Fertilize bermudagrass greens smartly and safely

When you intensively manage turfgrass, you must not only be attentive to the requirements of the turf but also to the potential environmental impact of your practices. Fortunately, research indicates that the intensive management practices that produce optimum growth generally produce no negative impact on the environment. This article presents strategies for managing the fertility needs of golf greens with virtually no environmental impact.

Strategy I: Nitrogen management Golf-course superintendents use nitrogen (N) at higher rates on golf-green turf than any other applied nutrient. Although N mobility in a golf-green root-zone mix limits the usefulness of an N soil test, you still must apply N according to the requirements of the plant. In most cases, this is a difficult task considering the influence of temperature, rainfall and play intensity. Generally, 1 to 2 pounds of N per 1,000 square feet every 30 days has been the recommended application rate for bermudagrass growing in a high-sand-content medium. However, you can choose from several possible strategies to achieve this, and some are better than others.

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In research we conducted at the University of Florida, we found that an application of 0.25 pound of N per 1,000 square feet as ammonium nitrate (AN) in granular form every 7 days was not sufficient to produce a uniform, acceptable-quality turf. This is because the low number of fertilizer granules per unit area caused mottling of the turf and increased dollar-spot incidence. At least 0.5 pound of N in granular form per application was necessary for an acceptable-quality response.

On Poa trivialis, an application of 1 pound of N per 1,000 square feet every 28 days as granular ammonium sulfate (AS) produced a higher-quality turf than did an application of 1.5 pounds N per 1,000 square feet every 96 days followed by a spray application of 0.5 pound N per 1,000 square feet every 28 days as potassium nitrate (KN). And spraying a low rate of N (0.125 pound N per 1,000 square feet) as KN every 7 days produced inferior results compared to spraying 0.25 pound N per 1,000 square feet every 14 days. Thus, low N rates on a weekly basis produced inferior turfgrass quality compared to higher rates on a biweekly basis. Apparently, bermudagrass requires a minimum threshold for a uniform response to N fertilizer. Low-quality turfgrass and dollar-spot incidence were persistent problems on the plots receiving a spray application every 7 days.

Transition from cool- to warm-season turfgrass almost always is a problem. Frequent, light spray applications of KN are one management strategy that reduces the disturbance you normally encounter during transition. We found that in combination with either AS or AN in granular form at the equivalent of 1.5 pounds N per 1,000 square feet every 96 days, a spray application of 0.5 pound of K per 1,000 square feet every 28 days as KN produces a smooth transition from cool- to warm-season turfgrass.

* Selection of N fertilizer sources. An array of fertilizer N sources is commercially available for use on golf-course greens. These include the standard soluble sources such as AN, AS, urea and KN. When fertigation is the primary means of N fertilization, these soluble sources are the products of choice. When you apply them at the proper rate on a frequent basis, these materials do an excellent job of maintaining turfgrass quality. However, when applied at the wrong rate or frequency, they may produce unacceptable-quality turf and have a negative environmental impact.

One of the negative aspects of AS is a growth flush, which may occur soon after application. However, AS in combination with controlled-release N sources can provide uniform turfgrass quality and growth over extended growth periods without any negative environmental impact.

Because of the low height of cut-0.125 to 0.10 inch in some ultra-dwarf-bermudagrass maintenance programs-many coated products are too large for use on greens. Certain mini-prill sulfur-coated-urea products are the only suitable coated N sources you can use. Some synthetic methylene-urea (MU) products are small enough for greens use but may not release the N at a sufficient rate during the cool-season growth period. This is because microbial action releases N from MU products, and temperature governs microbial activity. Under cool conditions, microbial activity declines, and the N-release rate drops. For this reason, MU products generally work best during the warm season.

