soil test report does consider the various forms of nitrogen losses and natural inputs. Thus, a soil test report will suggest a nitrogen application program based on soil test results and information provided by the user at the time the soil sample was submitted.
Quick release vs. slow release N sources. There are many sources and formulations of nitrogen found in lawn fertilizers. However, those sources can be broken up in to slow release sources (water insoluble nitrogen) or quick release sources (water soluble nitrogen). In the latter, the fertilizer is readily soluble in water and, once dissolved, the nitrogen is immediately available for grass plants to take up and use. Common examples of quick release sources include ammonium sulfate, ammoniacal nitrogen and urea. As a result, grass plants green up very quickly and grow vigorously (sometimes too much so) for a relatively short period of time following an application of quick release nitrogen.
An over application of quick release nitrogen and the accompanying excessively lush grass growth also increases the need for more moisture and more frequent mowing and can make the lawn more susceptible to certain diseases, insect problems and environmental stresses. In order to avoid this situation, it is highly recommended that quick release nitrogen fertilizer be applied at no more than one pound of actual N per 1000 2 of lawn per application.
In contrast to the rapid green up and rapid growth rate resulting from quick release sources of N, slow release sources result in a slower rate of green-up and growth. In addition, they can provide a longer period of sustained nitrogen release to the lawn. This extends the length of time the grass exhibits a healthy green color and sustains a more uniform, but slower growth rate. For these two reasons, it is also likely that the time between fertilizer applications will be lengthened and may even reduce the need for nitrogen fertilizer applied in a growing season.
Organic, synthetic organic and inorganic sources. Slow release sources can originate from natural sources such as those produced from plant or animal processing byproducts or be synthetically manufactured. The former are known as natural organic nitrogen sources while the latter are known as synthetic organic nitrogen sources. The usage of the word organic in this case simply means that the products contain the element carbon (C) in their molecular structure(s). When carbon is absent from the fertilizer molecule, they are known as inorganic fertilizers.
Photo 5: Well-planned shoreline landscape including a healthy lawn area. Brad Pedersen.
Most of the quick release fertilizers are inorganic while slow release fertilizers are mostly organic in origin. The one notable exception is the fertilizer material known as urea. It does contain carbon in its molecular structure and therefore is considered an organic material but it reacts like a quick release nitrogen source when used in a lawn fertilizer. Urea is often the starting point for many of our synthetic organic nitrogen sources used in lawn fertilizers. These products are produced by either further chemically reacting the urea such as in the case of producing methylene urea or coating the urea such as in the case of sulfur coated or polymer coated urea. Both methylene urea and sulfur coated urea are commonly available in homeowner lawn fertilizers.
Slow release fertilizers and water quality impacts. There are also positive water quality implications when using slow release sources of N. Lawns, landscape plantings and shorelines can be compatible partners in helping to protect water resources. See Picture 5. Using slow release fertilizers can be an important tool in managing our lawns to protect water quality. Since smaller amounts of N are released over a longer period of time, grass plants have a greater chance of taking up and using the N before it potentially leaches through the soil and has a chance to contaminate ground and/or surface water resources. Because the nitrogen is released more slowly, slightly higher rates of fertilizer can be applied. As always however, be sure to follow the directions on the fertilizer container for proper spreader settings and application rates.
For more information
For a comprehensive listing of nitrogen sources and a more thorough discussion on when and how to correctly apply lawn fertilizers including calibrating a lawn fertilizer spreader for accurate application, see the Home Lawn Care Chapter on Understanding and Using Home Lawn Fertilizers in the Sustainable Urban Landscape Information Series website.
Nitrogen is an important element in sustaining normal, healthy grass plant growth and aiding recovery from injury. Used wisely and applied at appropriate times of the year, the likelihood of creating adverse effects to the lawn or the surrounding environment will be minimal.