Feast or Famine Fertility

No single nitrogen fertilizer program is ideal for all lawns, grounds, athletic fields or golf courses. The type and amount of nitrogen fertilizer used and the timing of your applications will depend on the turfgrass species, soil type, management, use of the turf and yours and your clients' expectations.

NITROGEN FERTILIZER PROGRAMS FOR COOL-SEASON TURFGRASSES

By Peter Landschoot, Pennsylvania State University

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Cool-season turfgrass species differ in the amount of nitrogen (N) fertilizer needed for optimum performance. Kentucky bluegrass and perennial ryegrass typically need 3 to 4 pounds of N/1,000 square feet per year, whereas fescues respond best to about 2 pounds N/1,000 square feet per year. If you fertilize Kentucky bluegrass with only 1 or 2 pounds N/1,000 square feet during the entire growing season, it will usually become light-green, thin and more susceptible to disease damage (rust, dollar spot and red thread). On the other hand, if you fertilize with fine fescue turf more than 4 pounds N/1,000 square feet per year, it can become more susceptible to drought, heat stress and leaf spot diseases. Therefore, you must identify the species you are managing and adjust your nitrogen fertility program accordingly. In turf containing mixtures of species, you can design your nitrogen program to favor the desired species (see Table 1, page 16).

FACTORS AFFECTING N

Turfgrass cultivars also vary in their nitrogen needs. Unfortunately, seldom are there absolute recommendations for individual cultivars because nitrogen needs have not been determined for most new cultivars and many turf managers have no way of knowing which cultivars are present in lawns.

Nitrogen fertilizer programs will vary with soil quality and type. Turfgrasses growing on sites where much of the topsoil has been removed prior to establishment or in sandy soil usually require more nitrogen fertilizer than turf growing in good quality topsoil. This is due to the low amounts of nitrogen found in poor-quality soil and the fact that nitrogen is more easily leached from sandy soil. Improving poor-quality soil with the addition of organic amendments, such as good-quality compost, can improve soil quality and add nutrients, thus reducing nitrogen needs. Use of a slow-release nitrogen fertilizer on turf growing in sandy soil can help avoid nitrogen leaching when compared to equal amounts of quick-release nitrogen fertilizer.

Management practices, such as mowing and irrigating, can significantly influence the amount of fertilizer needed by turfgrasses. By returning grass clippings to your lawn, you can reduce nitrogen fertilizer needs by up to one third compared to collecting clippings and removing them from the site. Turf that you frequently irrigate during the summer months will use more fertilizer than non-irrigated turf because it is growing and producing new tillers.

Turf use will also determine the amount of nitrogen needed for good turf growth. For instance, turfgrasses growing in high-traffic areas, such as athletic fields, usually require more nitrogen for better recovery from wear than low-traffic areas. High-use athletic fields may require up to 5 pounds of nitrogen/1,000 square feet during the growing season to help with turf recovery from wear. In these cases, make sure you can mow frequently because high amounts of nitrogen will force rapid leaf growth. Turf used along roadsides (to create a buffer between lanes on highways) or turf growing on steep banks for erosion control generally receive little or no fertilizer because aesthetics are not a primary goal and you usually keep mowing in these areas to a minimum.

Ultimately, users will have different expectations concerning the function and appearance of turfgrass areas. Thus, nitrogen fertilizer programs will vary according to these expectations. Be sure to educate users on the possible outcomes of applying unusually high amounts (more mowing and reduced stress tolerance) or low amounts (thin turf and increased susceptibility to certain diseases) of nitrogen to turf.

APPLICATION FREQUENCY

The number of nitrogen fertilizer applications that you make during the growing season is just as important as the amount and type of fertilizer you use. To maintain high-quality turf, you should apply fertilizer two or more times per year. If you make only two applications, you usually need to use higher rates of nitrogen (1.0 to 1.5 pounds N/1,000 square feet per application). In this case, fertilizers containing slow-release nitrogen sources are desirable because the nitrogen is released gradually over extended periods and burning of turf foliage is less likely.

In most cases, nitrogen fertilizer programs involve more than two and as many as five applications per year. These programs allow more flexibility with respect to application rates and nitrogen sources than two-application programs because there is less time between applications. A four-application-per-year program, for example, can involve rates less than 1.0 pound N/1,000 square feet per application. These lower rates allow use of quick-release nitrogen sources. They also allow more uniform green color and growth throughout the growing season than two-application programs.

