Nitrogen and Phosphorus: fertilizing lawns to protect water quality – Turfgrass Science at Purdue University

Nitrogen and Phosphorus: fertilizing lawns to protect water quality

Healthy turf areas provide many environmental and recreational benefits. A properly fertilized turf has fewer weeds, is more resistant to insect feeding and is less disease prone than malnourished turf. Furthermore, a dense turf slow and decreases water runoff following storm events which helps reduce flooding in urban environments. Concern over water quality is growing over recent years recent years. While the exact causes are unclear, some of the problem is associated with nutrient enrichment. Increased nutrient loads stimulate excessive algae growth, which results in poor odor/taste, decreased clarity, and habitat loss and in the worst case, fish-kills. The exact source and contributions of excess nutrients is usually uncertain, complex, and situation dependent. However, traditional agricultural practices as well as urban sources such as septic system failure and lawn fertilization have all been suggested.  

Fertilizer Nutrients
Although turfgrasses require many nutrients to survive and thrive, nitrogen (N), phosphorus (P), and potassium (K) are applied in the greatest amount. Nitrogen promotes greenness and stimulates growth, phosphorus is necessary for the plant’s energy and reproductive systems, and potassium is used in many cellular processes. Most turf fertilizer programs are constructed around an annual N requirement which varies depending upon the owner’s desired appearance, turf vigor and resources available to manage growth. A mature cool-season (e.g. bluegrass or fescue) lawn will typically receive 2-4 pounds of actual N/1000 sq ft/yr, spaced over 2-5 applications. Individual fertilizer rates, sources and application timings are specifically applied to minimize growth flushes and maximize nutrient uptake. The target N rate for individual N applications is rarely more than 1 pound of actual N/1000 sq ft and most programs specify at least some slow release nutrient sources. For more information regarding lawn fertilization programs, refer to Purdue University’s publication: Fertilizing Established Lawns:AY-22

Focus on Phosphorus: How Does it Get into Surface Waters?
Among the “big-three” nutrients, P is required in smaller quantities than N or potassium, but its importance should not be overlooked. Deficiencies are rare in mature turf, but are a serious concern in newly established turf due to the lack of a root system. Thus “starter” fertilizers high in P are always recommended when establishing a new lawn from seed or sod. Once plants become established and develop a deep root system, annual P fertilization requirement decreases on soils that possess sufficient P. In unusual situations such as when overseeding, where the plant has suffered substantial root loss due to environmental stress like severe heat and drought, or during the spring when soils are cold and P uptake is limited, P fertilization may be necessary in order to promote healthy turf growth.  Realize though that P does not improve root growth as some would suggest, we provide it when the root systems are compromised only to ease the roots ability to find the tightly held P in the soil.  

The biggest risk for P loss from turf is during establishment or where mature turf density is low. Phosphorus readily binds with soil elements and may move with sediment as soil erodes during water runoff. Additionally, it is important to understand that P is naturally found in most soils, water and living organisms. Non-turf sources such as pollen or tree leaves can be a large source of P entering surface waters during spring or fall when they drop onto impervious surfaces such as roads and sidewalks and is carried into storm sewers during rainfall events. Additionally, the P level of domestic animal wastes or migratory birds like Canada geese is very high. Runoff losses from these sources may also significantly affect the P level in ponds and lakes.  

Choosing a Fertilizer Product 
Selecting a fertilizer product is often a confusing task, but understanding how to read the fertilizer label helps. As previously mentioned, most fertilizer programs are constructed around N applications to meet a specific need. To correctly apply the desired nutrients will require different amounts of individual fertilizer products based on the amount or percentage of nutrients in that specific product. Legally all fertilizers must display the percentage by weight of nitrogen, phosphate (P2O5) and potash (K2O) which is also called the analysis (e.g. 18-4-10). Depending upon your objective, different products should be selected and used. For example, if you are planting a new lawn from seed or sod then a “starter” fertilizer with a higher percentage P than N (e.g. 11-24-10) should be used to ensure proper root growth development. Only a soil test can determine your exact nutrient needs, however, a general recommendation is to apply 1.0-1.5 pounds of P2O5 per 1000 ft2 at planting and again four weeks later. 

If you turf is mature, “maintenance” fertilizers (e.g. 22-3-8) should be used because they supply mostly N and less P or potassium. When selecting individual products it is important to note that fertilizers with a high N content (e.g. 28-3-8) will cover more area than those with a low N content (e.g. 6-3-4) applied at the same target N rate. Additionally, to avoid excess P applications the % phosphate of individual fertilizers should be carefully evaluated. For example, although both previously mentioned product analyses contain the same % P, 3 %, nearly five times the P amount would be applied when selecting the 6-3-4 product and applying 1 pound of actual N per 1000 ft2. Fertilizers low in P are becoming readily available to homeowners and professionals.

Some communities have proposed limiting or even eliminating P-containing fertilizers. This might be acceptable on established lawns with adequate P in the soil, but does not make good agronomic sense on low P soils and/or newly established turf. Other communities have suggested only using natural organic fertilizer sources to limit P. Unfortunately, these fertilizers are byproducts of animal manures and usually contain significant percentages of P.  An alternative to the zero P ban is the concept of using fertilizer N to P ratios. For example, a 4:1 or 5:1 ratio would be appropriate for most mature turf areas. Thus, a 28-3-8 fertilizer product qualifies, whereas a 6-3-4 or 12-12-12 product would not. One tool that may help determine proper fertilizer product needs is Purdue’s fertilizer calculator:

Best Management Practices to Protect Urban Water Quality

  1. Do not apply unnecessary nutrients. The old adage: “Don’t guess, soil test” applies. The only way to truly know your soil nutrient status is to take a soil test. A list of regional accredited soil testing laboratories can be found at:
  2. Keep fertilizer on the turf. If granular fertilizer particles land on hardscape (e.g. sidewalks, driveways, patios etc.) sweep or use a landscape blower to move them back into the turf where they can be used by the plant.
  3. Never apply nutrients to frozen soil or dormant (e.g. brown) turf.
  4. Return lawn clippings back to the turf during mowing. This recycles nutrients like N and P back to the soil and can reduce annual fertilizer needs.
  5. Maintain a 5 to 10 foot vegetative buffer strip around all surface waters (ponds, lakes, streams, etc.). Avoid direct fertilizer applications to these areas.
  6. Keep excess lawn debris, tree leaves and twigs, lawn clippings, etc. out of gutters, storm sewers, streets, ditches and any surface waters.
  7. Pick up pet waste promptly. Pet and animal wastes contain nutrients as well as harmful bacteria.

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