Turfgrass Fertilization: A Basic Guide for Professional Turfgrass Managers

Introduction

Dollar for dollar, fertilization does more to improve poor-quality turfgrass or maintain good-quality turfgrass than any other management practice. Proper fertilization practices produce a dense, medium- to dark-green turf that resists pests and environmental stresses.

Careless application techniques or excessive amounts of fertilizer applied at the wrong time of year can result in serious turf damage and contamination of water resources. Successful turf maintenance fertilization requires that you assess your turf's nutritional requirements, understand fertilizers, know how much to apply and when, and use proper application techniques.

Nutrient Requirements of Turfgrasses

Turfgrasses require at least 16 nutrients for normal growth and development. Some nutrients are needed in large amounts, other nutrients only in minute quantities. Regardless of the amount required, a deficiency of any of these nutrients will limit the growth and development of your turf. Thus, a calcium deficiency can be just as detrimental to the plant as a lack of nitrogen, even though turfgrasses use more nitrogen than calcium.

Nine of the 16 required nutrients are needed in much larger quantities than the other seven. These nine nutrients—carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur—are called macronutrients. Carbon, hydrogen, and oxygen make up about 90 to 95 percent of the plant's dry weight. They are never deficient in turfgrasses because they are derived from carbon dioxide (CO₂) and water (H₂O).

Nitrogen, phosphorus, and potassium are referred to as primary nutrients and are supplied periodically to turf through fertilizer applications. Calcium, magnesium, and sulfur, the secondary nutrients, are needed only occasionally in the form of fertilizer or lime.

The micronutrients iron, manganese, zinc, boron, copper, molybdenum, and chlorine are required only in minute amounts and are rarely supplied to turfgrasses through fertilization. Exceptions are if turfgrasses are planted in soils high in sand content, such as golf course putting greens, or if iron applications are used to provide a darker green turf without stimulating excessive foliar growth.

Table 1 lists the 16 essential nutrients, the form in which grass plants are able to use them, and the approximate amounts of each found in healthy turf. The exact amounts of needed nutrients vary among species, cultivars, soil moisture contents, and time of year; hence, the amounts listed under "sufficiency range" should not be used as indicators of deficiencies or excessive concentrations.

Table 1. The essential turfgrass nutrients.

*Bold type indicates the form more commonly available to turfgrasses.

**Sufficiency ranges are expressed as percentages or parts per million (ppm) on a dry weight basis. Values were obtained from publications by J. B. Jones, "Turf Analysis," Golf Course Management, 48, no. 1 (1980): 29-32; H. Marschner, Mineral Nutrition of Higher Plants (New York: Academic Press,1995); and E. Epstein, Mineral Nutrition of Plants: Principles and Perspectives (New York: John Wiley, 1972). In some cases, ranges are based on general observations and not necessarily applicable to all turfgrasses or every growing condition or management situation.

Each of the 16 essential nutrients has specific roles or functions in turfgrass plants. Some nutrients, such as nitrogen and phosphorus, affect many important plant functions, whereas others may only activate a few chemical reactions. Regardless of how large a role each nutrient plays, all are needed for the plant to develop normally. The primary functions of the essential mineral nutrients are presented in Table 2.

Deficiencies of nutrients in turfgrass plants can be expressed in numerous ways. The most obvious is a reddening or yellowing of leaf tissue. Deficiencies can also appear as a thinning of the stand, stunted growth, and increased susceptibility to disease. Ideally, fertilizer should be applied before deficiencies occur. The best way to assess nutritional requirements of turf is through soil testing, tissue testing, or both.

Table 2. Functions of the essential mineral nutrients.*

*Based on information from H. Marschner, Mineral Nutrition of Higher Plants (New York: Academic Press, 1995); E. Epstein, Mineral Nutrition of Plants: Principles and Perspectives (New York: John Wiley, 1972); and F. Salisbury and C. Ross, Plant Physiology, 2nd ed., (Belmont, CA: Wadsworth Publishing,1978).

Soil and Tissue Testing

Soil Testing

Soil testing is an important first step in developing a turfgrass fertility program. For some nutrients, it is the only way you can accurately determine how much fertilizer your turf needs. Most land-grant universities and many commercial laboratories provide soil testing services, although prices and services vary among labs. For a nominal fee, Penn State's Agricultural Analytical Services Lab offers a standard soil test for phosphorus, potassium, and lime requirements. More comprehensive soil analyses are available upon request.

A soil test program involves sampling, laboratory analysis, interpretation, and recommendations. The results obtained from a soil test are only as good as the sample submitted. Sampling directions vary from lab to lab, so follow instructions on the test kit carefully. Instructions should tell you how many subsamples are required per test, the sampling pattern, the sampling depth, and whether thatch should be included in the sample.

Figure 1. Soil sampling pattern for turfgrass areas. Take soil samples about 4 inches in depth from 12 or more location in an area with relatively uniform soil characteristics and management practices.

Penn State's Agricultural Analytical Services Lab sampling instructions suggest collecting 12 or more subsamples per location in a regular grid pattern (Figure 1). If the site varies in soil type, previous lime or fertilizer treatment, or other past maintenance practices, take separate samples accordingly. Test kit instructions suggest sampling soil 3 to 4 inches in depth and discarding thatch. Mix all subsamples together to make one sample, then take about 1/3 pint of this mix and place it in the mailing kit (Figure 2). Be careful not to contaminate the sample with lime or fertilizer during sampling and mixing.

Figure 2. Penn State soil test sampling instructions suggest mixing all subsamples together to make on sample.

Typically, soil tests should be taken every two or three years. If you wish to monitor nutrient levels over many years, take the samples at about the same time of year every time you sample. Always test the soil before establishing or renovating a turfgrass area.

Soil test labs vary in how they analyze soil and interpret test results. The greatest variation in analysis is usually among labs from different areas of the country. Be sure to send your samples to a laboratory that is familiar with the nutrient requirements and growing conditions of turfgrasses in your region. If you are sending samples to a national commercial laboratory, note your location.

Interpretation of soil test results allows your nutrient levels to be placed into categories such as low (deficient), adequate, or high based on the research and experience of turfgrass specialists. Recommendations are usually provided as pounds of fertilizer per 1,000 square feet (also based on research and the experience of turfgrass specialists). Make sure you understand the recommendations before applying the fertilizer; that is, determine whether the recommended amount of fertilizer is to be applied in several separate applications or provided in one application.

Recommendations offered by Penn State's Agricultural Analytical Services Lab are based on research with turfgrass in Pennsylvania and the experience of turf specialists at the University. It is not surprising that recommendations from other states differ since soils, research procedures, and specialists' opinions differ from those of Penn State specialists. To maintain consistent soil test results and recommendations, work with one lab that is convenient to use and whose recommendations you can understand.

Tissue Testing

Testing of turf leaf tissue allows you to monitor nutrient levels, which in some cases can be related to the need for fertilizer. Leaf tissue testing is also a means of diagnosing nutrient deficiencies, verifying diagnosis made from visual deficiency symptoms. Tissue nutrient levels can be determined for most or all nutrients, or for only one or two. Some turfgrass managers sample leaf tissue for nitrogen to determine fertilizer nitrogen requirements. As with soil testing, proper sampling of leaf tissue is critical. Samples must be representative of the area, collected according to lab instructions, and, above all, free from soil and other contaminants.

Fertilizer Basics

Cost is a primary concern in deciding which fertilizer product to use. Selecting the least expensive fertilizer, however, does not necessarily mean you have found the best value. Fertilizer should be purchased on the basis of quality rather than bag size or price. Quality is determined by the amounts and types of nutrients contained in the bag and the product's physical characteristics.

