Intense scrutiny of pesticides has forced changes in their use, and concerned citizens who worry about harming our water resources are turning their focus to the application of fertilizers to turfgrass.
Now more than ever, proper fertilization practices are necessary to protect the environment.
In addition, the price of fertilizer has risen over the years, making it a sizeable portion of every turfgrass manager’s budget. Using the right fertilizer at the right time not only maximizes turf performance, but economic efficiency.
What nutrients do turfgrasses need to maintain proper health?
Eighteen nutrients are recognized as essential for plant growth. Three of these (carbon, hydrogen and oxygen) are derived from things like air and water. Six are considered “macronutrients” because the turf uses them in higher quantities. These are nitrogen (N), phosphorus (P), potassium (K), calcium, magnesium and sulfur. Most fertilizer applications to turfgrass focus on nitrogen, with a secondary emphasis on phosphorus and potassium. Generally speaking, nitrogen will produce leaf growth and color, phosphorus aids seed germination and seedling establishment, and potassium aids tolerances to stresses such as traffic and heat. The other macronutrients are occasionally applied if a deficiency is indicated on a soil test.
Micronutrients (boron, chlorine, copper, iron, manganese, molybdenum, nickel, sodium and zinc) are just as important for plant growth, but the plant requires smaller quantities of them. Among the micronutrients, iron is most often applied because turf response to iron is a rapid, but temporary, greening of the leaf tissue. This is because iron is a co-factor in the enzymes that produce chlorophyll, the green pigment in plants. The other micronutrients may be applied if a soil test recommends them.
Nitrogen is key
Turfgrass fertility programs focus on nitrogen because it’s the mineral needed in greatest quantities by the turf and the one for which there is the most dramatic visual quality response. Nitrogen is a component of amino acids, nucleic acids, chlorophyll and molecules like ATP and NADH that are important in photosynthesis. Nitrogen is found in many different forms, but only nitrate (NO3-) and ammonium (NH4+) can be taken up by the plant. The nitrogen cycle is complicated, however, and microorganisms are constantly converting nitrogen into its different forms. For this reason, there is no accurate test of soil nitrogen status. Rather, nitrogen is applied on a predetermined schedule or in response to a loss of color or quality of the turfgrass.
Nitrogen can be lost to leaching, runoff or gaseous loss to the atmosphere, or tied up in forms that are unavailable for plant uptake. Environmentalists are especially wary of runoff and leaching, although research shows that, in establishing turfgrass, leaching is of relatively minimal concern. It is usually not until after a turf has reached a certain age (10 years or so) that leaching due to excessive fertilization can be an issue. So, one advantage of an athletic field is that it basically is in a constant state of turnover, which may reduce long-term leaching potential. Runoff is not a major concern, either, since research shows that core aerification and other typical management practices improve infiltration and greatly reduce the likelihood of fertilizer runoff. When it comes to nitrogen’s gaseous loss to the atmosphere, though, turfgrass managers should be aware that more than 20 percent of surface-applied urea fertilizer may be lost to the air via volatilization. Because of this, fertilizer application should be made prior to rainfall or irrigation (that is not excessive).
Nutrients are lost from turf by removing clippings, since they contain about one-third of the turf’s annual nitrogen needs. Returning clippings to the turf will reduce your annual fertility requirements.
Choose the right nitrogen fertilizer for agronomic goals
Nitrogen comes in many different forms and each has its agronomic advantages and disadvantages (See Tables 1 and 2). Inorganic nitrogen sources are generally less expensive but also have quicker release characteristics because they are easily soluble in water, increasing the chance of fertilizer burn. Organic sources such as sulfur-coated urea, polymer-coated urea and short-chain methylene ureas are slowly available, water-soluble sources of nitrogen. Most blended fertilizers include a mixture of different forms of nitrogen in order to achieve an even release of nitrogen over time. Read and understand the fertilizer label to make sure that it meets the goals of supplying the forms of nitrogen that you desire, as well as other nutrients. For nitrogen, you can either apply larger amounts of a slow release fertilizer less often or smaller amounts of a quick-release fertilizer more often. Either can be effective and depends on your budget and desired turf quality.
