Developing a fertility program is one of the most important and complex decisions a turf manager will have to make. While it would be convenient to have a generic program that could be applied to all situations, there is no one program which will produce excellent turf under all conditions. Fertilizer needs vary according to existing soil nutrient levels, species conditions, level of quality desired and many other factors. Thus, rather than providing a "cookbook" recipe for success, it is more practical to discuss the components of a sound fertility program and then adapt those principles to a particular situation. The four main variables in any fertility program are: nutrient to be applied, frequency of application, rate of application and timing of applications throughout the season.

Soil Testing:

The addition of fertilizer and liming materials to a lawn should be based on a soil test. A soil analysis for plant nutrients, texture and pH is especially important at turf establishment or renovation. When working with an established, healthy turf, a pH test will generally be sufficient.

The following guidelines describe how to sample a turf area for accurate soil analysis:

1. A separate sample should be submitted for each distinct area of the lawn where soil properties may differ. Poorly drained areas, previous gardens, etc. should be sampled separately. An average lawn will require only one or two samples.

2. Each sample submitted should be a composite of several smaller samples taken within the area. Six to ten cores taken at a depth of 6 inches (the root zone) will usually provide a good sample of the area. A half-pint sample will be sufficient. Thatch and other debris should be removed from the sample prior to submission.

3. Contact the University of Rhode Island Cooperative Extension for information on how to submit soil for a soil test, or see GreenShare Factsheet on soil testing for an order form in PDF format.The report for your soil will be sent to you by mail, along with instructions as to what nutrients to apply and at what rate. Be sure to specify whether the test is for existing turf or for an area to be established at a later date.

Nutrients:

Of the 16 essential mineral elements necessary for growth, nitrogen (N), potassium (K), and phosphorous (P) are the principal nutrients applied in most fertility programs. Fertilizers which contain N, P and K are considered to be "complete fertilizers." Turfgrasses require N, P and K in a ratio of approximately 4-1-2. When fertilizing existing turf, it is a good idea to use a fertilizer which provides N, P and K in roughly this ratio, rather than a fertilizer which provides more P than K, unless specified by soil test results. Nitrogen is the backbone of a fertility program and is applied in the greatest amount because of its many effects on turfgrass growth. Adequate nitrogen maintains a dark green color, high shoot density and the ability to recuperate from stress or pest injury. Excessive N, however, can increase disease problems, reduce tolerance to high and low temperature and result in moisture stress due to increased growth exceeding available moisture. Thus it is important to provide N at a frequency and rate which will assure vigorous growth without overstimulation.

A source of N can be chosen which is either quickly available to the plant [water soluble nitrogen (WSN)] or slowly available to the plant over a period of months or even years [water insoluble nitrogen (WIN)]. Both types of N have advantages and disadvantages. WSN comprises the majority of N applied by turfgrass managers (Table 1). Of these fertilizers listed, urea is the most commonly used source of N in most complete fertilizers. Ammonium nitrate, ammonium sulfate and potassium nitrate have a higher salt index and are more likely to burn turf than urea. Mono- and diammonium phosphate are used in fertilizers when P is also desired.

WSN provides a rapid increase in both color and growth rate. It can be applied in either granular or liquid form and is less expensive than the WIN sources. Because the release of N from WSN sources is relatively independent of temperature, WSN can be used throughout the season with acceptable response.

Compared to more insoluble N sources (WIN), some disadvantages of WSN include: high foliar burn potential when applied at excessive rates or during periods of high temperature, relatively short period of plant response (six to eight weeks at normal rates) and greater potential for loss via leaching or volatility (gaseous losses).

Water insoluble nitrogen (WIN) can also be incorporated into a fertility program. WIN releases its N slowly over a longer period of time than WSN. Because of their ability to meter out N slowly over a long period of time, slow release N sources have a low potential for foliar burn. They also provide a longer residual plant response than WSN and do not result in flushes of rapid growth. WIN is less reactive than WSN so that less is lost via leaching and gaseous losses.

Potential disadvantages associated with WIN include: slow color response, high cost per unit of N and potential carryover of N into the following growing season. Carryover of N may or may not be a problem, depending on the situation. In a low-maintenance situation where the frequency of fertilizer applications is low, carryover of nitrogen would be beneficial. Nitrogen would be available for a longer period to supply nutrients to the slowly growing turf. For homeowners, inclusion of some WIN in a fertilizer is desirable, especially on sandy soils prone to leaching.

Typical slow release N sources include: ureaformaldehyde products (UF), isobutylidene diurea (IBDU), sulfur coated urea (SCU), and products derived from natural organic materials such as activated sewage sludge, seaweed and other plant and animal residues (Table 2). Ureaformaldehyde (UF) fertilizers (38% N) depend upon microbial activity to release N from complex organic polymers (long chain compounds). Thus, factors which favor microbial activity also favor N release. Soil temperatures higher than 55 0F, adequate moisture, adequate aeration and pH between 6.0 and 7.0 allow UF products to work properly.

Sulfur Coated Urea (SCU, 32-36% N) and plastic or resin-coated urea products release N slowly because the urea pellet (prill) is covered with a coating of sulfur, plastic or resin. N leaks through the pores at a slow rate compared to uncoated urea. Prills which have an incomplete or cracked coating behave like WSN. Thinly-coated prills release N more rapidly than thickly-coated prills. Adequate moisture and warm soil temperatures (warmer than 550F) favor release of N from SCU and other coated urea fertilizer products. Isobutylidene diurea (IBDU, 38% N) releases N as a result of very slow solubility in water. The physical process is essentially similar to dissolving sugar or some other soluble product, only at a much reduced rate. Finer particle size products are available for use on low-cut areas or where a more rapid response is desired. Because they rely upon moisture for release, IBDU is not a good choice for non-irrigated turf areas. It is not commonly found as the nitrogen component in most complete fertilizers. Because its release is not affected by temperature, it is a good choice for early spring or late fall fertilization when adequate natural rainfall is usually plentiful.