Nutrients in Fertilizers

Turfgrass fertilizers contain one or more plant nutrients; the most common of these are nitrogen, phosphorus (designated on labels as available phosphate, or P₂O₅), and potassium (designated as water-soluble potash, or K₂O). These three nutrients are represented on the fertilizer container as three numbers, which indicate the percentages by weight of nitrogen, phosphate, and potash—always in that order (Figure 3). The three numbers are referred to as the fertilizer grade.

Figure 3. The three numbers on fertilizer containers indicate the percentages by weight of nitrogen, phosphate, and potash in the fertilizer.

When nitrogen, phosphorus, and potassium are all present in the container, the fertilizer is called a complete fertilizer. Sometimes one or two of these nutrients are not present, and the missing nutrient(s) are simply listed as "0" in the grade. Occasionally, turfgrass fertilizers contain other nutrients such as sulfur, iron, and/or calcium. These are usually listed on the label, but they are not part of the fertilizer grade.

A fertilizer grade is used to determine the percentage by weight of plant nutrients in the product. For example, a 100-pound bag of fertilizer with a grade of 30-0-10 contains 30 pounds of nitrogen, no phosphate, and 10 pounds of potash. A 50-pound bag of the same product would yield 15 pounds of nitrogen, no phosphate, and 5 pounds of potash. Knowing the fertilizer grade is important when determining how much fertilizer to apply to your turf.

Sometimes, a fertilizer ratio is specified on soil test reports or in fertilizer recommendation sheets. The fertilizer ratio indicates the proportion of nitrogen, phosphate, and potash in the product. For example, an 18-6-6 fertilizer contains three parts nitrogen to one part phosphate to one part potash. Thus, this fertilizer has a 3-1-1 fertilizer ratio.

How much phosphorus and potassium are really in your fertilizer?

The chemical formulas P₂O₅ and K₂O are the traditional means of expressing the amount of phosphorus (P) and potassium (K) in fertilizer. In fact, no such compounds exist in fertilizers. In the rare event that you have to determine the actual amounts of phosphorus and potassium in your fertilizer, use the following formulas:

% P₂O₅ x 0.44=%P

% K₂O x 0.83 = % K

Examples

A fertilizer containing 20% P₂O₅ has about 9% P (20% P₂O₅ x 0.44 = 8.8% P)

A fertilizer containing 10% K₂O has about 8% K (10% K₂O x 0.83 = 8.3% K)

Note: Typically, you do not need to perform these calculations because fertilizer recommendations are almost always provided as pounds P₂O₅ per 1,000 square feet and pounds K₂O per 1,000 square feet.

How to calculate a fertilizer ratio

If your soil test report recommends applying 1.5 pounds of nitrogen, 0.5 pound of phosphate, and 0.5 pound of potash per 1,000 square feet, you should apply a fertilizer with a ratio of 3-1-1 since you need three times as much nitrogen as phosphate and three times as much nitrogen as potash. The simplest method of determining a ratio is to divide the weights of nitrogen, phosphate, and potash by the lowest weight of the three.

Example:

To determine a fertilizer ratio for a recommendation of 1.5 pounds of nitrogen, 0.5 pound of phosphate, and 0.5 pound of potash, divide the weight of each of the three nutrients (1.5 pounds, 0.5 pound, 0.5 pound) by the nutrient with the lowest weight (0.5 pound).

1.5 ÷ 0.5 = 3

0.5 ÷ 0.5 = 1

0.5 ÷ 0.5 = 1

Thus, the fertilizer ratio that best fits this recommendation is 3-1-1. You can also determine fertilizer ratios from fertilizer grades by dividing the percentage of nitrogen, phosphate, and potash by the lowest percentage of the three nutrients (unless the lowest percentage is 0, in which case divide by the lowest whole number that is not 0).

Example:

Determine ratios for the following fertilizer grades: 21-7-7, 22-0-8, and 18-5-9.

Grade = 21-7-7

• 21 ÷ 7 = 3
• 7 ÷ 7 = 1
• 7 ÷ 7 = 1
• Ratio = 3-1-1

Grade = 22-0-8

• 22 ÷ 8 = 2.8
• 0 ÷ 8 = 0
• 8 ÷ 8 = 1
• Ratio = 2.8-0-1

Grade = 18-5-9

• 18 ÷ 5 = 3.6
• 5 ÷ 5 = 1.0
• 9 ÷ 5 = 1.8
• Ratio = 3.6-1-1.8

Nitrogen Sources in Fertilizers

The source of nitrogen in a fertilizer is important for determining your turf's growth rate, density, and color. Nitrogen fertilizers can be divided into two categories: quick-release and slow-release. Quick-release nitrogen sources are soluble in water; hence, nitrogen is available to plants immediately. They can also burn turf more easily than slow-release sources. Slow-release nitrogen sources typically release a portion of their nitrogen over relatively long periods (several weeks to several months).

The relative amounts of quick- and slow-release nitrogen in a fertilizer product are listed on the label as percentages of the total nitrogen (Figure 4). Quick-release nitrogen is designated as ammoniacal nitrogen and/or urea. Slow-release nitrogen is designated as water-insoluble nitrogen (WIN), slowly available nitrogen, or controlled-release nitrogen. For a more detailed explanation of nitrogen sources, see “Turfgrass Nitrogen Sources," below.

Figure 4. A fertilizer bag may carry the following label:
Guaranteed Analysis:

Physical Characteristics of Fertilizers

A fertilizer's physical characteristics determine how easy it is to handle and how evenly it is applied to turf surfaces. Granular fertilizers that contain significant amounts of dust and broken particles make for poor distribution of nutrients, especially when applied through rotary spreaders. Similarly, products containing granules of different sizes are not as evenly distributed by rotary spreaders as those with uniform sizes because the larger, heavier particles are thrown farther from the spreader than smaller, lighter particles. When purchasing a fertilizer, look for a product with uniform particle sizes and minimal amounts of dust and broken granules.

The size of fertilizer granules is another important physical characteristic that can influence distribution of nutrients and appearance of turf. Fertilizer granule sizes for turf managed at low mowing heights (e.g., golf course greens, tees, and fairways) should be smaller than granules used for turf mowed at heights typical for lawns. On low-cut golf turf, mowers can pick up or damage large granules. Small granules designed for golf turf provide a better distribution of nutrients because there are more particles per unit area of turf. Having more granules per unit area on low-cut turf also reduces the potential for speckling due to particles being too far apart.

Granular fertilizer used by professional turf managers is usually assigned a size guide number (SGN), which indicates the median diameter of the fertilizer granules (in millimeters) multiplied by 100. It is determined by passing a sample of the fertilizer through a series of sieves; the sieve opening size (in millimeters) that retains 50 percent of the weight of the fertilizer is multiplied by 100 to determine the SGN. For example, a fertilizer having 50 percent of its particles retained on a sieve with 2-millimeter openings will have a SGN of 200. Most granular turfgrass fertilizers have SGNs ranging from 75 to 250. Granular fertilizers designated for golf greens have SGNs in the low range (75 to 100) and those for tees and fairways typically range from 125 to 150, whereas lawn fertilizers have SGNs around 200 to 250.

The density of granular fertilizers can influence swath width and particle distribution. Lightweight fertilizers are thrown for only a short distance by rotary spreaders, resulting in narrow swaths and, thus, the need for more passes by the spreader operator. Also, lightweight particles are easily carried by wind, resulting in poor distribution patterns on windy days. Most professional fertilizer applicators prefer high-density fertilizers because of their improved spreading characteristics.

