Turfgrass Maintenance Fertilization
Department of Crop and Soil Sciences - Cooperative Extension
Introduction
A regular fertilization program is necessary to maintain good quality turfgrass. Dollar for dollar, fertilization does more to improve poor quality turfgrass or maintain good quality turfgrass than any other single management practice.
First, a soil test should be made to establish the basis for a regular fertilization program. Soil testing service is available from The Pennsylvania State University Soil and Forage Testing Laboratory and from private testing laboratories. Soil test mailing kits for University testing may be obtained from county Cooperative Extension Service offices at a nominal cost. The soil sample is forwarded to the University. After analysis, recommendations for fertilization are made.
Grass plants normally need nitrogen, phosphorus, and potassium (potash) in larger amounts than can be supplied naturally from the soil. Nitrogen, which is essential for vegetative growth and good green color, is a constituent of plant proteins, chlorophyll, amino acids, and other plant substances. Phosphorus is necessary for good root development and important in many vital growth processes. Potassium is required for physiological functions and promotes disease resistance and winter hardiness in grasses.
Fertilizer should be bought on the basis of its quality rather than on bag size and price. Value depends on the total amount of plant food contained in the bag and the source of the nitrogen-carrying portion of the fertilizer. The law requires that the total amount of plant nutrients be shown on the bag. The bag may or may not indicate the source of nitrogen used; ask your dealer or county agent. If the fertilizer contains slow release nitrogen materials, the percent water insoluble nitrogen (WIN) or controlled release nitrogen (CRN) must be stated on the bag. A guarantee that 30% or more of the total nitrogen is water insoluble or controlled release nitrogen indicates a quality turfgrass fertilizer.
Fertilizer Definitions
Complete fertilizer. A complete fertilizer contains the three major fertilizer elements - nitrogen, phosphorus (phosphate), and potassium (potash).
Fertilizer grade. The fertilizer grade designates the percentages of nitrogen, available phosphate, and water soluble potash in the product. A 20-5-10 grade fertilizer contains 20 percent nitrogen, 5 percent available phosphates, and 10 percent water soluble potash. Thus, a 40 pound bag of 20-5-10 contains 8 pounds of nitrogen (20 percent of 40), 2 pounds of available phosphates (5 percent of 40), and 4 pounds of water soluble potash (10 percent of 40). The law requires that the grade be given on the container and always in the order of nitrogen, available phosphates, and water soluble potash.
Fertilizer ratio. The fertilizer ratio indicates the relationship among the percentages of nitrogen, available phosphates, and water soluble potash found in fertilizer. A 10-5-5 grade contains twice as much nitrogen as available phosphates or water soluble potash and has a 2-1-1 ratio. Grade of 16-8-8 and 20-10-10 also have a 2-1-1 ratio. A grade of 12-4-8 has a 3-1-2 ratio. A ratio may be determined by dividing each number of the fertilizer grade by the lowest number of the grade.
Turf type fertilizer. A turf type fertilizer is usually defined as a complete fertilizer having an approximate 2-1-1, 3-1-2, or 4-1-2 ratio containing a minimum of 10% nitrogen, and having 30% or more of the total nitrogen as water insoluble or controlled release nitrogen.
Lightweight fertilizer. Lightweight fertilizers have the nutrients impregnated in or carried on such materials as vermiculite, ground corn cobs, peat, composted garbage or trash, synthetic foams, and similar materials. They often have a higher nitrogen guarantee than normal weight fertilizers, and a given area can be treated with a smaller total weight of material. Many of these materials contain soluble (quickly available) nitrogen rather than the more desirable slowly available nitrogen.
Liquid fertilizer. Fluids in which fertilizer nutrients are in true solution are liquid fertilizers. However, this term is applied also to suspension and slurry fertilizers, which are fluid mixtures containing dissolved and undissolved plant nutrients. Unlike true liquid fertilizers, they require continuous mechanical agitation during application. The value of these materials is based on the grade and weight of the liquid rather than on the fluid content. The law requires the weight to be stated on the container. The plant nutrient content may be calculated in the same manner as dry fertilizers. If, for example, a gallon of 10-5-5 liquid fertilizer weighs 10.5 pounds, then that gallon contains 1.05 pounds of nitrogen (10% of 10.5), .525 pounds of available phosphates (5% of 10.5), and .525 pounds of water soluble potash (5% of 10.5).