Particle size and moisture influence N release from isobutylidene diurea (IBDU) more than temperature. Therefore, IBDU usually produces a higher-quality, more rapidly growing turfgrass than MU during the cool season (see Figure 1, at left). Additionally, greens-grade IBDU particles are small enough for use on closely mowed turfgrasses. However, conditions of high rainfall, such as happen in the Southeast during the summer months, may diminish the response period for IBDU. Thus, other sources of controlled-release N may produce a longer-term response in the warm season.

You also can use natural-organic-N fertilizers on golf greens with success. Several organic-N materials are commercially available, and most perform well as controlled-release-N sources when you use them in combination with other N sources. As with the MUs, microbial activity mediates N release. Thus, natural organics produce better results during the warm-season growth period. Because microbial action is necessary for N release with nearly all organic-N sources, turfgrass response tends to be similar among such products, except that those containing higher levels of N tend to produce a more rapid and intense green-up than those that contain low or marginal N levels.

*Liquid vs. dry materials. Basically, a pound of soluble N is a pound of N. If you manage two different soluble sources according to the same guidelines, no difference in turfgrass growth and quality should occur. With fertigation, you can apply small N rates frequently to avoid potential N runoff and leaching. However, properly applied, dry granular fertilizers can produce the same results. Research has shown that little, if any, N leaches below the turfgrass root system when you use either N source-liquid or granular-in the proper manner.

*Leaching losses. A high-quality stand of bermudagrass is one of the most N-efficient soil covers in existence. Under proper management, very little N will leach. However, under special conditions, such as heavy rainfall shortly after application, or application to slowly growing or poor-quality turfgrass with a limited root system, N can leach below the root zone. Under these conditions, proper selection of the N source may reduce the environmental impact of the N application. Controlled-release N sources leach less than soluble N sources. N sources such as IBDU and Nitroform leach less N than AS under the same conditions. However, at the proper rate on high-quality turfgrass and under conditions of normal rainfall and irrigation, you would not expect to see large differences in N-leaching losses among N sources.

Strategy II: Phosphorus management Phosphorus (P) is often referred to as the "energy element." It is responsible for a number of energy and genetic transfer reactions within the plant and is not generally noted as a "growth element." Under most conditions, large increases in top growth or color will not occur in response to a P application. Some dark, purplish discoloration may occur in the tillers under conditions of extreme deficiency. The most visible responses to applied P occur during establishment of newly planted turfgrass. More tillering, rooting and matting during establishment are common responses to P application.

*Cool- vs. warm-season response. Growth responses to applied P differ according to the type of turfgrass. In two studies, cool-season turfgrass (ryegrass) responded positively to applied P, even on phosphatic soils. Conversely, P application reduced bermudagrass growth. This related to turfgrass N status. The ryegrass accumulated more N in response to applied P, whereas the bermudagrass accumulated less N. Therefore, on overseeded turf, you should make a P application shortly after establishment of cool-season turfgrasses.

A limited number of P fertilizer sources are available that you can use on golf greens. A greens-grade monoammonium phosphate (MAP) contains N as well as P (11-52-0). Applying 1 pound of P as MAP per 1,000 square feet shortly after establishment should ensure optimum P nutrition for the cool-season turfgrass.

Historically, courses have used sands coated with a layer of iron or aluminum oxide in their greens' root-zone mix. These coated sands react with applied P and aid in its retention. If the golf-green's root-zone mix uses an uncoated sand that is susceptible to P-leaching losses, you may have to give more attention to the P requirements of the warm-season turfgrass. Soil tests effectively determine the root-zone P status.

When you need to apply P, MAP is perhaps the only granular source small enough to avoid mower pickup. Another P source, monopotassium phosphate (MKP), has recently become commercially available. You generally apply MKP as a liquid solution. It compares favorably with other sources of P in promoting growth and P uptake.

