Nitrogen applications to golf course turf are essential to provide sufficient growth to recover from intense traffic, to minimize the potential for disease incidence, and to maintain sufficient turfgrass density that minimizes weed encroachment, surface water runoff, and soil erosion. Three areas of N applications that are interrelated need to be addressed to develop a sound N management program:
- Source of N in a fertilizer.
- Rates of application (per application and total annual N applied).
- Timing of applications during the year.
A wide range of N-containing fertilizers is available to the turfgrass manager. These fertilizers generally fall into one of two broad categories:
- Fertilizers that contain only soluble, quickly available N.
- Fertilizers that contain some N in a slow release form that is not immediately available for plant use.
The amount of N fertilizer that can be used in any single application is dependent on the type of N fertilizer, as defined by Maryland regulations.
Water Soluble Nitrogen
Fertilizers with N that can immediately go into solution, and thus have N that is rapidly available for turf uptake, are categorized as water soluble N fertilizers. These fertilizers, while quickly available for turf use, have the most potential for leaching if used improperly.
The most common water soluble forms used for golf course fertilization contain N in the ammonium form (NH+4). Soluble N fertilizers that contain ammonium N include urea, ammonium sulfate, and ammonium chloride. These fertilizers can produce excellent quality turf without leaching or runoff problems if used properly. The ammonium N can be absorbed by the soil, reducing the potential for N movement. Ammonium sulfate can be particularly useful in suppressing diseases, such as take-all patch in young bentgrass, and other common patch diseases of turfgrass, such as spring dead spot in bermudagrass.
Some water soluble N fertilizers contain N in the nitrate (NO3) form. N leaching and runoff potential is much higher for NO3-N than other forms of N. Thus, where conditions exist that are conducive to leaching or runoff, fertilizers that contain significant amounts of NO3-N should not be used. These conditions include sandy sites (sands and loamy sands) with high water tables, times when turf is not actively growing, and sites that are highly sloped. Fertilizers high in NO3-N include ammonium nitrate, potassium nitrate, and calcium nitrate. Fertilizers that contain predominantly NO3-N should only be used on sites not prone to runoff or leaching, where very rapid response is essential, and on turf that is actively growing. Turfgrass uptake may occur within a few days with NO3-N containing fertilizers compared with seven to 10 days with NH4-N fertilizers. Generally, fertilizers containing significant amounts of NO3-N are not recommended for turfgrass fertilization.
Excessive rates of soluble N per application can result in excessive growth of turf (which can eventually affect tolerance to environmental stress and pest resistance) and can increase the potential for N loss through leaching, particularly on sandy soils. As discussed in the Regulatory Considerations section, the 2011 Maryland regulations on turfgrass fertilization limit the application of water soluble N fertilizers to 0.7 pounds actual N per 1,000 ft2 per application.
Slow Release Nitrogen
Slow release N fertilizers contain N in a form that delays its availability for plant uptake after application. It extends N availability significantly longer than a rapidly available nutrient source such as urea. Slow release N fertilizers include sulfur coated urea (SCU), polymer coated ureas, ureaformaldehyde (UF), methylene ureas, isobutylidene diurea (IBDU), and natural organics. To be considered a slow release N fertilizer, the fertilizer must contain at least 20% water insoluble or controlled release N. The N in all slow release fertilizers used for turfgrass maintenance, including natural organics, is ultimately converted in the soil to NH4-N.
Slow release fertilizers are less prone to N leaching and runoff, compared with soluble fertilizers applied in excess of recommended rates. While varying considerably in individual characteristics and release patterns, slow release N fertilizers typically provide more even turfgrass response and provide N for turfgrass uptake over a longer period of time. The use of slow release fertilizers should particularly be considered on sites that are prone to leaching or runoff and when an N application needs to be made to turfgrass during non-optimum growing conditions.
The 2011 Maryland turfgrass fertilization regulations limit the application of slow release N fertilizers to 0.9 pounds actual N per 1,000 ft2 per application.
Natural Organic Nitrogen
Natural organic fertilizers are slow release N fertilizers that are derived from either a plant or animal product and do not contain synthetic materials. They have not been altered from their original state except by physical manipulation (drying, cooking, chopping, grinding, shredding, or pelleting). Most natural organic fertilizers contain P and thus have additional regulations imposed on their application.
Enhanced Efficiency Nitrogen
Enhanced efficiency N fertilizers are a type of slow release N fertilizers that further decrease the potential for nutrient loss to the environment and release less than 0.7 pounds N per 1,000 ft2 per month. If a turfgrass fertilizer is classified as an enhanced efficiency N fertilizer, Maryland regulations allow up to 2.5 pounds of actual N per 1,000 ft2 to be applied in one application, as long as 80% of the annual rate for a given turfgrass species is not exceeded.
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