Some turfgrass fertilizers are sold as liquids or dry formulations that can be dissolved in water for spray applications. Some liquid fertilizer formulations separate into layers when stored for extended periods in cold temperatures. Be sure to follow storage directions carefully when using liquid formulations. Dry fertilizers used for spray applications should not contain impurities that can clog or abrade spray nozzles.

Calculations Used in Turfgrass Fertilization

Proper fertilization practices require that precise amounts of nutrients be delivered to turfgrass areas. Small mistakes in area measurements or fertilizer rate calculations can produce poor results and, sometimes, serious turf injury. Sample problems are designed to provide the simplest methods of solving the most frequently encountered questions regarding area measurements and fertilizer rates.

Determining the Area of a Site to Fertilize

Square:

                a (100 feet)

                b (100 feet)

                Area = a × b

                Area = 100 ft × 100 ft

                Area = 10,000 sq ft

 

Rectangle:

                w (100 feet)

                l (200 feet)

                Area = l (length) × w (width)

                Area = 200 ft × 100 ft

                Area = 20,000 sq ft

 

Right triangle:

                h (40 feet)

                b (30 feet)

                Area = 0.5 × b (base) × h (height)

                Area = 0.5 × 30 ft × 40 ft

                Area = 600 sq ft

Circle:

                r (5 feet)

                Area = πr²

                Area = (π is always 3.14) × r²

                Area = 3.14 × 5²

                Area = 3.14 × 25 ft

                Area = 78.5 sq ft

To calculate the area of irregularly shaped lawns, divide the lawn into common shapes, calculate the areas of the shapes, and then add the areas together.

Example Calculations Used in Turfgrass Fertilization

Problem 1. A fundamental problem in turfgrass fertilization involves determining how much fertilizer is needed to supply a specified amount of nitrogen (or any other nutrient) per 1000 square feet. Use the following examples to learn how to solve this type of problem.

Example:

You have a 50-pound bag of 26-5-10 fertilizer that you want to apply to a lawn at a rate of 1.0 pound nitrogen per 1000 square feet. How much of the 26-5-10 fertilizer will you need to apply per 1000 square feet?

The quickest way to solve this problem is to ignore the weight of the fertilizer bag and simply divide the amount of nitrogen desired (1.0 pound nitrogen per 1000 square feet) by the percentage of nitrogen in the bag (26%). When using percentages in calculations, convert the number to its decimal form (for example, 26% = 0.26; 5% = 0.05).

(1.0 pound nitrogen per 1000 square feet) ÷ 0.26 = 3.8 pounds of a 26-5-10 fertilizer is needed to supply 1.0 pound nitrogen per 1000 square feet

Example:

Find out how much phosphate and potash you are applying to the turf when you apply 3.8 pounds of the 26-5-10 fertilizer per 1000 square feet.

Multiply the amount of fertilizer you are applying (3.8 pounds per 1000 square feet) by the percentage of phosphate in the bag (5%). Do the same for potash (10%). Remember to convert the percentages of phosphate and potash to their decimal forms.

(3.8 pounds fertilizer per 1000 square feet) x 0.05 phosphate = 0.19 pound phosphate per 1000 square feet

(3.8 pounds fertilizer per 1000 square feet) x 0.10 potash = 0.38 pound potash per 1000 square feet

Problem 2. Another common problem involves determining the area that a bag of fertilizer can cover and how many bags are needed to cover large sites.

Example:

How much area can be covered with a 50-pound bag of 26-5-10 at the rate of 1.0 pound nitrogen per 1000 square feet?

Now that you know 3.8 pounds of 26-5-10 fertilizer will cover 1000 square feet, determine how many times 3.8 pounds goes into 50 pounds.

50 pounds ÷ 3.8 pounds = 13.2

Now multiply 13.2 by 1000 square feet: 13.2 x 1000 square feet = 13,200 square feet.

Thus, a 50-pound bag of 26-5-10 covers 13,200 square feet at a rate of 1.0 pound nitrogen per 1000 square feet.

Example:

How many 50-pound bags of 26-5-10 will you need to fertilize a 30,000 square feet lawn at 1.0 pound nitrogen per 1000 square feet?

If a 50-pound bag of 26-5-10 fertilizer covers 13,200 square feet at 1.0 pound nitrogen per 1000 square feet, determine how many times 13,200 goes into 30,000.

30,000 ÷ 13,200 = 2.3 bags of 26-5-10 will cover 30,000 square feet.

Problem 3. Occasionally, fertilizer recommendations given as pounds nitrogen per 1000 square feet must be converted to pounds fertilizer per acre.

Example:

You are treating a large sports turf complex and would like to determine how many pounds of a 16-8-8 fertilizer should be applied per acre if the recommendation calls for 0.75 pound nitrogen per 1000 square feet.

First: Find out how much fertilizer will be needed per 1000 square feet (see examples in problem 1).

(0.75 pound nitrogen per 1000 square feet) ÷ 0.16 = 4.7 pounds fertilizer per 1000 square feet

Second: Since there are 43,560 square feet in an acre, multiply the amount of fertilizer needed per 1000 square feet by 43,560, then divide by 1000. (4.7 pounds fertilizer x 43,560) ÷ 1000 = 205 pounds of a 16-8-8 fertilizer per acre

Nitrogen in Turf

Nitrogen is an essential element for all living things and the mineral element needed in the largest amounts by turfgrasses. Although nitrogen is abundant in the atmosphere (about 80 percent of the air surrounding us is nitrogen gas), it is in limited supply in soils and available to plants only after it has been converted to nitrate (NO₃^-) or ammonium (NH₄⁺) by microorganisms or industrial processes. In most cases, nitrogen fertilizer must be applied regularly to maintain high quality turf.

Although nitrogen fertilizer is required for healthy lawns, it can also contaminate ground- and surface waters through leaching and runoff. Excessive nitrate concentrations in drinking water are a health risk, especially for infants, pregnant and nursing mothers, and young children. Nitrogen movement into water can also accelerate degradation of ponds, lakes, coastal bays, and estuaries through a process called eutrophication. Eutrophication refers to the addition of nutrients to surface waters, resulting in algae blooms, dense aquatic plant growth, depletion of oxygen, and, in advanced stages, fish kills.

The goal of a nitrogen fertility program is to optimize plant uptake while minimizing leaching, runoff, and gaseous losses. To achieve this goal, you should understand how nitrogen behaves in the environment and know the conditions that influence its fate.

Optimizing Nitrogen Use

Although soil testing can provide guidelines for how much phosphorus, potassium, and lime turfgrasses need, it does not give reliable information about nitrogen requirements. Just how much nitrogen should be applied depends on the species you are attempting to maintain (and, in some cases, the cultivar), the soil conditions at the site, how the turf is managed, and how the site is used. Also, the amount of nitrogen that turfgrasses take up is influenced by application timing, the source(s) of nitrogen, and the amount of nitrogen applied per application. Designing fertilizer programs for maximum uptake and use of nitrogen by turf is discussed in "Fertilizer Programs".

Leaching

Leaching occurs when irrigation or rainfall carries nitrogen, primarily in the nitrate form, downward through the soil profile. As nitrate moves below plant root systems, it continues to move downward, eventually ending up in groundwater. How much nitrogen is leached from a lawn depends on the soil type; the amount and rate of precipitation; and the nitrogen source, rate, and timing of application.

