By Dr. Tom Samples
When establishing a wear-resistant and well-rooted bermudagrass sports field from sprigs or seed, timely fertilization and balanced mineral nutrition are just as important as light, water, and appropriate air temperatures.
Photosynthesis. Thanks to chlorophyll contained in chloroplasts in some plant cells (not roots), once leaves emerge from seed or nodes on sprigs, they begin capturing light energy and use it to create compounds that provide energy or are stored in reserve for use by the plant at a later date. Bermudagrasses produce carbohydrates through photosynthesis, the combination of carbon (C), hydrogen (H) and oxygen (O) from carbon dioxide and water in the presence of light. Carbon, H and O make up about 90 to 95 percent of the dry weight of a bermudagrass plant. These three macronutrients are not deficient in bermudagrasses. Bermudagrasses obtain carbon from atmospheric carbon dioxide. Carbon, a component of amino acids, proteins, sugars and starch, is also found in the walls of plant cells. Water provides bermudagrasses with both H and O. In addition to C, organic compounds including carbohydrates, fats, proteins, enzymes and hormones also contain H and O.
Essential Mineral Nutrients. The soil supplies bermudagrasses with essential mineral nutrients. In order to be considered an essential mineral nutrient 1) A plant must be unable to complete its life cycle when the nutrient is not available; 2) The function of the nutrient cannot be replaceable by another nutrient; and 3) The nutrient must be part of an essential plant constituent or directly involved in the physical and chemical events of photosynthesis, respiration, and/or the production and breakdown of organic compounds. Essential mineral nutrients are classified as macronutrients or micronutrients according to their concentration in plant tissue.
The concentration of a macronutrient in dry turfgrass tissue must be greater than
1,000 parts per million (ppm), while the concentration of many micronutrients in dry turfgrass tissue seldom exceeds 100 ppm. Nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg) and sulfur (S) are classified as macronutrients. Of these, N, P and K are primary essential nutrients. Calcium, Mg and S are secondary essential nutrients. Although the amount required by bermudagrasses varies among these six macronutrients, each is equally important. The amount of each primary essential nutrient found in turfgrass tissue in descending order, is N > K > P. The essential micronutrients are boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn) and most recently recognized, Nickel (Ni). Some essential nutrients affect many plant processes while others may be required to activate a specific chemical reaction or are involved in a very limited number of processes within the bermudagrass plant.
Soil Testing. The first step in developing effective and nutritionally balanced bermudagrass ‘grow in’ fertilization and liming plans is soil testing. A soil sample should be submitted for testing several weeks before planting. The primary goal of a soil-testing lab is to accurately predict the pH and the amount of each nutrient in the soil sample being tested that is available to turfgrasses. After drying, grinding and weighing, a solution is used to saturate and extract nutrients from the soil sample. The soil extract is then analyzed by a laboratory instrument (for example, an automated plasma atomic emission spectrometer) to determine the amount of each nutrient present. An automated pH analyzer is often used to determine soil/water pH (WpH).
Soil pH. The pH directly affects the solubility and plant availability of essential nutrients in the soil. Slightly acidic soils (pH range between 6.0 and 6.5) are preferred when managing bermudagrass in native soils. The soil pH decreases with increasing soil acidity. In acidic soils, several nutrients including P, become less available to plants because of reactions with Fe and aluminum that result in the formation of precipitates that are not in a form that bermudagrass plants can use. As the soil pH increases above 6.5, a lack of Mn may limit bermudagrass growth. Similarly, less Fe, Cu and Zn are available for plant uptake at high soil pH levels. However, as the soil pH rises above 6.5, P and Mo may become more available to plants. The amount of lime needed to neutralize acids in a soil depends on the soil’s pH, and the ability of the soil to resist or buffer against a change in pH. Clayey soils have a greater buffering capacity than sandy soils. If lime is recommended, the buffer pH (BpH) will appear on the soil test report along with the WpH. The BpH is a value generated in the laboratory in order to develop specific lime recommendations.
Lime. Lime is available for turf applications in both pulverized and granular forms, and can be purchased in bulk quantities or in bags. Granular or pelletized lime with uniform particles is usually preferred when applying lime with a walk-behind or 3-point hitch, rotary spreader. Calcitic lime is manufactured by grinding rock containing large amounts of calcium carbonate, while rock with a combination of both calcium and magnesium carbonates is used to produce dolomitic limestone. Dolomitic limestone is often about 50 % calcium carbonate and about 40 % magnesium carbonate. Dolomitic limestone is usually recommended when both the soil pH and the available Mg level in the soil are low. Generally, no more than 50 lbs. of lime per 1,000 sq. ft. is recommended per application to established bermudagrass sports fields. However, if necessary, much more lime can be broadcast and tilled or disked into the soil before sprigs or seeds are planted.
