Are Fertilizers Polluting Our Water Supply?

Jeff Higgins, Ph.D.
Director of Market Development
Pursell Technologies Inc. (PTI)
Sylacauga, Alabama

                 Every perceived threat inevitably finds a source for people to point fingers at and lay blame, and right now you may be faced with just such a situation.

                The issue is concern about fertilizer runoff in the water supply, and golf courses make an easy, high-profile target. In reality, golf courses in the U.S. represent a small percentage of the areas in question. Of approximately 30 million acres of turfgrass in American urban environments, golf courses comprise only about 1.3 million acres—far less than residential lawns or parks and sports facilities.

                However, golf courses are often faced with more scrutiny because the public believes they cause water pollution with their use of fertilizers. Nitrogen receives the most attention because it’s the most widely used nutrient on turfgrass, and also the most difficult to manage due to its many biological and chemical reactions. 

                One form of nitrogen that is applied and/or produced from these reactions is nitrate (NO₃^-), which is very mobile and prone to leaching.  Surface water contamination with nitrate nitrogen has been responsible for eutrophication (excess nutrients in the water stimulating aquatic plant growth and upsetting the oxygen balance), and nitrate in drinking water has been linked to “Blue Baby” syndrome, in which infants have problems extracting oxygen from their blood.

                But are golf courses at fault? Usage data from the Golf Course Superintendents Association of America (GCSAA) indicate the average annual expenditure for fertilizers has gone from $17,588 in 1994 to $22,858 in 1999.  This could show that fertilizer usage is increasing, or could simply mean fertilizer costs have risen.

                That data also provided information on which types of fertilizer are being used on golf courses, with fast-release nutrient sources accounting for roughly 28% of the total (Figure 1).  

 Figure 1.  Nutrient Sources Utilized on Golf Courses

                Unfortunately, we have limited data evaluating nitrogen sources and what happens to them after application. There are two types of nitrogen: organic and inorganic.  In soil, ammonium (NH₄⁺), nitrate (NO₃^-), and nitrite (NO₂) are the most important compounds, and originate either from aerobic decomposition of organic matter or from the addition of fertilizers.  Inorganic nitrogen (ammonium and nitrate) is the only forms utilized by turfgrass.  No matter what type of nitrogen source(s) applied, it must be converted to one of these two forms for turfgrass utilization. 

                Once nitrogen has been applied it can take several paths.  Ideally, it will be taken up by turfgrass (utilization), but nitrogen can also be bound to the soil, immobilized by soil microbes, lost to the atmosphere (volatilization and denitrification), lost to surface waters (run-off), or lost to the groundwater (leaching).            

                Given so many variables, it is important for superintendents to understand nitrogen sources, including the transformations in the soil that determine their fate (Figure 2).

 Figure 2.  Fate of Nitrogen Applied to Turfgrass.

 Turfgrass Utilization and Clipping Removal

 Turfgrasses can absorb both forms of inorganic nitrogen from the soil solution. This uptake is termed utilization.  If turfgrass clippings are removed during mowing, inorganic nitrogen will be lost.

 Mineralization 

Mineralization is the process where organic matter, organic fertilizers, and reacted synthetic organic nitrogen fertilizers (methylene ureas, urea formaldehydes) are broken down by soil microorganisms to provide available ammonium and nitrate.

 Leaching 

Leaching occurs when nitrate solubilized in the soil solution moves with the water flow down through the soil.  Unless turfgrass absorbs this form of nitrogen, it will be lost through leaching if excess moisture is applied or available in the soil pores.

Denitrification

Denitrification is the conversion of nitrate under anaerobic conditions to gaseous nitrogen, which is lost to the atmosphere. Applied nitrate fertilizers can be lost at the rate of 10 to 30% within 2 to 3 days through denitrification in soils that are compacted, waterlogged, or have high pH (pH > 7.5).

 Volatilization

The conversion of ammonium to ammonia gas (NH₃) which escapes to the atmosphere is called volatilization. This occurs in high pH soils (pH > 7.5) under hot, windy conditions.  Volatilization can usually be avoided by watering after fertilizer application.

 Understanding Nitrogen Fertilizer Sources

                Often, superintendents and managers measure nitrogen needs based on turfgrass response.  However, it is frequently the effects of fertilizers on other  aspects of a turfgrass program that influence its success or failure.

                Turfgrass response from nitrogen fertilizers is affected by application rate and the release mechanism of the nitrogen source (availability).  Therefore, response time can be altered from as short as a few weeks to as long as several months by manipulating these parameters. 

                The different release mechanisms for fertilizers include hydrolysis (by water), microbial, and temperature-dependent.  Table 1 lists the releases associated with various nitrogen fertilizers. 