Flowable fertilizer. Fertilizer nutrients of extremely small particle size are carried in liquid suspension. Because of the particle size and added suspension agents, they do not require constant mechanical agitation during application.
Water insoluble and controlled
release nitrogen. A fertilizer bag may carry the following label:
In this example, the 20% represents the total percentage of nitrogen contained in the bag. The 8% represents the total percentage of nitrogen in the bag that is water insoluble. The percentage of the total nitrogen that is water insoluble must be calculated from this label information. This value can be obtained by dividing the percentage of water insoluble nitrogen indicated by the total percentage of nitrogen contained in the bag (also indicated on the label) and multiplying by 100. In this case, 8% ÷ 20% x 100, or 40% of the total nitrogen is water insoluble. This fertilizer meets the requirements of a turf grade fertilizer. If the grade were 20-10-10, having 4% water insoluble nitrogen in the bag, the percentage of the total nitrogen that is water insoluble would be 4% ÷ 20% x 100, or 20%, and the fertilizer would not meet the requirements. Calculations are made in the same manner for controlled release nitrogen (CRN).
Types of Nitrogen
The ideal fertilizer program provides uniform growth over the entire growing season. Although this ideal is never fully reached due to temperature and moisture fluctuations, the types of nitrogen carrying materials in a fertilizer are important in moving toward this ideal. Basically, nitrogen materials are divided into two broad groups - quickly available and slowly available.
The quickly available materials are water soluble, and the nitrogen is immediately available to the plants provided there is adequate soil moisture. Results are a sudden flush of growth and a rapid depletion (two to six weeks) of the available nitrogen. It is necessary to make frequent light applications of these materials to obtain uniform growth and to lessen the "burning" potential. Heavy applications of most quickly available sources of nitrogen may cause "burning" (dehydration) of the turf. Quickly available nitrogen materials include ammonium sulfate, ammonium nitrate, nitrate of soda, ammonium phosphate, calcium nitrate, urea, and others.
Slowly available nitrogen materials release a major portion of their nitrogen over relatively long periods. These materials depend on microbial decomposition alone or physical and/or chemical processes in combination with microbial activity to provide nitrogen in a form available to the plant. The activity of soil microorganisms is highly dependent on soil moisture, pH, and temperature conditions. Under high temperatures and adequate moisture supply, microbial breakdown of these materials is accelerated. Under conditions of high temperature and low moisture or low temperatures, the breakdown is much slower. Materials dependent on physical processes (such as hydrolysis) for nitrogen release are relatively unaffected by temperature but are highly dependent on adequate soil moisture. Among the slowly available sources of nitrogen are natural organic materials and synthetic organic materials dependent on microbial decomposition alone, and synthetic organic materials and coated nitrogen products dependent on physical and/or chemical processes in combination with microbial activity.
Natural organic materials include activated or processed sewage sludge, animal and vegetable tankage, manures, soybean meal, and cottonseed meal. Because these natural organic materials vary greatly in their chemical composition, there is a wide variation in the rate of breakdown, although all of them release their nitrogen at a slower rate than the quickly available nitrogen sources.
Ureaform compounds are synthetic materials made by the chemical union of urea and formaldehyde. Within a given ureaform material is a series of chemical compounds with varying degrees of solubility and/or resistance to decomposition. As the ratio of urea to formaldehyde increases, the length of the methylene urea chains or polymers decreases. As soil microorganisms decompose these materials, the short chained compounds break down first, followed successively by each longer chained compound. Thus, a small amount of nitrogen is released over a relatively long period. "Standard" ureaform has a urea:formaldehyde ratio of approximately 1.3:1.0 and contains about 25% water soluble nitrogen and 75% water insoluble nitrogen. These materials may be applied at heavy rates and at relatively infrequent intervals. Currently, there are commercially available methylene ureas having higher urea:formaldehyde ratios (shorter chained compounds) that contain approximately 75% water soluble nitrogen and 25% water insoluble nitrogen. As the amount of water insoluble nitrogen decreases, the application rate must be decreased and the frequency of application increased. Fluid ureaformaldehydes containing short chain methylene ureas are commonly used by the lawn service industry. These materials generally contain quick release nitrogen but do offer safety from "burning" in comparison to other fluid or liquid nitrogen carriers. Some of these fluid materials contain small amounts of slowly available nitrogen. Care must be taken not to confuse urea (quickly available nitrogen) with "standard" ureaform (slowly available).