Soil test results. A soil test report provides information to assist in the selection and purchase of appropriate fertilizers. The pH of this particular soil is 6.6 and no lime is needed at this time. A Mehlich 1 extractant solution was used to estimate the amounts of P, K, Ca, Mg, Zn, Cu, Fe, Mn and B, which are reported in pounds of nutrient per acre (lbs/A). Procedures to determine the amount of soil organic matter and the level of soluble salts were not performed. This report notes that the soil is low in P (16 lbs/A), low in K (43 lbs/A), and sufficient in Ca (199 lbs/A), Mg (40 lbs/A), Zn (1.5 lbs/A), Cu (10 lbs/A), Fe (10 lbs/A) and Mn (2 lbs/A). The soil also contains 0.2 pounds of B per acre and 10 pounds of sodium (Na) per acre.
Bermudagrass nitrogen requirement. When fully established, bermudagrass usually requires from 0.5 to 1.0 or more pounds of N per 1,000 sq. ft. per growing month depending on weather, soil and the amounts of sand, silt, clay and organic matter in the rootzone. During bermudagrass grow in, the most effective N fertility programs have flexibility built in. For best results, both readily available, highly water-soluble N (WSN) sources and controlled-release N sources may be applied.
Highly water-soluble N. These sources are less expensive per pound of N compared to controlled-release N sources, and stimulate rapid aerial shoot growth and field coverage. Water-soluble N sources, often referred to as agricultural-grade fertilizers are more likely to cause foliar ‘burn’ and leach from the bermudagrass rootzone. To limit the potential for fertilizer burn, no more than 1 lb. of WSN should be applied per 1,000 sq. ft. per application. Urea (46-0-0), ammonium sulfate (21-0-0 w/ 24 % S), diammonium phosphate (18-46-0), monammonium phosphate (11-48-0) and potassium nitrate (13-0-44) are WSN sources. Ammonium sulfate, monammonium phosphate, diammonium phosphate and potassium nitrate are available in granular and in some cases, sprayable forms. Urea is formulated as a solid granule or hollow prill, and may also be applied as a foliar spray.
Once bermudagrass is fertilized with urea, and if the soil is moist, this WSN source reacts quickly (usually within 7 to 10 days) with water to form ammonium-N (NH4+). The naturally occurring enzyme urease speeds this reaction. If bermudagrass is growing in alkaline (high pH) soil, ammonia (NH3) may volatize from ammonium-containing N sources as well as urea. Nitrogen losses by ammonia volatilization as high as 30% have been reported. In addition to highly water-soluble N, the K sources muriate of potash (0-0-61) and potassium sulfate (0-0-52 w/ 17 % S) are also considered readily available after application.
Controlled-release N. Although controlled-release fertilizers have a range of release rates, it is most common that ‘slow-release’ fertilizers contain a minimum of 50 % of the total N in water-insoluble form. Urea formaldehyde and polymer-coated urea are examples of controlled-release N sources. Compared to WSN products, fertilizers containing controlled-release N sources are more expensive per pound of N, and the color responses and rate of bermudagrass growth are more gradual following application. Advantages of using controlled-release fertilizers include less potential for N leaching and foliar burn, and fewer fertilizer applications. Fewer fertilizer applications means there are fewer times during grow-in that irrigation must be interrupted so that the soil can dry and the field can be fertilized. Application rates are often of 1.5 to 2 times those of quick-release fertilizers. Milorganite, a natural organic source of controlled-release N, and is one of the oldest branded fertilizers in the US.
Plant analysis. In addition to soil testing, many laboratories also offer plant analysis to estimate the nutrient content of turfgrass tissue. This test can be very helpful as bermudagrass is approaching or at 100 % total groundcover. Plant analysis serves as a ‘snapshot’ of the nutritional balance in the aerial shoot tissue of bermudagrass. If necessary, nutrients identified as being low or deficient may be applied as a foliar spray. In fact, it is often easier to uniformly apply small amounts of nutrients as a spray. For best results when applying nutrients in solution, it is essential that the spray tips and sprayer operating pressure deliver the intended amount of product and thoroughly moisten the foliage. Interestingly, nutrients enter bermudagrass leaves through tiny cracks or pores in leaves rather than the stomates, and leaf penetration is better at night. To allow for maximum nutrient uptake, bermudagrass should be sprayed during the day and irrigation should be delayed until late in the morning the following day.
Tom Samples, PhD, focuses on turfgrass science and management in his role with the Department of Plant Sciences at the University of Tennessee in Knoxville.