 Table 1.  Release mechanism for various nitrogen fertilizers.

 Soluble Nitrogen Sources

                Soluble nitrogen sources are characterized by fast release of nitrogen within two days after application, with turfgrass response peaking in seven to ten days.  Nitrogen in these soluble sources is in either ammonium or nitrate form.  The ammonium is prone to volatilization, with nitrate more prone to leaching.  Examples of soluble sources are urea, ammonium nitrate, ammonium sulfate, and potassium nitrate. 

                Urea is among the most widely used nitrogen sources due to its low cost.  Once applied, urea is broken down into ammonium, which can then be absorbed by turfgrass or converted to nitrate via nitrification.  Nitrogen loss can occur through volatilization or leaching.  Irrigating immediately after application reduces volatility loss.

Reacted Synthetic Organic Nitrogen Sources

                This group of slow-release fertilizers is sometimes called urea formaldehyde (UF). UF products are comparable to soluble sources in terms of nitrogen use efficiency.  However, under conditions favorable for leaching and/or volatilization, UF products may be more efficient. 

 Isobutylidene Diurea (IBDU)

                IBDU is considered a reacted synthetic organic product, but its release mechanism is quite different.  In the presence of water, IBDU is hydrolyzed to urea, with the release of urea controlled by soil moisture and particle size. Nitrogen loss from IBDU is generally leaching and ammonia volatility. 

 Natural Organic Nitrogen Sources

                Natural organic nitrogen sources are various composted or waste materials.  Organic nitrogen sources are converted first to ammonium and then to nitrate via mineralization.  Nitrogen loss with natural organic sources is very low due to their limited water-soluble nitrogen and the fact nitrogen is made available so slowly through microbial breakdown. 

 Sulfur Coated Urea Nitrogen Sources

                Sulfur coated urea (SCU) is urea granules with a thin coating of sulfur to protect the surface from microbial breakdown and decrease the initial rate of urea release. Once water is absorbed into SCU granules, the urea is released within 48 hours; however, the nitrogen release rates are affected by non-uniformity in the sulfur coating. Approximately 20% of SCU granules are “locked off,” meaning they’re coated too thick and can’t release nitrogen.

 Polymer Coated Nitrogen Sources

                A relatively new technology for controlling the release of nitrogen is polymer coating.  This process involves coating a soluble nitrogen source—such as urea, ammonium sulfate, or potassium nitrate—with a polymer.  These polymer-coated sources release nitrogen via osmosis.  Water is absorbed across the polymer membrane and once inside, forms a highly concentrated nitrogen solution. This solution then diffuses across the polymer membrane, releasing nitrogen to the turfgrass.  This diffusion continues until 100% of the nitrogen has been released. 

                Since the release of nitrogen from these polymer-coated products is strictly osmotic, temperature is the only variable that affects their nitrogen release.  Nitrogen release can vary from as short as 10 weeks to as long as 12 months.  The longevity of release is controlled by the thickness of the polymer coating.  The nitrogen substrate that is polymer coated and its water solubility determine which form of nitrogen is made available.

                This unique mode of action should be preferable if you want to diminish or remove many risks associated with other types of fertilizer. By giving you more control over how the nitrogen is released into the turfgrass, you lessen the possibility of fertilizer becoming a pollutant in surface water or groundwater.

 Best Management Practices  (BMPs)

Always read and follow label recommendations

Use controlled release sources

Always calibrate application equipment prior to use

Calibrate and fill spreader on hard surface to allow collection if a spill occurs

Never apply fertilizers directly to water

Use drop spreaders around water bodies

Never apply fertilizer to hard surfaces such as cart paths, parking lots

Irrigate lightly after fertilizer application

Use soil and tissue testing

Establish and maintain buffer strips and setbacks around water bodies

Prevent organic matter (clippings, leaves) from reaching water bodies

Use proper size fertilizer granules

Initiate a water testing program

Maintain accurate, timely records

Always apply fertilizers during active turfgrass growing conditions

Apply fertilizers when soil moisture is not at field capacity

                Many golf course superintendents do most of their fertility planning based on schedules or visual appearance of the turfgrass, both of which can lead to over-fertilization!

                The concern about fertilizer contamination is very real, and several states have legislation pending which could ultimately affect you. In some areas, golf course development is already being delayed or restricted, often in response to public environmental fears.

                Developing a good understanding of nitrogen fertilizer sources—combined with a thorough knowledge of the specific golf course and the use of fertilization BMPs—can help minimize or eliminate adverse impacts on water quality.

                And that’s a good course for everybody.