Natural organic fertilizers vary in composition depending on source of nitrogen used. The most common natural organic products are derived from sewage sludge by dewatering, aerobic digestion, composting and/or further processing or some combination of these techniques. Other natural organic fertilizers include: seed meal (7% N), fish meal (7-12% N), pelletized sewage sludge (3-6% N), dried blood (13% N) and various other composted materials. Because both WSN and WIN have desirable attributes, it is usually a good idea to use a fertilizer which contains some of each type. Many commercially available fertilizers contain both WSN and WIN. The ratio of each in the product vary according to release rate desired, price and other factors. The percentage of each type in a fertilizer material is listed on the fertilizer label.

Rate of Application:

Once an N fertilizer has been chosen, one must decide the rate to apply the product. Fertilization rate depends upon many factors such as: N source to be applied, time of year and fertility requirement of the species and cultivars present. When using solely WSN, application rates of 0.75 to 1.5 lbs. N per 1000 sq. feet per application are recommended to avoid undesirable growth surges and potential foliar burn. WIN sources may be applied at rates as high as 3 lbs. N/1000 sq. feet per application without fear of burning the turf (we suggest going no higher than 2 lbs. N/1000 sq. feet). Fertilization during July and August should be approached cautiously to avoid excess growth during periods of high temperature and moisture stress. The proportion of WIN in a fertilizer is often increased when approaching the summer or during the summer to protect against foliar burn.

Timing of Application:

Fertilization programs utilizing only WSN require more frequent applications to maintain acceptable turf than programs which incorporate some WIN. Fertilization programs can include up to five applications to a lawn per season with three or four of those applications containing N fertilizer. The timing of those fertilizer applications during the season is an important consideration.

Many agronomists consider late summer (August/early September) and late fall (late November, when the turf has gone dormant) fertilizations to be the most important for cool-season grasses. Recovery from summer stress injury, as well as increased tillering and rhizome production, occur during the fall. N must be provided to maximize these processes. The late fall application should be made after the last mowing when turf has stopped growing, but before it has lost its green color. An application at this time enhances winter color and provides spring greenup three to four weeks earlier than programs without a late-season application. This concept has also been shown to improve Kentucky bluegrass rooting the following spring. While late season fertilization is increasing in popularity, it must be approached cautiously. In areas which may be environmentally sensitive, late-season fertilizing should be avoided.

An N application during late spring is also widely practiced in order to enhance color going into summer and to encourage growth before the high temperatures and moisture stress of summer occur. It is important that this application does not stimulate the turf to rapid growth during the stressful summer months (especially if irrigation is not available). Fertilizer applied at this time should contain a high percentage of slow-release or water-insoluble nitrogen. Many programs also employ an early spring (April) application of N. This application is used primarily to enhance greenup at the time when preemergent weed materials are being applied. If a late-season application is used, the N rate during early spring can be reduced substantially, since color will be markedly enhanced from the late season application. Iron applied without nitrogen at this time of year also enhances color development without stimulating growth.

Phosphorous & Potassium:

While N is the main component of all fertility programs, several other elements also bear mention. Potassium (K) is needed by turf in amounts second only to N. Adequate K fertility improves wear tolerance, heat and cold tolerance, stolon and rhizome growth and rooting (improving water uptake). While every fertilizer application may not include K, those applications preceding stress periods are good times to supplement K. The early fall application, in particular, is often made with a fertilizer containing N and K in a 1:1 ratio to improve winter survival without overstimulating growth.

Phosphorus (P) improves both rooting and winter hardiness of turf. It is essential when establishing new seedings of turf. Adequate P in the seedbed helps to ensure rapid establishment. On mature turf, where P is rarely found to be deficient, there is little value to applying P unless a soil test shows it to be deficient. P is strongly bound to the soil and does not easily move downward. It is a good idea to apply P in conjunction with some process which will facilitate its incorporation into the soil (aeration, dethatching, etc.).

Another element which can be incorporated into a fertility program is iron (Fe). Iron, which functions in chlorophyll synthesis, provides a rapid increase in green color when growth stimulation associated with N is undesirable. Early spring or summer applications of Fe are becoming increasingly popular. Iron is very strongly absorbed to the soil, making it often unavailable to the plant for use. In order to increase plant availability, iron is applied as a liquid rather than as a granular product. "Chelated" iron is often chosen as a source of Fe. A chelating agent is a soluble organic compound that binds with Fe to make it more available to the plant. Intelligent use of Fe can result in a healthier plant and reduced need for N.

Lawn Fertilization Programs:

A typical fertilization program results in the application of about 2 to 4 lbs. N per 1000 sq. feet per season. The main objective is to produce acceptable quality turf. If you are satisfied and the turf is healthy at a rate lower than 3 lbs. N per 1000 sq. feet per season, reducing the rate is justified. A material containing both WSN and WIN provides some of the advantages of WSN (quick greenup, release independent of temperature and lower cost than WIN) while incorporating some advantages if WIN (lower burn potential, less leaching and less frequent application than WSN). There are many factors which must be taken into consideration when formulating a fertility program. A basic knowledge of plant nutrition, fertilizers and soils, as well as a willingness to experiment with your current program, are keys to finding the program best suited to your situation.

Adapted from the University of Massachusetts Extension, 2000