SCHEDULING FERTILIZER APPLICATIONS

The best times of year to fertilize cool-season turfgrasses in northern climates are in late summer (late August to mid-September), late fall (early to late November), and mid- to late spring (May to early June). In some cases, two spring applications may be desirable — one in early spring (March or early April) and another in late spring (late May or early June). Nitrogen fertilizer applied to non-irrigated turf during periods of heat and drought in mid-summer can lead to turf injury. You can apply low rates of nitrogen to irrigated turf (i.e., golf course fairways and athletic fields) during mid-summer to promote limited growth and recovery from divots and wear.

TABLE 1. ANNUAL NITROGEN REQUIREMENTS FOR TURFGRASS SPECIES USED IN THE NORTHERN U.S.
Cool-Season Turfgrass Species Amount of Nitrogen Required each Growing Season* ( lb/1000 sq ft )
Creeping bentgrass 3 - 6
Kentucky bluegrass 3 - 4
Perennial ryegrass 3 - 4
Annual ryegrass 2 - 3
Tall fescue 2 - 3
Fine fescues (creeping red, Chewings, hard, and sheep) 2 - 3
* Use rates in the high range for turf grown in infertile soils, when clippings are removed from the site, and in high traffic areas. Rates in the low range can be used for turf grown in fertile soils and when clippings are returned to the turf.

The most important time of year to fertilize turfgrasses is late summer. Nitrogen is extremely important at this time of year because it promotes recovery from drought and heat-related injury sustained during mid-summer. Late summer and early fall is the time of year that cool-season grasses manufacture and store carbohydrates for use in winter and the following spring. Carbohydrates are used by turfgrasses for leaf, root and rhizome growth; disease and stress tolerance; and protection from winter injury. Nitrogen applied during late summer will stimulate foliar growth, but not to the extent that occurs following spring nitrogen applications. Thus, you can use slightly higher rates of nitrogen (1.25 to 1.5 pounds N/1,000 square feet) for late summer applications, provided that some of the nitrogen is from a slow-release source.

An application of nitrogen fertilizer in late-fall can serve as a replacement for an early spring application. Late fall, in this case, is when leaf growth slows or stops, but before soils are frozen. In most areas of the northern United States, late-fall fertilization should take place in November. If done correctly, late-fall fertilization will provide early and noticeable turf green-up in spring with less leaf growth compared to an early-spring fertilizer application. Excess leaf growth is often associated with high rates of nitrogen applied in early spring. You should avoid late-fall fertilizer applications when soils are frozen because the nitrogen remains on the surface and is subject to runoff into streams and lakes where it can cause pollution problems.

The main disadvantage of late-fall fertilization is that, in some situations, nitrogen leaching may occur. You should not perform late-fall fertilization on sandy soils with quick-release nitrogen fertilizers. Slow-release nitrogen sources, such as IBDU, natural organic or sulfur-coated urea, are ideal for late-fall applications mostly because their nitrogen is not as likely to leach as quick-release nitrogen sources.

If you do not make late-fall fertilizer applications, you may need to consider applying some nitrogen fertilizer in early spring. Because high rates of nitrogen applied to turf in early spring will produce excess leaf growth and force plants to use up valuable food reserves (carbohydrates) needed for root growth, stress tolerance and disease resistance, you should use lower rates of nitrogen. Typically, rates of 0.5 to 0.75 pound N/1,000 square feet will allow early-spring green-up of lawns without excess leaf growth. Since 0.5 pound N/1,000 square feet in early spring will not supply enough nitrogen to carry the turf through the summer months, you will need to also make a late-spring application in late May or early June and rates can vary from 0.75 lb to 1.5 pounds N/1,000 square feet. A fertilizer containing slow-release nitrogen is desirable at this time of year because it can supply small amounts of nitrogen to turf well into the summer.

NITROGEN FERTILIZER PROGRAMS FOR WARM-SEASON TURFGRASSES

By Grady Miller, University of Florida

Nitrogen (N) is the key element for turfgrass growth and is the fertilizer nutrient most often applied by homeowners and professional turf managers. Nitrogen provides an aesthetic as well as a functional role in turfgrass culture. When applied properly, nitrogen helps ensure the darker green color most people prefer, and at higher rates it can promote rapid recovery of worn turf areas. Excessive nitrogen application leads to excessive shoot and leaf growth, reduced root growth, low plant-carbohydrate reserves, increased susceptibility to environmental stress and some diseases, coupled with potential negative environmental implications. To most effectively use this nutrient, you need to have a good understanding of how nitrogen is involved in both plant and soil. So, how much nitrogen is enough? How much nitrogen is too much?

FACTORS AFFECTING N

The amount of nitrogen that you should apply to turf is not an easy question to definitively answer without looking at the big picture. A general response is that you should apply nitrogen at the rate necessary for healthy grass, without excesses that could be lost in the environment. Therefore, the appropriate rates of nitrogen should be based on turf use and condition, knowledge of how a given turf responds and nitrogen carrier for a given grass. The recent trend has been to reduce the total yearly amount of nitrogen applied, especially on golf course greens and fairways.

