Keeping an Eye on Nitrogen

The almost automatic response to the sight of stressed turf is to apply nitrogen. But sometimes this is exactly the worst thing that you can do. Although nitrogen deficiencies are sometimes the cause of turf quality problems, it turns out that turf damage due to excessive nitrogen on the properties you manage is a more common problem than most people realize (see Figure 1, page 14). Once nitrogen levels are high, it takes a long time for turf to recover. It's clear that avoiding the build-up of soil nitrogen in the first place should be a key goal in all fertility programs.

NITROGEN SOURCES

Nitrogen is delivered to soils in a variety of forms. Nitrate-based fertilizers (ammonium nitrate, calcium nitrate, potassium nitrate, etc.) deliver nitrogen to the plant in the form of nitrate (NO3-). On the other hand, ammonium-generating fertilizers such as proteinaceous organic fertilizers, urea products (urea, sulfur coated urea, polymer coated urea, ureaformaldehydes and isobutylidene diurea [IBDU]) and ammonia products (ammonium nitrate, ammonium sulfate, mono and di-ammonium phosphates, etc.) release both ammonium and nitrate following their breakdown by nitrification in the soil. Nitrogen may also arrive in the soil more stealthily: through breakdown of soil organic matter or via use of a water source (either well water or reclaimed water) that is high in nitrogen.

DEFINING OPTIMAL NITROGEN LEVELS

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As for many other parameters of turf growth and health, there is no one single nitrogen concentration that will insure healthy turf. Instead, there is a wide range of values that can support good growth (see Figure 2, page 18). A look at PACE Turfgrass Research Institute's database of soils from around the country will give you an idea of the broad spectrum of nitrogen (N) concentrations found on good-performing greens and fairways (see Figure 3, page 18). You can gain some helpful insights by studying these summary graphs:

The majority of good-performing turf samples had 20 parts per million (ppm) or less of total plant available N.

Nitrogen levels that are too low (less than 3 ppm) can also compromise turf health.

Fairways consistently had higher levels of nitrogen than greens; our conclusion is that fairways are consistently over-fertilized and should be managed to have less than 20 ppm total nitrogen in the soil.

A small number of good-performing greens and fairways had high levels of total nitrogen — up to 40 ppm. So yes, it is possible to have high-quality turf at high nitrogen levels, but your odds of creating a problem are much, much higher under these conditions.

THE ROLE OF AMMONIUM-BASED NITROGEN

As we've described above, levels of total nitrogen (a sum of nitrate and ammonium nitrogen) that are above 20 ppm can cause damage to turf. But sometimes, even when total nitrogen readings are less than 20 ppm, we see problems that are related to the presence of ammonium in the soil. We have found that even relatively low concentrations of ammonium (7 ppm or more) are related to turf damage. Thus, you could have a soil with 15 ppm total nitrogen: 5 ppm nitrate and 10 ppm ammonium. This would be a problem soil, even though the total values are below 20 ppm.

Under normal conditions, ammonia-based fertilizers should break down in the soil to nitrate, a form of nitrogen that is less toxic to plants than ammonia. However, this break-down process, known as nitrification, can be inhibited if the soil is low in oxygen (due to compaction, black layer, poor drainage, high microbial populations, etc). Accumulation of toxic levels of ammonium is more likely to occur during the summer, when soils are typically more oxygen depleted than usual.

Healthy turf also appears to be correlated with a specific balance between nitrate and ammonium levels. Keeping this balance at three or more times more nitrate than ammonium appears to be optimal for turf health. For example, if nitrate levels are 9 ppm, then ammonium levels shouldn't be higher than 3 ppm.

SPECIAL PROBLEMS ON FAIRWAYS AND ROUGHS

Fairway and rough turf typically has higher soil nitrogen levels than greens or tees, even when they are all fertilized similarly. How is this possible? The most likely explanation is due to the higher levels of organic matter (in the form of thatch, dead roots or other plant material, such as clippings) in these areas. As the organic matter decays, carbon, nitrogen and water are released into the soil, thus contributing to rising nitrogen concentrations.

How much nitrogen is stored in soil organic matter? The average nitrogen release from organic matter is 1.5 pounds of N per 1,000 square feet for greens, and 3.8 pounds N per 1,000 square feet for fairways over the course of the year. From these values, you can see that nitrogen release from organic matter can provide significant amounts of nitrogen. Your soil reports should provide a rough estimate of nitrogen release from organic matter. If you have fairway soil that contains more than six percent organic matter, apply nitrogen cautiously, if at all. The reason is that a soil containing six percent organic matter will release most of its nitrogen during the warm summer months. The warmer the weather, the more rapid the release. The ideal organic matter content for fairways is below six percent and for greens is below two percent.

HOW WATER CONTRIBUTES

In addition to the role of organic matter in higher nitrogen levels, a high nitrogen water source can also play a role in delivering too much nitrogen. If you use either well water or recycled water with nitrogen levels greater than 8 milligrams per liter, turf may be at risk. You need to monitor both soil and water nitrogen levels regularly to avoid toxic build-up.

