Depending on which plant physiology textbook you read, there are about 18 elements that are classified as essential to plant growth. All of them are equally important – without them the life cycle of the plant couldn’t be completed.
Despite their essentialness to plant growth, many of these 18 elements aren’t applied as a part of our routine fertilization programs on athletic fields. Some of them aren’t minerals (carbon, hydrogen and oxygen). Many of them in native soil are assumed to, in most cases, be present in adequate quantities such that applying them is only justified if a soil test indicates. These include the macronutrients calcium, magnesium and sulfur, along with the micronutrients boron, chlorine, copper, iron manganese, molybdenum, nickel, sodium and zinc. Even phosphorus and potassium should technically be applied only when a soil test indicates need.
In this article, we’ll focus on one element: nitrogen. It’s one of only two elements in which application results in a visible agronomic response from the grass (iron is the other). There are some concerns about nitrogen in the environment and applications to turfgrass have been implicated as potentially contributing to this problem.
Keep in mind that we don’t have a soil test for nitrogen, thus we make applications based on a schedule, which is impacted by many different factors.
Why is it important?
If we look at the composition of a typical plant, we see that most of its weight (just under 90 percent) is composed of the atoms carbon, hydrogen and oxygen. These elements are parts of both structural (i.e. cell walls) and nonstructural (i.e. sugars) carbohydrates. That means that the other 15 elements deemed essential comprise the remaining just over 10 percent of the weight of the plant. By far, the next most abundant is nitrogen at about 4.6 percent. It’s a component of amino acids, proteins, chlorophyll, nucleic acids and co-enzymes. Thus, it serves many biological functions within the plant – the level of nitrogen affects root and shoot growth, turfgrass stand density, color, recuperative potential and disease proneness.
A potential problem?
By the 1990s, most folks within the industry were keenly aware of the controversies associated with the use of some of our pesticides for turfgrass management. At that time, though, not as much attention was paid to whether our fertility management practices might contribute to unwanted environmental contamination.
In the present, we know that misapplication of certain nutrients can and does lead to unwanted environmental contamination. The extent of the contribution by turfgrass management practices is often debated.
Nonetheless, turfgrass management is implicated. At first, all the attention was on phosphorus, due to its contribution to eutrophication of lakes.
Turfgrass scientists have also studied whether there could be unwanted movement of nitrogen from a turfgrass system. This work became very complex because the fate of nitrogen in a plant-soil system is very complicated, with nitrogen existing in many different forms. There was some concern about the potential for fertilizer nitrogen to leach out of turfgrass in the form of nitrate nitrogen. But, at that time, many turfgrass researchers were interested in the fate of applied nitrogen to understand and make nitrogen use more efficient in turf. For example, they wanted to understand if gaseous loss of nitrogen occurred and if it did, to what extent. Many of the initial studies that were done during the 1980s and early 1990s were encouraging from an environmental standpoint. What was found was that if nitrogen was properly applied to turfgrass, then it was immobilized very quickly and very little of it was detected in water that leached out of the soil profile.
Because of these studies, it was thought that turfgrass was not a significant contributor to nitrate leaching.
What many of these studies shared was that they were conducted on newly-established turfgrass, where much of the nitrate nitrogen is quickly converted into organic forms via immobilization. If the nitrogen is immobilized, then it can’t leach. These conversions between different nitrogen forms are governed by microbial populations in the soil. But over time, the balance of the different forms of nitrogen in the soil shifts. Additionally, it was found on older turfgrass systems that fertilizer nitrogen can in fact leach out of the system.
One of the first projects that shed light on this was a long-term project conducted at Michigan State University. Using the same lysimeter plots where it was first concluded that nitrate leaching might not be an issue, researchers began to find excessive levels of nitrate in the leachate after about a 10-year period. It was also found that the level of nitrate detected in the leachate could be significantly reduced simply by applying less nitrogen. Therefore, it’s recommended that on older turfgrass stands, the amount of nitrogen applied should be reduced to cut down on leaching potential. Note that this shouldn’t result in a decrease in turfgrass quality. The rest of the nitrogen is supplied from what had been tied up in organic forms, becoming available again through a process called mineralization.