Another consideration in addition to the type of nitrogen applied is the fertilizer’s particle size. The Size Guide Number (SGN) is based on the average particle size in millimeters, multiplied by 100. The smaller the particle, the more particles will be applied to a given area, which will result in more uniform coverage of the nutrient. The Uniformity Index (UI) is another important measure. Higher UI means greater particle size uniformity, which results in more uniform fertilizer distribution.
Test your soil to know what other nutrients your turf needs
In almost all cases you should apply a “starter” fertilizer (higher in phosphorus) when seeding. Otherwise, general maintenance fertilizers for sports fields typically have N:P:K ratios of 4-1-2 or 4-1-3. On most soil-based fields, conduct a soil test every one to two years to check on fertility status, especially phosphorus and potassium levels. Sand-based fields should be tested more often. Remember that changes in growth and color of the grass may not be due to a nutrient deficiency. Turfgrass species have different fertility requirements and application schedules. Fertility requirements will vary by location; consult your local extension literature for specifics. They also will vary by cultural practices, use of the turf and environmental conditions. In other words, a perennial ryegrass football field requires more nitrogen than the same species grown as a home lawn.
Apply the right fertilizer at the right time
After choosing a fertilizer, follow the recommended application schedule, which will be influenced by species used, fertilizer used and location. Warm-season turfgrass field fertility recommendations typically are based on a set number of pounds of nitrogen per growing month, which depends on the species utilized. For example, common bermudagrass is typically fertilized at 0.4 to 0.7 pounds N/1000ft2/growing month, while zoysiagrass is fertilized at 0.3 to 0.6 pounds N/1000 ft2/growing month.
Cool-season turfgrass fields are fertilized on more of a set schedule that recognizes that growth is maximized in spring and fall. A typical application schedule for a cool-season turfgrass sports field, where leaching and runoff of nitrogen are not of concern:
- 1 lb N/1000 ft2
- 1 lb N/1000 ft2
- 1 lb N/1000 ft2
- 1.5 lbs N/1000 ft2 with a quick-release source of N
In addition, a half-rate application can be made in early spring and July where turf recovery is required.
On cool-season turfgrass fields, the key is to avoid excessive nitrogen in spring. The plant’s response to nitrogen applied in spring is to produce more leaf tissue at the expense of roots and carbohydrate reserves, which will decrease the grass plants’ stress and pest tolerance going into summer. Early fall applications aid in recovery from summer stress. An application in late fall can be most advantageous to the turfgrass if made when the grass is still green but shoots are no longer growing.
Research has shown that the late fall application is very important in cool-season turfgrass. But there also are concerns about the environmental impact of applying excessive amounts of nitrogen in late fall. To balance these concerns, if you have a high sand field or are in an area that is prone to potential contamination of waterways through leaching or runoff, the late-fall application should be modified so that about 1 pound of N/1000 ft2 is applied in late October/early November, with up to 50 percent in a slowly releasing form.
It is also important to note that correcting nutrient deficiencies with applications of fertilizer may not make the deficiency problem go away. While foliar feeding is a practice for sand-based sports fields, the majority of nutrient uptake is from the soil and, if the root system is poor, uptake is adversely affected. If the turfgrass plant cannot take up the nutrients, simply applying fertilizer will not increase the amount of nutrient in the grass. Management practices that maximize root growth and prevent poor soil conditions, such as drought, compaction or wet soils in spring, will increase the turfgrass root’s ability to take up nutrients.
Last, but not least, make sure to properly calibrate the spreader or sprayer to apply the fertilizer at half the desired rate. Then, apply in two, preferably perpendicular, directions. This will help to ensure that the fertilizer is being applied uniformly and avoid striping. To avoid fertilizer burn, do not apply more than the maximum recommended application rate and avoid applications during unfavorable weather conditions such as high heat or drought.