The major goal of turfgrass management programs is to produce attractive turf that is healthy and able to withstand the rigors of its intended use. Fertilization is one of the most basic and important components of a turfgrass management program.Turf fertilization contributes greatly to lawn color, density, uniformity and growth. Properly fertilized grass competes with weeds and recovers from damage caused by environmental stresses faster than improperly fertilized lawns. When developing a turf fertilization program, a number of factors need to be considered. These factors include:
Turfgrasses require 16 chemical elements for growth and development. These elements are divided into two main groups based on how they are obtained by turfgrass plants. The first group — carbon (C), hydrogen (H) and oxygen (O) — comes from atmospheric carbon dioxide and water, and makes up most of the turfgrass plant. The second group is composed of minerals taken from the soil or fertilizer applications. This group can further be divided into three sub-groups based on the quantities used by turf plants. The macronutrients — nitrogen (N), phosphorus (P), and potassium (K) — are used in relatively large quantities by turfgrasses. The secondary nutrients — sulfur (S), calcium (Ca), and magnesium (Mg) — are used in somewhat smaller amounts. The micronutrients — iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), boron (B), and chlorine (Cl) — are used in the smallest amounts. Of these mineral elements, nitrogen, potassium, phosphorus, sulfur, and iron are most commonly applied in supplemental form to the lawn.
Determining the natural fertility of the growing site by testing the soil is the first step in developing a turfgrass fertility program. A basic soil test usually includes analysis of soil pH, phosphorus, and potassium levels, but testing labs may or may not include recommendations for the basic soil test fee. Knowing the fertility conditions allows you to make informed decisions regarding other aspects of turf fertilization such as fertilizer application rates and frequencies.
Soil pH adjustment is the second step. The optimum soil pH level for turfgrass growth is 6.0 to 7.0, indicating soil conditions that are slightly acidic. Slightly acidic soil reaction is desirable because it is at this pH that most soil nutrients are most available to growing plants. Over time, the leaching of acidic materials in the soil and applying acidifying fertilizers tends to gradually reduce the pH. Conversely, irrigation water tends to add bases (calcium, magnesium, potassium, and sodium) that gradually increase the pH level.
Periodic soil tests are the only way to monitor changes. Alkaline soils (when pH is above 7.0) are not desirable for turfgrass growth because of the reduced availability of plant nutrients. An alkaline soil problem can sometimes be corrected by applying elemental sulfur according to the guidelines presented in Table N-15. Individual applications of elemental sulfur on established turf should not exceed 5 pounds per 1,000 square feet. The best time for such applications is in the spring or fall, preferably in conjunction with core aeration.
Sulfur reacts slowly, so retest soil pH after 6 to 12 months to measure changes. Unfortunately, the addition of sulfur may not lower the pH in all soils; some soils are highly buffered and resist attempts to lower the pH. Attempting to lower the pH rapidly with acidifying materials on established turf can cause problems. One problem is an excess buildup of soluble salts at levels harmful to turfgrass.
Another problem is deterioration of soil tilth (the physical condition of soil, especially ease of tillage and suitability for root penetration). If hard water is used for irrigation purposes, it may neutralize or partially reduce the acidifying effect of sulfur applications. Acidifying fertilizers, such as ammonium sulfate, can be used in fertilization programs to offset the alkalinity of irrigation water.
Excessively acidic soils (when pH is below 6.0) can be fixed by adding agricultural limestone. The amounts are given in Table N-16. Like sulfur, the amount to add depends on soil type and the desired change. To be effective, sulfur or limestone should be incorporated thoroughly into the soil by tilling. Avoid using slaked lime and burned lime because they are dangerous to handle.
Fertilizer analysis is the percentage, by weight, of nitrogen (N), available phosphoric acid or phosphate (P2O5), and water-soluble potash (K2O) and is represented by the three numbers on fertilizer packages. For example, if you purchase a 50-pound bag of fertilizer with the three numbers 18-6-12, it means you are purchasing nine pounds of nitrogen, three pounds of available phosphoric acid, and six pounds of water-soluble potash.
Other mineral nutrients can be present in the fertilizer and are normally shown on the back of the package. Sulfur, iron, calcium, and magnesium are possible examples of mineral nutrients that may be present. Fertilizer ratio is the proportion of nitrogen, available phosphoric acid, and water-soluble potash in the package. For instance, in the previous example, there are three parts nitrogen to one part phosphoric acid to two parts potash. This fertilizer’s ratio is 3:1:2. In a fertilizer with an analysis of 10-10-10, the ratio is 1:1:1. In a fertilizer with an analysis of 20-5-10, the ratio is 4:1:2. This information is useful because it is often recommended that fertilizers with a 3:1:2, 4:1:2, or 5:1:2 ratio be used as general-purpose turf fertilizers.
Nitrogen is the most important element in turfgrass culture because it is present in larger percentages than other minerals in turf tissues. This large quantity of nitrogen is used by turf for the formation of chlorophyll — a substance necessary for photosynthesis. Nitrogen also comprises portions of plant proteins, amino acids, enzymes, and vitamins. Nitrogen is important for turf development and health. Turf plants primarily absorb nitrogen in the nitrate (NO3-) form, although they can also absorb the ammonium form (NH4+).
Nitrogen fertilization is also important for turfgrasses because it elicits the strongest growth response of any mineral element. Nitrogen mineral fertilization is often used to enhance green color and increase or maintain high density, both of which improve turf appearance. Response to nitrogen fertilization can be quick.
To take advantage of periods of active turf growth, supplying the soil with mineral nutrients is recommended. For cool-season turfgrasses, mineral nutrients should be supplied to correspond to the active periods of growth in the spring and fall. In fact, some turf managers believe that one-half or more of the annual fertilizer application should be made after September 1st each year to take advantage of the active growth that occurs in cool periods. Fertilizing in the fall helps turf plants recover from summer and store mineral nutrients for use during the following season’s growth. Finally, fertilizing at the time of cultivation (such as aerification or dethatching) can help the turf recover more readily.
Warm-season turfgrasses, such as zoysiagrass, bermudagrass and buffalograss, grow actively when temperatures are warmer, usually from mid-spring through mid-fall depending on latitude. Warm-season grasses are usually fertilized at least once per year in the spring at the initiation of growth. Successive applications can be made monthly during active growth.
Turfgrass fertility programs often revolve around the quantity and timing of nitrogen applications. Nitrogen is used in large quantities by turf plants, and because it is rapidly used up, it should be applied to most turfs one to four times per year. Most turf fertility recommendations will indicate the pounds of actual nitrogen to be applied per 1,000 square feet of turf per year.