The greatest potential for leaching is in sandy soils during periods of wet weather or under excessive irrigation, and following applications of quick-release nitrogen at high rates. Leaching can be reduced by using slow-release nitrogen sources on high-sand-content soils or by using low rate applications of quick-release nitrogen sources. Leaching can also be curtailed by restricting nitrogen applications when plants are not actively growing (during midsummer and winter) and/or during extremely wet periods of the year. Since leaching of nitrogen can sometimes occur even in loam soils, be sure always to follow good fertilization and irrigation practices.

Runoff

When nitrogen is applied to turf, some may be carried in runoff into surface or groundwater. Runoff is water that reaches the turf-soil surface and is not absorbed into the ground or accumulated on the surface, but runs downslope. The rate of runoff is determined by the amount and rate of precipitation, slope, infiltration capacity of soil, geological features of the site, vegetation cover, and cultural practices.

Runoff is most likely to occur following sudden, heavy rainstorms on soils with poor infiltration characteristics that support little or no vegetation. The most significant runoff threat, however, is from impervious surfaces such as sidewalks, driveways, roads, and frozen soils. Some runoff from impervious surfaces is carried into storm sewers and finds its way into surface or groundwater.

Research conducted at Penn State has shown that where a dense, well- established turf exists, the amount of nitrogen removed from the site via runoff is very low--provided the site has good infiltration characteristics. The dense cover of leaves, stems, and thatch of turf slows the rate of surface flow, allowing water and nutrients to infiltrate the soil.

Atmospheric Losses: Volatilization and Denitrification

Volatilization and denitrification can cause atmospheric losses of nitrogen fertilizer. Although these losses usually are not considered a health or pollution hazard, they can reduce the efficiency of nitrogen fertilizer applications, resulting in greater costs and reduced turf quality.

Volatilization occurs when nitrogen is converted to ammonia gas (NH₃) and escapes to the atmosphere. It is more likely to occur following surface applications of urea or ammonium-containing fertilizers. Losses are favored by high soil pH (basic or alkaline conditions), high temperatures, sandy soils, and thatch. Watering-in applications of urea and ammonium-containing fertilizers will reduce volatilization in turfgrass.

Denitrification takes place in saturated soils when anaerobic bacteria (bacteria that survive in the absence of oxygen) convert nitrate to N₂, a gaseous form of nitrogen that escapes into the atmosphere. Turf that survives in poorly drained soils often turns yellow in wet weather owing to denitrification. Improved drainage at these sites will reduce N₂ losses.

Tips for Getting the Most Out of Your Nitrogen Fertilizer

The following are suggestions for maximizing the efficiency of your nitrogen fertilizer program while minimizing losses to leaching, runoff, and the atmosphere.

1. Soil test. Applications of phosphorus, potassium, and lime according to soil test recommendations allow more efficient use of nitrogen fertilizer by turfgrasses.
2. Apply nitrogen in amounts needed by the species you are trying to maintain--more is not necessarily better.
3. On turf, apply nitrogen fertilizer in multiple applications over the growing season so as to meet the needs of your turf at the appropriate time--usually mid to late spring, late summer, and late fall.
4. Returning clippings to lawns can cut nitrogen fertilizer use by up to one-third.
5. Don't overwater--too much water can leach nitrogen below root systems and into groundwater.
6. Use slow-release fertilizers when making infrequent, high-rate applications in areas where soils are prone to leaching.
7. Keep nitrogen on the lawn and not on pavement. Shut off your spreader when moving across driveways or maintenance roads, or blow or sweep up granules from pavement. In small lawns enclosed by sidewalks and driveways, use a drop spreader or a liquid application for greater accuracy.
8. Do not apply nitrogen to lawns under summer dormancy or on frozen surfaces in winter.
9. Water-in urea or ammonium fertilizers, especially when applications are made in warm weather.
10. Fill and empty fertilizer spreaders in an area where spills can be easily cleaned up. Use your spilled fertilizer--don't wash it into the street or storm sewers.

Turfgrass Nitrogen Sources

Developing a nitrogen fertility program is an important decision that can affect the quality and durability of your turf. Because of differences in site conditions, uses of turf, level of turf quality desired, and cost considerations, no single program will fit all situations. Fortunately, there are many different turfgrass nitrogen sources that you can use to develop a program to fit your needs.

Before selecting a nitrogen source(s) for your program, understand how quickly the nitrogen in the product is released and under what conditions this occurs. It is also helpful to know how the product is formulated and its potential for burning turf.

Quick-Release Sources

Quick-release nitrogen sources are also called "quickly available," "fast-acting," "soluble," "readily available," and other terms that indicate rapid availability of nitrogen to turf after application. This group includes compounds containing ammonium, nitrate, or urea. Quick-release sources have nitrogen contents ranging from 11 to 46 percent (Table 3) and generally are less expensive than slow-release sources. Being water soluble, they may be applied in liquid as well as in dry form. They give a rapid green-up response, and frequent applications at low rates are suggested for reducing excessive growth, fertilizer burn, and nitrogen loss to the environment. Quick-release nitrogen sources are often combined with slow-release sources to provide a fast but moderate green-up and growth response while extending the duration of the response. 

Table 3. Some quick-release nitrogen sources used for turfgrass fertilizers.

* Salt index is a relative measure of the salinity of fertilizers and indicates the relative burn potential of nitrogen sources (a high salt index indicates a high potential to burn turf). Sodium nitrate is the benchmark value against which all other materials are compared, with a salt index of 100. Salt indices may vary with formulation.

Salts that contain ammonium and nitrate (potassium nitrate, ammonium sulfate, monammonium phosphate, etc.) are available in granular and, in some cases, sprayable formulations. In water, these nitrogen sources readily dissolve into their positively and negatively charged components. For example, ammonium sulfate [(NH₄)₂SO₄] fertilizer mixed with water forms ammonium (NH₄⁺) and sulfate (SO₄^-2). In soils, bacteria convert ammonium into nitrate through a process called nitrification. Plants may use nitrogen in either the ammonium or the nitrate form, but most nitrogen is taken up as nitrate.

Urea is a synthetic organic fertilizer that contains 46 percent nitrogen. It is available in granular and prilled forms for dry applications and, since it is water soluble, it can be applied as a liquid. Provided there is adequate moisture following application, it reacts quickly with water and the naturally occurring enzyme urease to form ammonium-nitrogen. This reaction usually takes place within 7 to 10 days. Under high pH (alkaline) conditions, volatilization of nitrogen as ammonia may occur from urea and ammonium. Volatilization is also favored by low soil-cation-exchange capacity (sandy soils), drying of moist soil, and high temperatures. Watering-in fertilizer keeps ammonia volatilization losses to a minimum.

Urease Inhibitors and Nitrification Inhibitors in Quick-Release Sources

Fertilizer products are now available that contain urease inhibitors and, in some cases, nitrification inhibitors. Sometimes referred to as “stabilized" nitrogen fertilizers, these products are essentially quick-release urea nitrogen sources that can, under certain circumstances, improve fertilizer efficiency by reducing nitrogen losses via ammonia volatilization and/or nitrate leaching.

When urea is applied to turf and watered into the soil, it undergoes hydrolysis and is rapidly converted to ammonia and then ammonium, which is a relatively stable form of nitrogen that can be taken up by the plant. If the urea is not watered-in, some of the hydrolyzed urea-nitrogen can be lost to the atmosphere through ammonia volatilization. The urea hydrolysis process is hastened by the urease enzyme, which is abundant in soil and thatch.

