FALL FERTILIZATION: Fall Fertilization Facts

Throughout the cool-humid region and upper transition zone of the United States, cool-season turf species like the fescues, bluegrasses and ryegrasses are the primary turfgrasses planted for lawns. These grasses are favored over warm-season species like zoysiagrass and Bermudagrass because they remain green for most of the year and rarely go completely dormant (e.g. turn straw-brown) during the winter months.

Maintaining a persistent, high quality, and aesthetically pleasing cool-season lawn requires properly timed fertilizations that maximize and maintain green color, shoot density and promote consistent growth, all of which create dense turf areas that are less prone to weed invasion and can better withstand heat, drought, traffic and pest stresses. Exact fertilizer needs for individual turf areas will vary by site and the prevailing weather conditions. Additionally, specific fertilization programs need to be periodically adjusted to apply slightly more or less nutrients depending on the following factors:

• Desired appearance: A dark green, dense lawn will require more fertilizer than a thin, lighter colored lawn. Remember, any time more fertilizer is applied, more mowing and irrigation may also be required. Choosing naturally dark green turfgrass cultivars may be one solution for reducing nutrient applications while ensuring a dark green turf. These cultivars can be identified in the National Turfgrass Evaluation Program turfgrass species database at www.ntep.org.
• Turfgrass species: Compared to Kentucky bluegrass and perennial ryegrass, deeper rooted species like turf-type tall fescue may perform adequately with lower annual fertileizer because they are able to extract nutrients from greater soil depths.
• Turf maturity and existing health: Newly planted lawns, both seeded and sodded, usually require 25 to 50 percent more annual fertilizer for the first few years to encourage rapid turf coverage and deep rooting. In addition many turf areas are established on disturbed urban soils which lack sufficient readily available nutrients. The same principle holds true for neglected or thin lawns that are being renovated with increased fertilization on a temporary basis.
• Geographic location: The growing season in the upper transition zone is longer than in Northern U.S. states. Therefore, slightly more fertilizer may be needed to sustain similar turf quality in more Southern U.S. states.
• Growing environment: Turf in moderate to heavy shade will require approximately 50 percent less annual nitrogen than turf grown in full sun with irrigation. Shaded turf simply grows slower and should not be heavily fertilized to minimize disease incidence and ensure turf persistence.
• Soil type: Turf grown on high sand content soils, like those found near the ocean, the Great Lakes or heavy clay soils, will require more fertilizer than turf grown on silt loams or organic soils. Sandy soils are prone to nutrient leaching losses and many heavy clay soils sometimes bind and complex nutrients making them more slowly available.
• Weather and irrigation: Readily available moisture stimulates shoot growth and frequent rain or irrigation can flush nutrients from the soil. Thus, more frequent fertilization will be required in wet years or irrigated sites than where the soil is typically dry.
• Mowing height and clipping management: Higher mowed turf, more than 3 inches, may require less frequent fertilization because the more extensive and deeper root system is able to extract more nutrients from the soil. Where clippings are regularly removed, 25 to 50 percent more annual fertilizer will be required to sustain turf quality.
• Traffic and use: Turf areas receiving heavy use will require more annual fertilizer to maintain shoot density, promote growth and recover from damage.

All of the aforementioned factors are important considerations when developing a fertilizer program. Keep in mind, however, that the overall goal of any turfgrass fertilizer program should be sustainability – in other words, implementing a program that maximizes turf health, balances aesthetic needs (color, density, etc.), minimizes mowing requirements and limits nutrient losses.

WHICH NUTRIENTS DOES TURF NEED? Turfgrasses require 16 essential elements to maintain proper health. These nutrients are often classified as macro and micro nutrients. Just because a nutrient is classified as a macro nutrient, do not assume that this nutrient is more important than a micronutrient; macronutrients are simply required by the plant in greater amounts. For micronutrients like iron or copper, very small deficiencies can result in poor turf appearance and slow growth. Whenever the topic of lawn fertilization arises the discussion usually revolves around three major plant nutrients – nitrogen (N), phosphorous (P) and potassium (K), contained in a complete fertilizer. In reality, however, most fertilizer programs are constructed around an annual N requirement. This is because N is the most abundant nutrient in the plant tissue, normally 3 to 5 percent by dry weight and elicits the strongest growth response. Nitrogen helps produce chlorophyll, the plant pigment that confers the green appearance to leaf tissue. Generally, the higher the tissue N content, the more green the leaves appear. Turf with a high leaf tissue N content will require more N to sustain that level of greenness.

In general, a mature, high quality turf will typically require three or more annual N applications totaling 2 to 5 pounds of actual N per 1000 square feet to maintain greenness and density. For cool-season grasses the bulk (50 to 75 percent) of the annual N needs should be applied using several applications, the first occurring in late-summer (first week of September) and continuing once or twice more through the autumn months. Nitrogen applications during this period maximize turf health while minimizing unwanted growth surges. It works because during the autumn months cool-season turfgrass shoot growth gradually slows and the plant is able to store carbohydrates made during photosynthesis. This enables the plant to better survive winters and results in faster spring green-up. The remaining 25 percent of annual N needs should be applied during the middle to late spring months (mid-May-early June). This application helps maintain green color and shoot density into summer. Little to no N should be applied during the summer months to minimize disease incidence and prevent possible damage where turf may be under heat and/or drought stress.

