Lawn Fertilization

3 September 2007

Virginia Cooperative Extension, Charlottesville/Albemarle County Office
460 Stagecoach Road, Charlottesville, Virginia 22902
phone: 434.872.4580   fax: 434.872.4578

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Fertilization of lawns is essential for the production of quality turf in Virginia. However, exceeding recommended fertilizer application rates or improper application timing can negatively impact surface water and groundwater quality and the health of the turf.

Proper timing of nitrogen applications is different for warm-season and cool-season turf grasses because of their different growth cycles. Excessive spring application of nitrogen to cool-season grasses in Virginia is detrimental because it leads to excessive leaf growth at the expense of stored food reserves and root growth. This increases the injury to lawns from summer disease and drought. Late summer and early fall applications of nitrogen to bermudagrass without applying adequate phosphorus and potassium can increase winter injury. The best time to fertilize cool-season grasses (what most of us are growing) in Virginia is from August 15 through November. Warm-season grasses perform best when fertilized between April 1 and August 15 in Virginia.

Soil tests taken every three or four years provide important information about the fertility of your lawn soil. The results will indicate the amounts of phosphorus, potassium, calcium, and magnesium your soil can provide to your turfgrass. It will also indicate the pH of your soil and thus whether lime is needed. A soil test may indicate you do not need to apply some nutrients or it will indicate the specific amounts of lime, phosphorus, and potassium your soil needs to provide adequate nutrition for your turfgrass. The nitrogen requirements of turfgrass cannot be reliably evaluated by a soil test. Therefore, the soil test report will not contain a nitrogen recommendation. Nitrogen applications on lawns in Virginia are best made following the different recommendations developed for cool-season grasses and warm-season grasses by Extension Turf Specialists at Virginia Tech. For more information on these recommendations call your local Extension office.

Fertilizers are often described by utilizing three numbers, such as 12-4-8 or 46-0-0. These three numbers indicate, respectively, the percent by weight of nitrogen (N), phosphate (P2O5), and potash (K2O) in the fertilizer and are required to be on every fertilizer bag or container. For example, a 12-4-8 fertilizer would contain 12% nitrogen, 4% phosphate, and 8% potash on a weight basis. Complete fertilizers contain nitrogen, phosphorus, and potassium. If soil tests indicate that high levels of phosphorus and potassium are available currently, then fertilizers supplying only nitrogen need be applied.

If a soil test indicates additional phosphate or potash is needed, it may be applied with a complete fertilizer or in separate applications from phosphate or potassium fertilizers. Fertilizers normally utilized to correct severe phosphorus and/or potassium deficiencies are 0-20-20, 0-28-0, 0-0-54, or 0-0-60. Never apply more than 3 lbs. of 0-0-54 or 0-0-60 per 1000 sq. ft to an established turf in hot weather without watering in the material to prevent foliar burn. Since water use is restricted in some areas this year this is an important consideration.

If the fertilizer analysis is 16-4-8, the fertilizer ratio is 4-1-2; similarly, a 14-7-14 analysis would have a 2-1-2 ratio. Mature lawns generally require more nitrogen than phosphorus and potassium; therefore, ratios of 4-1-2 or 4-1-3 are commonly recommended. Turf maintenance fertilizers vary in nitrogen content and may contain a portion of the nitrogen as water insoluble or slowly available nitrogen.

The source of nitrogen in fertilizers influences nitrogen availability and turf response. There are two categories of nitrogen sources; quickly available and slowly available. Quickly available materials are water-soluble, can be readily utilized by the plant, are susceptible to leaching and have a relatively short period of response. Quickly available sources include ammonium nitrate urea, ammonium sulfate, and calcium nitrate. Slowly available nitrogen sources release their nitrogen over extended periods of time and are applied less frequently and at somewhat higher rates than the quickly available nitrogen sources. Slowly available sources are less susceptible to leaching and are preferred on sandy soil types, which tend to leach. Slowly available sources include urea formaldehyde (UF), UF based products (methylene ureas), sulfur coated urea (SCU), IBDU, natural organics (bone meal, fishmeal, dried blood, and animal manures), and activated sewage sludge.

If a fertilizer contains a slow release nitrogen source it will be listed on the label. For UF based fertilizers the portion of the nitrogen that is slowly available is listed on the fertilizer bag as Water Insoluble Nitrogen (WIN). For instance, a 20-10-10 fertilizer with 5% WIN has 5/20 or 1/4 of the nitrogen in the slowly available form. If you chose a fertilizer that provides nitrogen in a slowly available form, you should understand how to calculate WIN in order to determine which fertilizer best fits your lawn. If WIN is not listed on the fertilizer label, one should assume it is all water-soluble or quickly available nitrogen, unless the fertilizer label indicates it contains sulfur-coated urea. Sulfur-coated urea fertilizers do provide slowly available nitrogen, but the fertilizer label does not list it as WIN. If the fertilizer contains sulfur-coated urea, include that portion as water-insoluble nitrogen when determining the amount of nitrogen that is slowly available.

Statements on a fertilizer bag such as "contains 50% organic fertilizer" do not mean the fertilizer is 50% slowly available. Calculation of WIN as noted above or determination of the amount of another slowly available nitrogen source is the only reliable method of determining the portion of the fertilizer that is slowly available.

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For more information about this and other landscape topics contact your local Virginia Cooperative Extension office. The local Virginia Cooperative Extension office numbers are Albemarle 872-4580, Fluvanna 591-1950, Greene 985-5236, Louisa 540-967-3422, and Nelson 263-4035.