Chemical additives called urease inhibitors block urease enzyme activity and dramatically slow the conversion from urea to ammonia, thereby reducing volatilization. The most common urease inhibitor is N-(nbutyl) thiophosphoric triamide, commonly referred to as NBPT. Examples of urea-based products where NBPT is an additive include LSN, UFLEXX, and UMAXX fertilizers. Urea products containing urease inhibitors are not slow-release fertilizers, and any potential improvement in nitrogen efficiency is a result of reduced ammonia volatilization. Benefits obtained with urease inhibitors will ultimately depend on the potential for ammonia volatilization following application. If conditions favor volatilization after application (hot days, high pH, leaving urea on the soil surface or in thatch with no follow-up irrigation or rainfall), using urease inhibitors will likely result in more nitrogen uptake and less volatilization.

Nitrification inhibitors can, in some circumstances, reduce nitrogen leaching through their inhibitory effects on specific soil microorganisms. When nitrogen fertilizer is applied to turf, it is converted from ammonium to nitrate through a process called nitrification. Nitrification is a natural process in soils that is mediated by two specialized soil bacteria. One of these bacteria, Nitrosomonas spp., transforms ammonium to an intermediate nitrogen compound called nitrite, while the other (Nitrobacter spp.) converts nitrite to nitrate. Nitrification inhibitors are designed to specifically target Nitrosomonas bacteria so that the nitrogen fertilizer remains in the more stable ammonium form. Ammonium tends to be stable because it is positively charged and adheres to clay and organic matter. Nitrate is negatively charged and more susceptible to leaching during rainy periods and in sandy soils.

Only one nitrification inhibitor, dicyandiamide (DCD), is currently used in turfgrass fertilizers. Products containing DCD include UFLEXX and UMAXX. The potential benefits of improved nitrogen efficiency from nitrification inhibitors will likely depend on the potential for nitrogen leaching following application. If conditions favor leaching after application, there is a higher probability of improved nitrogen retention in the soil using DCD. If conditions do not favor leaching, the benefits from using DCD would be limited.

Slow-Release Sources

Slow-release nitrogen sources, also called "controlled-release," "slowly available," "slow acting," and "water insoluble," are an important part of turfgrass fertility programs. They provide a longer duration of nitrogen release than the quick-release sources and are safer to use on turf because of their lower burn potential. Recent studies have shown that under certain conditions, slow-release nitrogen sources are less likely to leach into groundwater than quick-release sources when applied at high rates.

Disadvantages of slow-release nitrogen sources include their high price per unit of nitrogen and reduced efficiency (a lower percentage of the applied nitrogen is used by turf in the first year or two of use) compared to quick-release sources. The higher cost and low efficiency have prompted many manufacturers and turf managers to mix or blend both slow- and quick-release sources.

Slow-release nitrogen sources can be grouped into several categories, including the natural organics, ureaform, urea-formaldehyde products, triazones, IBDU, sulfur-coated urea, and polymer-coated nitrogen (Table 4).

Table 4. Some slow-release nitrogen sources used for turfgrass fertilization.

*Physical state of product, G = granular, P = powder, and L = liquid.
**Percentage water-insoluble nitrogen of the total nitrogen.
***Nitrogen that is not listed as WIN, ammoniacal nitrogen, or urea nitrogen on the fertilizer label.

Natural Organics

For the most part, natural organics are by-products from the plant and animal processing industries or waste products. Examples include bone, blood, and feather meal; fish scrap and meal; seed meals; dried and composted manures; activated and composted sewage sludges; and process tankage. Considerable variation exists in the physical and chemical properties of different natural organic fertilizers.

The natural organics can be characterized by relatively low nitrogen contents (usually below 10 percent), the presence of water insoluble nitrogen (WIN), and nitrogen release intermediate between that of quick-release nitrogen sources and extremely slow-release nitrogen sources such as ureaform. Release of nitrogen is dependent on microbial activity and is highly variable among products. Factors influencing nitrogen release are the chemical composition of the material and environmental conditions that influence microbial activity. Environmental conditions affecting breakdown of natural organic fertilizers include temperature, soil moisture and oxygen, and soil pH.

Ureaform

Ureaform is made by reacting urea with formaldehyde in ratios of about 1.3 to 1. Ureaform fertilizers should contain at least 35 percent nitrogen, with at least 60 percent of the total nitrogen being WIN. Urea-formaldehyde products not falling within these guidelines are referred to by other terms such as methylene urea and methylol urea.

Ureaform is divided into three, almost equal fractions based on solubility. Fraction I is soluble in cold water and contains urea, methylene diurea, and dimethylene triurea. Nitrogen availability in this fraction is similar to that of quick-release nitrogen sources, but the nitrogen is not as quickly available. Fraction II is insoluble in cold water but soluble in hot water; it is made up of the slow-release compounds trimethylene tetraurea and tetramethylene pentaurea.

Fraction III, the most slowly available, is insoluble in both hot and cold water and is made up of pentamethylene hexaurea and longer chain polymers. Studies have shown that over a six-seven month period about 4 percent of Fraction I, 25 percent of Fraction II, and 84 percent of Fraction III remain in the soil. The slow breakdown of Fractions II and III accounts for the low efficiency of ureaform during the first years of use. With continued use and buildup of ureaform, recovery of applied nitrogen improves.

Release of nitrogen from ureaform depends on microbial activity, and the same environmental factors that affect release from natural organics also affect release from ureaform. Because of low nitrogen recovery (efficiency) in the first years of use, you will usually need to use higher rates or supplement ureaform with soluble sources in these years. This low recovery and slow response during cool periods support the concept of fertilization with combinations of ureaform and quick-release nitrogen sources.

Other Urea-Formaldehyde Products

These are also reaction products of urea and formaldehyde but are made with wider ratios of urea to formaldehyde (more urea) than ureaform; thus, they release nitrogen faster. These products contain 30-35 percent nitrogen and are classified "slowly available." However, some contain enough water-soluble nitrogen to give a response closer to quick-release nitrogen sources, such as urea, than to slow-release nitrogen sources. Others can be expected to give a quick initial response, but they have a slightly slower release rate than the quick-release nitrogen sources. Any urea-formaldehyde product that does not claim WIN or claims CRN (controlled-release nitrogen) and not WIN as a percentage of the total nitrogen, will release nitrogen quickly (similar to urea).

Most urea-formaldehyde products are available as granular fertilizers. They contain mostly water-soluble compounds such as unreacted urea, methylol urea, and short-polymer methylene ureas (methylene diurea and dimethylene triurea). The amount of each compound in a product is largely dependent on the urea/formaldehyde ratio and the conditions under which the reaction takes place during manufacture. These nitrogen sources are typically more expensive than urea and ammonium and nitrate products, but they are safer since they have reduced fertilizer burn potential.

Triazones

Triazones are water-soluble compounds produced through a reaction involving urea, formaldehyde, and ammonia. On a dry weight basis, triazone products are about 30-36 percent triazones, about 40-50 percent urea, and the remainder, methylol and methylene ureas. Triazones are classified as slow-release nitrogen sources, even though their nitrogen-releasing properties are closer to those of urea than to slow-release nitrogen sources. Although more expensive than urea, triazone products are safer because of their reduced burn potential. Products containing triazones are liquids.

IBDU

IBDU is made by reacting isobutyraldehyde and urea. It contains 31 percent nitrogen, with 90 percent of the total nitrogen being WIN in the coarse (0.7-2.5 mm) product and 85 percent WIN in the fine (0.5-1.0 mm) product. IBDU breaks down slowly in soils because of low solubility, but once in solution, it is hydrolyzed and releases nitrogen. Particle size has a large effect on the release of nitrogen, with smaller particles releasing more quickly. The release rate is faster with higher soil-water content and, to a limited extent, higher temperatures.