Most grasses require a minimal amount of nitrogen to maintain a reasonable turf density. Warm-season species that fall into this category are centipedegrass, carpetgrass, bahiagrass and buffalograss. These grasses often are put in areas intended to be low-maintenance or areas where minimal turf quality is acceptable. Medium- to high-maintenance turfs, such as those found on golf course tees, fairways, greens, athletic fields and some highly manicured landscapes, typically require different warm-season grass species. The warm-season grasses most commonly used include bermudagrass, zoysiagrass and St. Augustinegrass (landscape uses). Because these grasses are used in very visible areas, or receive a high-degree of human contact, their yearly N rates are typically higher.

NITROGEN APPLICATION PROGRAMS

Turf that periodically receives heavy wear requires appropriate nitrogen rates and application timings for adequate recovery of turf density. The relationship between timing and amount is very important for turf when you desire controlled growth. Fertility programs for these high-use turf areas are often more sophisticated than with general turf. Due to their seasonal use, you should place the emphasis on increased growth rates to recover during part of the year and apply just enough nitrogen during other parts of the year to maintain modest growth.

The most effective way to promote recovery of worn turf areas is to use fertilizers with water-soluble nitrogen sources. Apply them at low rates (0.25 to 0.5 pound N/1,000 square feet) every 2 to 4 weeks during the most active period of growth. If you have areas of extremely high wear (e.g., on football fields between the hash marks, golf tees, etc), treat these areas separately. Spot treating worn areas also does not put as great a strain on your fertilizer budget.

Athletic fields often experience heavy use in the late fall and early spring, when turf growth is minimal. To encourage turf recuperation during these periods, fertilize worn areas a little later in the fall and a little earlier in the spring than less-trafficked areas. Remember to use lower rates because the turf is not as efficient at utilizing the applied nutrients. If the warm-season turf is overseeded with a cool-season grass, such as annual or perennial ryegrass, apply 0.2 to 0.3 pound N/1,000 square feet every 2 to 3 weeks to maintain density and color. Cool-season grass, such as the ryegrasses, do not need as much fertilizer during their growing season as most high-maintenance warm-season grasses. No fertility program can prevent turf loss in areas subjected to excessively high traffic, but reasonable nitrogen use, combined with good cultural practices will help ensure healthy turf all year.

Nitrogen application rates also depend on the soil type and weather conditions. In Florida, it is not unusual to use 7 to 10 pounds N/1,000 square feet annually on native soil bermudagrass, depending on location and winter overseeding. This is due to Florida's sandy soils and long growing season. In more northern areas that have heavier soils (more silt and clay) and a shorter growing season, turf managers should use appreciably less fertilizer. Regardless of location, the general rule is that the maximum nitrogen rate per application should not exceed 1 pound N/1,000 square feet. With the introduction of better controlled-release fertilizers this rate may be increased, but it is generally not recommended.

Your decisions regarding N rates should always include consideration of environmental ramifications. Applied nitrogen may be taken up by the plant, stored in the soil or thatch, lost to the atmosphere or lost to the groundwater. Nitrogen in excess of that taken up by the plant reduces the efficiency of the applied nitrogen and you should avoid doing that. Soils that are high in sand content are more prone to leaching; therefore, you must manage them to reduce the risk of nitrogen movement. Research substantiates that controlled-release fertilizers reduce N leaching in turfgrass. If you use a soluble nitrogen source, you can minimize the potential for nitrate-N leaching by applying it more frequently at a reduced rate, either by split-surface applications or fertigation. One of these programs is especially necessary where sandy soils are close to surface or groundwater resources.

Peter Landschoot, Ph.D., is associate professor of turfgrass management at Pennsylvania State University (University Park, Pa.) and Grady L. Miller, Ph.D., is associate professor of turfgrass science at the University of Florida (Gainesville, Fla.).

EXAMPLES OF LAWN FERTILIZER PROGRAMS

The following sample fertilizer programs are designed for medium to high maintenance lawns growing under environmental conditions and soils found in the northern U.S. These programs are only offered as examples and may or may not be suitable for your particular operation.