The main culprit for environmental contamination with nitrogen is nitrate leaching. Several states have placed restrictions on the application of nitrogen to reduce leaching losses. Some of these laws have produced positive results. The law in New Jersey, for example, requires a higher percentage of slow-release forms of nitrogen be applied. This has significantly improved turfgrass quality, reason being that in New Jersey’s predominately sandy soils, application of slow-release nitrogen results in more uptake by the turf and less loss to leaching.
If you live in a state that regulates applications of nitrogen, then these laws will no doubt have an impact on how you apply fertilizer.
If you don’t live in one of these areas, you’re probably applying nitrogen to your fields per some sort of schedule that’s based on research data. The difficulty with determining how much nitrogen to apply and when is that there’s no practical soil test to measure nitrogen status in the soil.
You can test for ammoniacal nitrogen and nitrate nitrogen, but studies have shown that these concentrations can vary literally over a period of days. In other words, by the time the soil test lab tells you the amount of nitrate or ammonium in the soil, it has probably changed. Knowledge of these soil test values isn’t very useful for predicating your turfgrass nitrogen needs.
Apply it to your turf
Nitrogen is important for plant growth, but it can be problematic if not applied correctly. Thus, the goal of your nitrogen fertility program should be to supply the turf with exactly the amount of nitrogen it needs – in a timely manner – while avoiding excess that’s both costly and potentially harmful to the environment.
Kentucky bluegrass, perennial ryegrass and bermudagrass require more annual nitrogen than tall fescue. In addition, the requirements can vary significantly among cultivars of the same species. Improved cultivars will require more fertilizer compared to the common varieties.
Mowing and irrigation management also have a big impact on nitrogen needs. Turf that’s mowed shorter (your gameday field) will require more nitrogen compared to the surrounding areas or fields that are mowed higher. Similarly, the fertility requirements will be higher if irrigation is optimized.
Turf that’s subject to more traffic stress will also require more nitrogen. The point to consider here is that if you have a large facility, the nitrogen needs of the gameday fields, practice fields and the surrounding areas are all a bit different.
You should also test your soil to determine fertility needs. The test won’t give information about nitrogen requirements, but it will help to determine how you should apply nitrogen. The number of interest here is the cation exchange capacity, or CEC. This is a measure of the number of exchange sites, or the nutrient carrying capacity of your soil. If your CEC is low, such as the case if you have a sandy soil, then you’ll want to apply less nitrogen but more often. If your CEC is high, typical of a clay soil, then you can apply more nitrogen less often.
On a high-maintenance, soil-based field with Kentucky bluegrass or perennial ryegrass where leaching of nitrate isn’t of concern, a generic application schedule is displayed in the table below.
This is a very generic schedule. On a sand-based field that’s mowed shorter, you would want to apply less nitrogen but more often.
For low-maintenance, soil-based fields, you might only make two applications per year. These applications would typically be in May to promote spring green-up and in August to help the turf recover from summer stress.
Other adjustments to this generic schedule should take into account your location, not only because of differences in grass growth during the year (the season and thus the fertility needs of the turf are different in warmer areas) but also if there are any laws or restrictions on fertilizer applications at any given time of the year.
This brings us to late-season fertilization. Much research has been done that says this application is both the most beneficial to the turf and yet the most potentially harmful to the environment, because of potential nitrate leaching and runoff. The recommendation for late- season nitrogen applications had typically been to use a 100 percent water-soluble source applied at about 1 to 1.5 pounds of nitrogen per 1,000 square feet after top growth ceases, but while the grass is still green and before the soil freezes. This has shown to provide many benefits to the turf, including more production of root tissue (either in the fall or in early spring) and less potential for excessive shoot growth in the spring. Thus, carbohydrate reserves aren’t depleted in the plant and it becomes more resistant to summer stress.
In response to environmental concerns, more recent research has shown that this late-fall application, where permitted and recommended, can include slow-release nitrogen, which is much less likely to leach or runoff. In locations where late-fall fertilization is either not recommended (for example, on a sand-based field) or not permitted, you may want to consider applying 0.5 pounds of nitrogen per 1,000 square feet of a water-soluble nitrogen source in mid-October instead.
These fall fertility recommendations vary considerably by state. Sports turf managers should consult their state turfgrass extension specialist, as they can provide recommendations for your location.
As important as nitrogen is to the health of an athletic field, it’s also important that it be applied correctly. This allows field managers to help to protect the environment, making an operation sustainable.
Another upside of making your nitrogen use more efficient is that you’ll also help protect your budget in the process.