Maintenance of adequate P in the root zone of golf greens constructed of uncoated sands without amendments can be a problem. Avoiding poor turfgrass quality in these situations is difficult. Additionally, P is attracting increasing attention today as a pollutant. Therefore, it is important to monitor P applications closely to ensure that you're not losing them to the environment due to inappropriate management strategies. At present, researchers are testing various root-zone amendments and P fertilizer sources to address this problem. Hopefully, some better management strategies will be forthcoming.

Strategy III: Potassium management Many authorities have emphasized potassium (K) fertilization recently. For the most part, recommendations for K fertilization rates have been increasing because many people believe this improves turfgrass cold and drought tolerance, disease and nematode resistance, and rooting. Because K is not a serious environmental contaminant, applicators have freely applied elevated rates in hope of achieving a positive response. It is true that K can enhance top and root growth (see Figure 2, below) when you apply it to a green deficient in K, but additional K (beyond what is necessary to alleviate the deficiency) may not result in additional growth and uptake. One 3-year study applied K at increasing rates from 0 to 7.5 pounds per 1,000 square feet every 90 days. As shown in Figure 3 (above), applying more than 1.5 pounds per 1,000 square feet did not result in an increase in tissue-K concentration. These data suggest that K rates equal to or greater than the N rate do little good. Additionally, in an intensive root-growth-evaluation study, additional K did not enhance root dry-matter accumulation.

Soil labs do not consistently use extractable soil K as a basis for K-application recommendations in the golf-course industry. However, research has shown that this can be a useful basis for determining the need for fertilizer K. When the soil tests very low to low in K, you can expect a response in turfgrass growth and quality from applied K fertilizer. However, sand-based greens typically test low to marginal in K, but this does not justify applying elevated levels of K. You cannot maintain high root-zone K levels in most sand-based greens because of uptake and leaching losses. Regardless, research shows that it is not necessary to maintain medium to high levels of soil K to grow quality bermudagrass. Therefore, excessively high levels of applied K are not effective. Conduct annual or semi-annual soil tests for K fertility.

*K fertility and transition. One common strategy superintendents use to transition from cool- to warm-season turfgrasses is spray applications of KN at a low rate. Application of either granular or liquid formulations of KN alone does not produce as good a result as when you combine it with AS, AN or urea. In one study, a 1.5-pound N application as granular AS every 96 days along with 0.5 pounds K as a KN spray every 28 days produced the best quality turfgrass and the smoothest transition.

*Leaching of K sources. Research has shown that different K-fertilizer sources leach differently. MKP appears to have great potential as a K and P source because of its diminished leaching potential. Researchers are currently evaluating the leaching characteristics of a number of K sources for turf. If proven under real turfgrass-growth conditions, MKP could be an additional management tool for maintaining K fertility.

Strategy IV: Micronutrient management A typical golf green has a pH higher than 6.0, with some approaching 8.0. Golf courses increasingly use effluent water for irrigation, and effluent water generally has a pH of greater than 7.0. Therefore, every irrigation neutralizes soil acidity, and most greens on such courses never become acidic. Under near neutral or alkaline conditions, iron and manganese (Mn) become sparingly soluble, and deficiencies of these two elements may occur. Research has shown that most bermudagrass greens with a pH of greater than 7.0 will respond to an application of Mn.

*Spray applications of iron sulfate (2- to 4-percent solutions) are common practice in golf-green maintenance. Being foliar-applied, you must repeat these spray applications frequently to maintain a dark-green color. Researchers recently have evaluated other iron products as possible sources of soil-applied iron. They've shown that you can reduce the N rate and still maintain turf quality by using a modified iron humate containing 7 percent N and 8 percent iron (7-0-0-8). Additionally, this resulted in excellent transition from cool- to warm-season turfgrass. To date, this product is experimental and unavailable to superintendents, but plans for its commercialization are complete, and the product should be available soon. Thus, superintendents may soon have another product that will fit nicely into their micronutrient-, N- and transition-management strategies.

Dr. Jerry Sartain is professor of soil fertility and turfgrass nutrition at the University of Florida-Gainesville.

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