TABLE 2. EXAMPLE OF A THREE-APPLICATION FERTILIZER PROGRAM FOR A PERENNIAL RYEGRASS OR KENTUCKY BLUEGRASS LAWN (CLIPPINGS RETURNED).
Dates of Application Nutrients*/1000 sq ft
May 1 - June 10 1.0 to 1.25 lb nitrogen (20% or more as WIN, CRN, or a coated nitrogen source**)
Aug. 25 - Sept. 20 1.0 to 1.5 lb nitrogen (20% or more as WIN, CRN, or a coated nitrogen source)
0.5 lb phosphate
0.5 lb potash
Nov. 1 - Nov. 30 1.0 lb nitrogen (50% or more as WIN, CRN, or a coated nitrogen source)
0.75 lb potash
* If soil test indicates high or excessive levels of phosphate and potash, omit from program and use nitrogen sources only. If soil test indicates phosphate and potash are needed, use fertilizer with high proportions of each nutrient.
** WIN = water insoluble nitrogen, CRN = controlled release nitrogen, coated nitrogen sources can include sulfur-coated urea or polymer-coated nitrogen.

TABLE 3. EXAMPLE OF A TWO-APPLICATION FERTILIZER PROGRAM FOR A FINE FESCUE LAWN (CLIPPINGS RETURNED).
Dates of Application Nutrients*/1000 sq ft
May 1 - June 10 1.0 lb nitrogen (20% or more as WIN, CRN, or a coated nitrogen source**)
Aug. 25 - Sept. 20 1.0 - 1.5 lb nitrogen (20% or more as WIN, CRN, or a coated nitrogen source)
0.5 lb phosphate
0.5 lb potash
* If soil test indicates high or excessive levels of phosphate and potash, omit from program and use nitrogen sources only. If soil test indicates phosphate and potash are needed, use fertilizer with high proportions of each nutrient.
** WIN = water insoluble nitrogen, CRN = controlled release nitrogen, coated nitrogen sources can include sulfur-coated urea or polymer-coated nitrogen.

TABLE 4. EXAMPLE OF A THREE-APPLICATION FERTILIZER PROGRAM FOR A HIGH TRAFFIC TALL FESCUE LAWN (CLIPPINGS RETURNED).
Dates of Application May 1 - June 10
Nutrients*/1000 sq ft 1.0 lb nitrogen (20% or more as WIN, CRN, or a coated nitrogen source**)
Aug. 25 - Sept. 20 1.0 lb nitrogen (20% or more as WIN, CRN, or a coated nitrogen source)
0.5 lb phosphate
0.5 lb potash
Nov. 10 - Nov. 30 1.0 lb nitrogen (50% or more as WIN, CRN, or a coated nitrogen source)
0.75 lb potash
* If soil test indicates high or excessive levels of phosphate and potash, omit from program and use nitrogen sources only. If soil test indicates phosphate and potash are needed, use fertilizer with high proportions of each nutrient.
** WIN = water insoluble nitrogen, CRN = controlled release nitrogen, coated nitrogen sources can include sulfur-coated urea or polymer-coated nitrogen.

TABLE 5. APPROXIMATE NITROGEN REQUIREMENTS FOR WARM-SEASON TURFGRASS SPECIES PER GROWING MONTH.
Warm-Season Turf Nitrogen Requirement Pounds N per 1,000 ft2 per Growing Month
General Turf Golf and Sports Turf
Bahiagrass 0.0 - 0.2 0.1 - 0.5
Buffalograss 0.0 - 0.2 0.2 - 0.4
Carpetgrass 0.0 - 0.3 ----
Centipedegrass 0.0 - 0.4 ----
Common bermudagrass 0.2 - 0.4 0.4 - 0.7
Hybrid bermudagrass 0.4 - 0.6 0.6 - 1.5
Seashore paspalum 0.2 - 0.4 0.4 - 0.8
St. Augustinegrass 0.2 - 0.8 ----
Zoysiagrass 0.2 - 0.3 0.3 - 0.6
Note: general turf includes lawns, amenity turf, general grounds, and utility turf continued from page 18

For any fertilization program the goal is to promote turf density and color without promoting excess top growth. In order to reach this goal you must match your fertilization program to the growth characteristics of the turfgrass you are dealing with. Growth characteristics differ between cool-season and warm-season turfgrasses (see Figure 1). Cool-season grasses, as the name implies thrive in cooler temperatures and slow down in summer. They exhibit rapid growth in spring, followed by a summer lull when temperature and heat stress is high and a more moderate growth spurt in the fall when cooler temperatures return. Warm-season turfgrasses, on the other hand, do best at higher temperatures. Consequently, as temperatures increase, so does growth of warm-season turfgrasses.

Based on these growth characteristics, it's important that you differentiate fertilizer timing between cool- and warm-season turfgrasses. Fall is the ideal time to apply nitrogen fertilizer to cool-season turf. Turf needs nitrogen in the spring as well, but you don't want to encourage excess growth at a time that the turfgrass growth is already at its peak. Conversely, for warm-season turfgrasses, you should apply nitrogen fertilizer on a more regular schedule throughout its growing season, without letting up in summer.

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