In tests at Penn State, we have observed a three- to four-week delay before obtaining a response from IBDU applications on Kentucky bluegrass, but not after applications to an aerated and topdressed putting green. Probably the close contact with wet soil and more liberal irrigation practices enhanced release on the putting green. If the delay in response is considered objectionable, a soluble nitrogen source can be used to supplement the IBDU. We have observed early spring greening with IBDU, and nitrogen recovery from IBDU exceeded that from ureaform during the first and second years of use. We have gotten a quicker response and greater nitrogen recovery from fine than from coarse IBDU.

Sulfur-Coated Urea

Sulfur-coated urea (SCU) is made by spraying preheated urea prills or granules with molten sulfur. A sealant, such as wax or a mixture of oil and polyethylene, is often applied to seal pores and imperfections in the sulfur. Nitrogen content is usually in the range of 32-43 percent and depends on coating thickness. Increasing the thickness lowers the nitrogen content.

Nitrogen is released from SCU by microbial degradation of the sealant and diffusion of soluble nitrogen through pores and cracks in the sulfur coating. The release rate quickens as coating thickness decreases and as temperature increases. Also, breakage of the coating as a result of mechanical damage or aging enhances the release of nitrogen.

Particles within a SCU product are not identical. If they were, one might expect all of them to release nitrogen at the same time. Quick release occurs with imperfectly coated particles; an intermediate rate of release takes place with particles in which the sealant has covered imperfections; and the greatest delay in release occurs with the more thickly and more perfectly coated particles. Once release begins from a given particle, it is quite rapid. Thus, the slow-release properties of SCU come from the variability in coatings among the individual particles. SCU with sealants have given good response from two applications per year on Kentucky bluegrass turf, and nitrogen efficiency has equaled that of quick-release nitrogen sources. Sealant-free SCU products typically release nitrogen at a slower rate since they have thicker sulfur coatings.

Polymer-coated nitrogen

Polymer-coated nitrogen fertilizers consist of urea, SCU, or other nitrogen sources coated with a thin layer of polymer (plastic) resin. Several types of polymer-coated nitrogen fertilizers are available. For nitrogen release to occur from polymer-coated urea, water is absorbed through the coating and dissolves the nitrogen. Nitrogen is then gradually released through the coating by osmosis. Different coating thicknesses may be used to obtain different nitrogen release rates. The thicker the coating, the slower the release. Release increases with a higher temperature and is not significantly influenced by soil moisture levels, volume of water applied, soil pH, or microbial activity.

Phosphorus in Turf

Phosphorus is one of three primary nutrients needed by turfgrasses as a regular fertilizer addition. Although it is present in small amounts in turfgrass tissues (0.3-0.55 percent on a dry weight basis), phosphorus is extremely important for rooting, seedling development, cell division, and the synthesis of various compounds used by plants. Phosphorus is available to turfgrasses as H₂PO₄^- and HPO₄^2- and is mobile in plants (meaning that it can move from one portion of the plant to another).

Phosphorus deficiencies in turf are usually expressed in the early stages of seedling development, appearing as a purple or red coloring of leaf blades and as reduced growth and tillering. A soil test taken prior to planting turfgrass seed can indicate how much phosphorus is needed for optimum establishment.

Phosphorus is present in inorganic and organic forms in mineral soils, and both are important sources for plants. Although the total amount of phosphorus in soils can be large, much is unavailable to turf because it forms insoluble complexes with other elements and/or because it is "fixed" to clay particles.

The most important factors affecting phosphorus availability to turfgrasses are soil pH and concentrations of iron, aluminum, manganese, and calcium in soils. In acid soils, the H₂PO₄^- form of phosphorus predominates and combines with iron, aluminum, or manganese to form insoluble compounds that are unavailable to turfgrasses. When the soil pH drops to 5.5 and below, enough phosphorus can be rendered unavailable to cause deficiencies in turf. Also, under acid conditions, some phosphorus can be "fixed" by silicate clays, resulting in reduced availability to plants.

In high-pH soils, HPO₄^2- is the most common form of phosphorus. In these soils phosphorus combines with calcium to form insoluble calcium phosphates. As the soil pH approaches 8.0 or above, significant amounts of phosphorus are unavailable to turfgrasses. Maximum amounts of plant-available phosphorus (both inorganic and organic forms) are obtained by keeping the soil pH between 6.0 and 7.0.

Phosphorus can be supplied to turf as inorganic and/or natural organic fertilizers (Table 5). Inorganic phosphorus fertilizers include superphosphates and ammonium phosphates and are manufactured by treating rock phosphate with various acids. Natural organic fertilizers typically contain phosphorus derived from plant or animal by-products. These fertilizers can contain as much as 13 percent phosphorus.

Table 5. Some sources of fertilizer phosphorus.

Phosphorus is largely immobile in soils, meaning that it takes a long time to move from the turf surface into the root zone. Phosphorus may take weeks or months to move just a few inches in soil. Because of its poor mobility, phosphorus should be incorporated into the soil prior to seeding or sodding at the amount recommended on your soil test report. Apply the phosphorus to the surface, then incorporate it 4-6 inches deep with a rototiller so that developing roots can use the fertilizer. On established turf, some phosphorus can be incorporated into soil either just before or just after cultivating with a core aerator. Perhaps the best approach to phosphorus fertilization of established turf is to soil test every three years to monitor your phosphorus levels and to use phosphorus-containing fertilizers periodically to maintain adequate levels.

Phosphorus, along with nitrogen, is one of the major nutrient sources contributing to surface water and groundwater pollution in the United States. Although phosphorus is not readily leached from turf soils into groundwater, recent studies of phosphorus fate on cropland have shown that this nutrient can enter surface water via erosion and runoff. Avoid applying phosphorus fertilizer where runoff is likely--such as on frozen soils and paved surfaces.

Potassium in Turf

Potassium is a primary turfgrass nutrient and is usually supplied annually as fertilizer to lawns. It makes up about 1.0-2.5 percent of the plant's dry weight, and its primary role involves regulating several important physiological processes. Potassium activates plant enzymes used in protein, sugar, and starch synthesis. It also plays a key role in maintaining turgor pressure in plants. Thus, it has a strong influence on drought tolerance, cold hardiness, and disease resistance of turfgrasses. Deficiencies of potassium in turf may be expressed as increased susceptibility to drought, winter injury, and disease.

Although large quantities of potassium are present in soils, only a small fraction is available to plants. Most soil potassium is in unavailable forms as feldspar, muscovite, and biotite minerals. Potassium is available to turfgrasses in the ionic form (K⁺) and occurs in the soil solution and on negatively charged soil particles. In general, more plant-available potassium is present in fine-textured mineral soils (soils that contain high amounts of clay) than in sandy soils, especially in areas that receive high amounts of rainfall or are regularly irrigated. The best way to determine potassium needs for turfgrass is through soil testing.

Potassium is mobile in plants and sometimes can be taken up in amounts greater than needed for optimum growth. This phenomenon, called "luxury consumption," is generally considered inefficient use of the nutrient. It is difficult to determine if luxury consumption is a problem in turf culture since very little information is available on the optimum concentrations of potassium in turfgrasses.

Potassium can be supplied to turf using inorganic fertilizers, natural organic fertilizers, or both (Table 6). However, most fertilizer potassium is derived from inorganic sources, in particular, muriate of potash (potassium chloride) and sulfate of potash (potassium sulfate). Both of these fertilizers are water soluble.

Table 6. Sources of fertilizer potassium.

Although it is readily leached into groundwater, potassium is not a major pollutant in surface water and groundwater in the United States. It rarely is present in concentrations toxic to people or aquatic life, and it does not deplete water of oxygen.