IBDU (isobutylidene diurea) is an example of a synthetic material that is dependent on hydrolysis to release its nitrogen. IBDU has extremely low solubility in water. Because it is relatively unaffected by temperature, it has the advantage of releasing nitrogen, provided adequate moisture is available, during periods of cool weather when microbial activity is limited. IBDU also has been shown to be more efficient (more of the nitrogen applied is recovered by the plant in the year of application) than natural organic or ureaform nitrogen.
Sulfur coated urea (SCU) is a slow release nitrogen material made by coating urea prills or granules with molten sulfur. Most SCU products also are coated with a sealant such as wax to seal cracks or other imperfections in the sulfur coating. The release of the nitrogen is determined by the thickness of the sulfur coating. Urea particles with varying thicknesses of sulfur coatings release nitrogen over a relatively long period (six to eight weeks). SCU does not qualify under the accepted definition of water insoluble nitrogen and is, therefore, often labeled with a controlled release nitrogen (CRN) statement or with a rate of dissolution statement. The dissolution rate refers to the amount of sulfur coated urea that will dissolve in water in seven days under a set of standard laboratory conditions. If a SCU product has a 30% dissolution rate, then 30% of the nitrogen acts as a quickly available source of nitrogen and 70% as a slow release source of nitrogen. Dissolution rates of 25% to 35% are considered best for turfgrass fertilization. Other coated products use plastic resins, waxes, asphalt, and latex as the coating, but none has shown value for turfgrass fertilization.
Recommended Fertilizer Programs
Proper liming is essential to a sound fertilization program. Lime should be applied in accordance with a soil test. Proper liming creates a favorable soil environment for plant growth and keeps plant nutrients available for plant use. Liming, therefore, provides the most efficient use of applied fertilizer materials.
Again it must be emphasized that a soil test to determine fertilizer requirements provides the best guide for proper fertilization. When soil tests are not used, one of the following recommendations should be followed. These recommendations are for average soil conditions and must be supplemented with additional fertilizer where soils are extremely deficient in phosphorus and/or potash.
Recommendations are given with lesser rates of nitrogen in the spring than in the fall to avoid overstimulation of the grass, which may result in an increased incidence of leafspot disease damage.
* Where the fertilizer contains 30% or more of the total nitrogen as water insoluble or controlled release nitrogen:
Apply in late spring 1.5-0.5-0.5 pounds of nitrogen, phosphate, and potash per 1,000 square feet.
Apply in early fall 2.0 to 2.5-1.0-1.0 pounds of nitrogen, phosphate, and potash per 1,000 square feet.
* Where the fertilizer used contains 15% to 29% of the total nitrogen as water insoluble or controlled release nitrogen:
Apply in late spring 1.0-0.5-0.5 pounds of nitrogen, phosphate, and potash per 1,000 square feet.
Apply in late summer and again in midfall 1.25-0.5-0.5 pounds of nitrogen, phosphate, and potash per 1,000 square feet.
* Where the fertilizer used contains less than 15% of the total nitrogen as water insoluble or controlled release nitrogen:
Apply in midspring and again in early summer 0.75-0.25-0.25 pounds of nitrogen, phosphate,and potash per 1,000 square feet.
Apply in late summer and again in midfall 1.0-0.5-0.5 pounds of nitrogen, phosphate, and potash per 1,000 square feet.
* Where the soil is known to be high in phosphorus and potassium, nitrogen alone may be applied:
Apply in late spring and again in late summer 6 pounds of a ureaform compound (38-0-0) per 1,000 square feet.
or
Apply in late spring and again in late summer 7 pounds of IBDU (approximately 31-0-0) per 1,000 square feet.
or
Apply in late spring and again in late summer 7 pounds of SCU (approximately 31-0-0) per 1,000 square feet.
or
Apply in midspring and again in early summer 20 pounds of a natural organic nitrogen (approximately 5% nitrogen) per 1,000 square feet.
Apply in early fall 40 pounds of the same material per 1,000 square feet.
or
Apply in midspring, again in early summer, again in late summer, and again in midfall 2 pounds of urea (approximately 45% nitrogen) per 1,000 square feet.