A wide variety of N sources are commercially available to supply turfgrass N needs. Traditionally, N sources have been divided into two broad categories, quick release or readily water soluble and slow release or water insoluble. A good N fertilizer program will utilize both sources and various quick and slow mixtures at different points during the growing season. Quick-release N sources like urea or ammonium sulfate dissolve easily in the presence of water and are capable of greening turf in a matter of hours. These N sources are relatively inexpensive but are also short lived (a few weeks) and produce unpredictable growth flushes. The traditional rule of thumb for quick-release N sources has been that you should never apply more than 1 pound of N per 1,000 square feet. This guideline was established long ago to minimize burn potential, avoid significant growth surges and minimize environmental losses. With this in mind, quick-release fertilizers by themselves are best used at reduced rates, applied frequently or as a smaller part of a fertilizer blend. When used alone at higher rates, these sources work best when applied during the late part of the growing season, just prior to winter dormancy. Applied at this time, most of the N taken up by the plant is partitioned into carbohydrate storage where it can be used by the plant as a slower release N source during spring regrowth.

The most commonly used slow-release N sources for lawns include sulfur-coated urea (SCU), polymer-coated urea (PCU), methylene ureas, and the natural organics (e.g. manures, etc.). Slow-release N sources require more than just water to release their N. Several mechanisms like protective coatings and microbial decomposition all control how quickly the N is released. Slow-release N sources can be applied at higher N application rates – 2 pounds of N per 1,000 square feet without significant risk of foliar burn or environmental losses. Unlike quick-release sources, they do not cause rapid greening but provide extended feeding, often for two to three months or more. This sometimes makes them more economical than the quick-release sources because they do not need to be applied as frequently, reducing overall labor costs.

OTHER NUTRIENT NEEDS. With the exception of nitrogen, the old adage: “Don’t guess, soil test” is appropriate to reliably determining appropriate nutrient needs. This is especially true for nutrients used in large quantities like phosphorus and potassium. In addition, there is growing public concern regarding nitrogen and phosphorus losses to the environment and their potential effects on drinking water quality. In fact, several large cities in the United States have limited or banned phosphorus use on turf areas because of its potential role in declining water quality. Research, however, shows that when applied correctly, lawn fertilizers remain where they are applied. The root system of an actively growing healthy turf is very efficient in taking up nutrients and few, if any, nutrients actually move off site. Improperly timed (e.g. dormant turf) or excess nitrogen, however, can leach into groundwater and both nitrogen and phosphorus may contaminate surface waters via runoff from low density turf under certain conditions. Completely eliminating phosphorus from many fertilizer programs to protect water quality is a double-edged sword since a poorly established, low density turf due to a phosphorus deficiency may exaggerate sediment and possibly other nutrient losses.

The importance of phosphorus to plant health is well documented. Adequate soil phosphorus is essential for newly planted turf, especially when planted on many urban soils. These soils typically comprise of construction spoils or “fill” which consist of severely disturbed soil profiles where the original topsoil has either been scraped off or buried. The new surface soil where the turf is being planted most often possesses an inherently low overall nutrient status. Thus, the most accurate way to determine nutrient needs for these soils is to submit a soil test for analysis. It is not uncommon for some soils to require 3 or 4 pounds of phosphate (P2O5) prior to planting. Your local county cooperative extension office will have recommendations for appropriate soil testing labs in your area. For new establishment either from seed or sod, sufficient phosphorus levels are critical for rapid turf cover and maximum root development. If soil phosphorus levels are deficient, seedlings will establish slowly, the stand will be thin and prone to weed invasion and the soil may easily erode due to poor turf cover. For new establishments, a general rule of thumb is incorporate approximately 1.5 pounds of P2O5 per 1,000 square feet into the top ½ inch of soil prior to seeding or sodding. Smaller surface applications may also be required throughout establishment – about the first year or two.

For established turfgrasses, in lieu of a soil test, a general recommendation is to apply one-fourth as much phosphorus and one-half as much potassium as nitrogen. For instance, if your turf requires 4 pounds of N per 1,000 square feet annually, you should also supply 1 pound P2O5 and 2 pounds K2O. Single application rates should be similar to N – 0.5 to 1.5 pounds per 1,000 square feet are appropriate. Grass clipping return or removal also affects P and K needs. Where clippings are returned or mulched back into the turf, 0.5 pounds of P2O5 will be required to maintain soil P levels. Where clippings are removed or the soil is severely deficient, approximately 1.0 pound of P2O5 should be supplied annually to replace removed phosphorus or until the soil reaches a sufficient status. Like N, K is subject to leaching losses and may need to be applied more regularly on sandy soils, during wet years or on regularly irrigated sites. Where soil phosphorus levels are sufficient, fertilizer products without phosphorus like an 18-0-18 should be used to supply potassium and avoid unnecessary phosphorus fertilization and potential environmental loss.

CALCIUM & MICRONUTRIENTS. Both lime and gypsum products are used to supply calcium to turf. Soil color may be a good indicator of whether or not these products are necessary. If the soil has a reddish color, you are most likely located east of the Appalachian Mountains where the soils are highly weathered and in general possess an acidic (less than 7) pH. Turfgrasses generally prefer a soil pH of about 6.5, and if the soil is seriously acidic (less than 5.5) it may be necessary to incorporate lime into your fertilization program to elevate the pH and ensure maximum nutrient availability. Lime will help increase soil pH but gypsum will not. If a soil test shows that magnesium is also deficient, dolomitic limestone, which has both calcium and magnesium, can be used. As a general rule of thumb, no more than 100 pounds of limestone should be applied per 1,000 square feet in any single application. It is best to incorporate slowly soluble nutrients like calcium or phosphorus directly into the soil by the roots prior to planting. If the turf is established, core cultivation prior to application may help speed movement into the rootzone.

The author is an assistant professor of agronomy and turfgrass science at Purdue University, West Lafayette, Ind., and he can be reached at 765/494-4692 or cbigelow@purdue.edu.