Secondary Nutrients in Turf: Calcium, Magnesium, and Sulfur

Calcium, magnesium, and sulfur are considered secondary nutrients because in most cases they only occasionally need to be supplied to turf in the form of fertilizer. Applications of calcium and magnesium are usually only necessary when your soil pH is below optimum for turfgrass growth. By liming soil when your soil test indicates a need, you are supplying your turf with calcium or calcium- and magnesium-containing limestone. When your soil test indicates a need for calcium but not magnesium, you can use a lime source containing only calcium carbonate. If the soil is low in magnesium, however, use dolomitic limestone since it contains both calcium carbonate and magnesium carbonate (Table 7).

Table 7. Some common sources of calcium, magnesium, and sulfur.

*Actual percentages of nutrients may vary depending on purity and source of product.

In the rare event that calcium is recommended for turfgrass growing in soil with an adequate pH, gypsum can be used as a source of calcium. Keep in mind that gypsum is not a liming source. Also, despite claims on some gypsum labels, it will not relieve soil compaction or break up clay soils in the northeastern United States. Gypsum improves soil structure in soils with high sodium concentration found in some areas of the western United States or soils that have been contaminated with irrigation water containing excessive sodium.

Sulfur is sometimes used to lower soil pH where a high soil pH can cause turf problems. Sulfur is usually only necessary in western states where arid conditions lead to alkaline soils. In the northeastern United States, high pH values are rarely a problem and there is usually enough sulfur in soils to supply turf needs.

Micronutrients in Turf

The seven micronutrients (sometimes called trace elements) required by turfgrasses include iron, manganese, zinc, copper, molybdenum, boron, and chlorine. As mentioned earlier, micronutrients are needed by turfgrasses only in minute amounts and rarely need to be supplied to turfgrasses growing in mineral soils. However, when turfgrasses are grown in soils with high sand content (golf course putting greens and some tees) or high in pH, micronutrient applications can be beneficial.

Iron

Iron is an important component of plant enzymes and proteins involved in respiration, nitrogen metabolism, and chlorophyll synthesis. In individual turfgrass plants iron deficiencies appear as chlorosis (yellowing) of the youngest leaves. Turf iron-deficiency symptoms show up as yellow mottling, as opposed to the uniform yellowing observed in nitrogen-deficient turf.

Most soils in the northeastern United States contain adequate levels of iron, and deficiencies are rare. In unusual cases where excessive liming has occurred or irrigation water with a high pH is used in large amounts, the uptake and/or translocation of iron by turf may be reduced. This problem, sometimes referred to as lime-induced chlorosis, can be corrected by acidifying the soil and by supplying iron-containing fertilizers.

In the northeastern United States, turfgrass managers apply iron fertilizer to enhance turf color without stimulating excessive leaf growth. Iron applications can produce darker green turf even when levels are adequate in plant tissues before applications are made. By reducing the rate of nitrogen fertilizer and supplementing with small amounts of iron, a noticeable turf green-up can be achieved with fewer of the negative aspects associated with excessive nitrogen fertilization, such as frequent mowing and outbreaks of certain diseases.

The most common forms of iron fertilizer for turfgrasses are inorganic iron salts and organic iron chelates (chelated iron) (Table 8). An inorganic iron salt is a water-soluble form of iron that contains iron or iron and ammonium paired with sulfate (e.g., ferrous sulfate, ferric sulfate, or ferrous ammonium sulfate). Since turfgrasses can absorb iron from these products through foliage, the products are typically applied as foliar sprays. In soil applications, much of the iron from inorganic sources is converted to insoluble iron hydroxides, iron phosphates, or iron carbonates--compounds that are unavailable to turfgrasses.

Table 8. Some common fertilizer sources of iron.

*Actual percentage of iron may vary depending on purity and source of product.

Chelated iron sources are usually more efficient at supplying plants with iron than inorganic iron salts. Recent studies have shown that about 2 pounds of iron per acre from iron chelate provides the same color enhancement of Kentucky bluegrass as 4 pounds iron per acre from inorganic iron sulfate. Since lower rates of chelated iron can be used to obtain a dark green turf, there is less chance of injuring turfgrass with an iron application.

Rates of iron fertilizer for lawn grasses can vary depending on the source, time of year, and number of applications. Generally, a rate of 2 pounds of iron per acre from chelated iron is adequate for a noticeable turf green-up. Turf green-up from iron applications can last between several weeks and several months, depending on weather conditions following application. Applications during cool, wet periods (when turf is growing rapidly) enhance color for only two to three weeks, whereas applications during cool, dry periods (when growth of turf is slow) may last for several months.

Excessive amounts of iron can cause noticeable discoloration (a black-green color) in turfgrasses and, in some cases, may injure them. The degree of injury depends on the type of turf, the rate of iron, and the environmental and management conditions at the time of application. Some temporary blackening of Kentucky bluegrass foliage has been observed with as little as 4 pounds of iron per acre, from both inorganic and chelated sources. Some dieback of Kentucky bluegrass foliage can occur with rates higher than 15 pounds iron per acre.

Chelated Micronutrients

Iron, zinc, manganese, and/or copper often occur in forms that are not taken up by plants. This problem is especially marked if the soil has a high pH (8.0 or above). One way of correcting this problem is to apply the nutrient as a chelate. Chelate comes from the Greek word “clawlike" and denotes a soluble and stable product formed when an organic compound called a chelating agent bonds to the nutrient. The chelating agent keeps the nutrient in solution and releases it at the root surface where it is absorbed into the plant. Chelated nutrients can also be absorbed through turf foliage.

The most common commercial chelating agents used in the turfgrass industry are EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid). EDTA chelates iron at a pH of less than 6.3; above a pH of 6.8, it reacts with calcium, rendering it ineffective. DTPA chelates iron up to a pH of 7.5; above 7.5, calcium interferes with solubility, making it ineffective.

Chelates have been shown to be superior sources of iron, zinc, manganese, and/or copper since lower rates of chelated micronutrients can achieve the same results as higher rates of inorganic sources. Because lower rates can be used, the potential for plant injury is reduced. However, the cost of chelated micronutrients may be considerably higher than that of inorganic sources.

Other Micronutrients

Unless your soil has a high pH (greater than 8.0) and the texture is extremely sandy, micronutrient fertilizer applications are probably not needed. In fact, micronutrients other than iron are rarely beneficial and are sometimes harmful when applied to turfgrasses. Boron, for example, is toxic to turfgrasses even when applied in small amounts. Indiscriminate use of copper can lead to deficiencies of iron in turfgrasses. If you are managing turf in high sand content soils, work with a reputable soil and tissue testing lab to determine if micronutrient supplements are needed. If they are, use high-quality turfgrass fertilizers containing only the micronutrients that you need to correct the deficiency (Table 9).

Table 9. Some common fertilizer sources of micronutrients (manganese, zinc, copper, molybdenum, boron, and chlorine).

*Actual percentages of nutrients may vary depending on purity and source of product.

Fertilizer Programs

No single turfgrass fertilizer program is ideal for all lawns, athletic fields, and golf courses. The type and amount of fertilizer you use and the timing of your applications will depend on many factors, including grass species and cultivars, soil type, management practices, how the turf is used, and the users' expectations.

Turfgrass species differ in the amount of fertilizer, especially nitrogen fertilizer, that they require for best performance (Table 10). Kentucky bluegrass and perennial ryegrass typically need 2-4 pounds nitrogen per 1000 square feet per year, whereas the fine fescues respond best to about 1-2 pounds nitrogen per 1000 square feet per year. If Kentucky bluegrass turf is not fertilized during the growing season, it will usually appear yellow-green, thin, and more susceptible to pest damage.