TESTING FOR NITROGEN

Regular monitoring (twice yearly is usually recommended) of soil nutrients should be the foundation for your turf management decisions. The monitoring can be performed by a variety of analytical labs across the country. To make sure that you receive enough information about your soils, be sure to request analysis (in parts per million) for nitrate and ammonium forms of nitrogen. From these values, you can calculate total plant available nitrogen (nitrate plus ammonium) and nitrate to ammonium ratios (nitrate divided by ammonium values). If you think that your water source may be high in nitrogen, a yearly analysis for total nitrogen (ammonium and nitrate) can also be very important.

For a “quick and dirty” test for soil nitrate that you can do in just a few minutes, you can use Hach water test strips for nitrate/nitrite content levels. For details on the procedure, see “Quick Test for Soil Nitrate,” left.

AVOIDING HIGH NITROGEN LEVELS

To keep nitrogen levels in check, follow these guidelines:

  • Monitor soil nitrate, ammonium and total plant available nitrogen one to two times per year. If irrigation water is a potential source of nitrogen, monitor annually for nitrate and ammonium.

  • Follow the guidelines for soil nitrate, ammonium and total plant available nitrogen shown in Figure 2, above.

  • Don't fertilize unhealthy turf unless you know that low nitrogen is the problem.

  • When you do apply nitrogen, base your program on low rates of nitrogen, applied more frequently. In general, an application should stay below 0.5 pounds per 1,000 square feet for fairways and below 0.25 pounds per 1,000 square feet for greens. Frequent application of lower rates of N will allow you to more safely judge the need for more N by monitoring turf growth, color and clippings yield (a good rule of thumb is to target less than ½ basket per 5,000 square feet of cool-season greens using a walk-behind mower. In areas with high rainfall, slow release fertilizers applied at higher rates of nitrogen may be necessary.

  • Use of ammonium-based and slow-release fertilizers should be limited in locations that have problems with excess nitrogen.

  • Maintain soil organic matter levels below six percent for tees, fairways and roughs, and below two percent on greens.

  • Increase soil aeration to promote higher oxygen levels and improved nitrification in soil. This is particularly important before and during hot weather, and can help to prevent build-up of excessive ammonia in the soil.

  • Don't forget that low levels of soil nitrogen (less than 3 ppm) can also cause turf stress.

Wendy Gelernter, Ph. D., and Larry Stowell, Ph. D., are the research directors for the PACE Turfgrass Research Institute (San Diego, Calif.).

QUICK TEST FOR SOIL NITRATE

We have developed a quick, easy, on-site test that can detect excessive levels of soil nitrates. Although this test does not replace analytical testing, it provides values that are rough estimates of nitrate levels, and should be a useful and rapid diagnostic tool.

Materials needed:

Hach water test strips for nitrate/nitrite (Cat. # 27454-25, Hach Company, P.O. Box 389, Loveland, CO 80539; 800-227-4224; www.hach.com). A bottle of 25 strips is $15.50 as of April 2006.
Small beaker or container
Tablespoon
Soil from problem area and from nearby area of healthy turf
Notebook for recording data

Procedure:

  1. Mix equal volumes of soil and tap water (for example, 1 tablespoon of each) in a small container and stir thoroughly. Allow the soil to settle for 1 minute.

  2. Dip the test strip into the soil suspension.

  3. Allow the strip to develop for 1 minute.

  4. Lightly rinse the strip with water to remove soil.

  5. Compare the color on the tip of the strip to the nitrate/nitrogen color chart on the test strip container. Write down the ppm value that you think is closest to the color you see on the strip. This value must be converted to obtain the nitrate concentration (in parts per million) of your soil, using the formula below:
    (Dipstick value × 1.5) + 2.9 = Soil nitrate concentration (ppm)

    For example, assume that the color on your dipstick indicates roughly 15 ppm nitrate. Remember, the dipstick value needs to be converted to one that is accurate for soil nitrate! The soil nitrate concentration is therefore = (15 × 1.5) + 2.9 = 24.9.

  6. If the final value is over 20 ppm as it is in the example above, nitrogen applications of all types should be halted until levels are well below 20 ppm. If possible, leach the area to help bring nitrogen levels down more rapidly.

Keep in mind that the dipstick test has several limitations, including:

It does not test for ammonium levels — only for nitrate levels. Therefore, even if you have high ammonium levels in your soils, you will get a low reading from the dipstick.

The test is designed to detect an excess of nitrogen, but it will not detect nitrogen deficits. To determine whether your soils are lacking in nitrogen, the soils should be sent to an analytical laboratory.

The values you get with this test are rough estimates, at best. For more accurate readings, soils should be sent to an analytical lab for testing.

Source: The PACE Turfgrass Research Institute Web site at www.paceturf.org (click on “Turf Guidelines”).

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