Table 10. Annual nitrogen requirements for optimum performance of turfgrass species used in Pennsylvania

* Rates in the mid- to high range for turf grown in new stands, infertile soils, when clippings are removed from the site, and/or in high traffic areas. Rates in the low to mid-range can be used for older turf stands growing in inherently fertile soils and when clippings are returned to the turf.

In contrast, if a fine fescue turf receives 3-4 pounds nitrogen per 1,000 square feet per year, it can become more susceptible to drought, heat stress, and some diseases. Therefore, be sure to identify the species you are managing and to adjust your fertility program accordingly. With lawns containing mixtures of species, fertility programs are usually designed to favor the most desirable species.

Turfgrass cultivars can also vary in their nitrogen requirements. However, specific recommendations for individual cultivars are seldom made because nitrogen requirements have not been determined for most new cultivars. In addition, many managers have no way of knowing which cultivars are present in the turf.

Turfgrass fertilizer programs will vary with soil quality and type. Turfgrasses growing on sites where much of the topsoil has been removed or in sandy soils usually require more fertilizer than turf growing in good-quality topsoils. This is because of the lower amounts of nutrients found in poor-quality soils and the fact that nitrogen is more easily leached from sandy soils. Improving poor-quality soils with additions of organic amendments, such as good-quality compost, can improve soil structure, add nutrients, and enhance nutrient retention, thus reducing fertilizer needs.

Management practices such as mowing and irrigation can significantly influence the amount of fertilizer that turfgrasses will need. By returning grass clippings to your lawn you can reduce nitrogen, phosphorus, and potassium fertilizer needs by up to one-third. Lawns irrigated often during the summer months will use more fertilizer than those not irrigated.

How the turf is used also dictates how much fertilizer is needed. For instance, turfgrasses growing in high traffic areas, such as athletic fields, usually require more fertilizer for better recovery from wear than low traffic areas. Roadside turf, used to create a buffer between lanes on highways and to control erosion on banks, generally receives little or no fertilizer since aesthetics is not a primary goal and mowing must be kept to a minimum.

Ultimately, users will have differing expectations concerning the function and aesthetics of turfgrass areas. Thus, fertilizer programs will vary according to these expectations.

Application Frequency

The number of fertilizer applications you make during the growing season is just as important as the amount and type of fertilizer you use. To maintain high-quality turf, two or more fertilizer applications per year are generally required. If only two applications are made, higher rates of nitrogen may be necessary. In this case, fertilizers containing slow-release nitrogen sources are desirable since the nitrogen is released gradually over extended periods and turf burning is less likely.

In many cases, fertilizer programs involve more than two and as many as five applications per year. The programs with more applications allow for more flexibility in application rate and nitrogen source than two-application programs since there is less time between applications. A four application per year program, for example, can involve rates less than 1 pound nitrogen per 1,000 square feet per application. These lower rates allow the use of quick-release nitrogen sources.

Scheduling Fertilizer Applications

The best times of year to fertilize cool-season turfgrasses are in late summer, late fall, and mid- to late spring. Sometimes, two spring applications may be desirable--one in early spring and another in late spring. Fertilizers applied to turf during periods of heat and drought in midsummer can stress plants and lead to injury.

The most important time of year to fertilize turfgrasses is late summer (early to mid-September). Fertilizer is very necessary at this time because it promotes recovery from drought and heat-related injury sustained during midsummer. Late summer to early fall is also the time of year that cool-season grasses begin to manufacture and store carbohydrates. Carbohydrates are used by turfgrasses for root and rhizome growth, disease and stress tolerance, and protection from winter injury. Nitrogen applied during late summer stimulates foliar growth, but not to the extent that occurs in spring. Thus, slightly higher rates of nitrogen can be used for late summer application. If using rates higher than 1 pound of nitrogen per 1,000 square feet, it is best to use fertilizers with a high proportion of slow-release nitrogen.

An application of fertilizer in late fall can serve as a replacement for an early spring application. Late fall, in this case, is the time that foliar growth slows or stops, but soils are not frozen. In most areas of Pennsylvania, late fall fertilization should take place in mid-November.

If done correctly, late fall fertilization provides early and noticeable turf green-up in spring, often with less foliar growth than accompanies early spring applications of nitrogen fertilizer. Excess growth is often associated with high rates of nitrogen applied in early spring, which can deplete carbohydrates needed for root growth, disease resistance, and tolerance to heat and drought stress later in the season.

The main disadvantage of late fall fertilization is that, in some situations, nitrogen leaching may occur. Consequently, this practice should not be performed on sandy soils with quick-release nitrogen fertilizers. Slow-release nitrogen sources, such as natural organics and IBDU, are ideal for late fall applications mostly because they are not as likely to leach as quick-release sources.

If late fall fertilizer applications are not made, a small amount of fertilizer may be desirable in early spring. Typically, rates of 0.5 to 0.75 pound nitrogen per 1,000 square feet allow early spring green-up of lawns without excessive foliar growth. Since 0.5 pound nitrogen per 1,000 square feet does not supply enough nitrogen to carry the turf through the summer months, a late spring application is probably needed. A late spring application can be made in late May or early June; rates can vary from 0.75 to 1 pound nitrogen per 1,000 square feet. A fertilizer containing some slow-release nitrogen is desirable at this time of year because it can supply limited amounts of nitrogen to turf in early to midsummer.

Phosphorus and Potassium

Recommendations from a soil test lab should specify the amounts of phosphorus and potassium (usually in pounds of phosphate and potash per 1,000 square feet) your turf needs. If a soil test indicates a need for phosphorus, the rate of phosphate applied in a single application should be similar to rates of nitrogen (0.5 to 1 pound per 1,000 square feet) or slightly higher (1.5 to 2 pounds of phosphate per 1,000 square feet), but it should not exceed 5 pounds phosphate per 1,000 square feet per application. If a soil test indicates a need for potassium, apply potash at rates of 0.5 to 2 pounds per 1,000 square feet.

Examples of Lawn Fertilizer Programs for Professional Turf Managers

The three sample fertilizer programs below are designed for medium- to high-maintenance lawns growing under environmental conditions and soils found in Pennsylvania and managed by a professional turfgrass manager.

Perennial Ryegrass and/or Kentucky Bluegrass Lawn (Clippings Returned)

*WIN = water insoluble nitrogen; coated-nitrogen sources can include sulfur-coated urea or polymer-coated nitrogen.

**If soil test indicates high levels of phosphate and potash, omit from program and use nitrogen sources only. If soil test indicates phosphate and potash are needed, use a fertilizer containing each nutrient.

Table. Tall Fescue Lawn (Clippings Returned)

*WIN = water insoluble nitrogen; coated nitrogen sources can include sulfur-coated urea or polymer-coated nitrogen.

**If soil test indicates high levels of phosphate and potash, omit from program and use nitrogen sources only. If soil test indicates phosphate and potash are needed, use a fertilizer containing each nutrient.

Table. Fine Fescue Lawn (Clippings Returned)

*WIN = water insoluble nitrogen; coated-nitrogen sources can include sulfur-coated urea or polymer-coated nitrogen.

**If soil test indicates high levels of phosphate and potash, omit from program and use nitrogen sources only. If soil test indicates phosphate and potash are needed, use a fertilizer containing each nutrient.

Prepared by Peter Landschoot, Professor of Turfgrass Science

Funding for Urban Nutrient Management is provided by the Chesapeake Bay Program through the Pennsylvania Department of Agriculture.