Liquid vs granular
Fertilizers for turf use include an ever-growing and widely varied selection of products. As a turf manager, you quickly can become awash with information about the many different formulations, analyses and grades of products available for purchase. Among the many choices, formulation-fluid or granular-is one of the most important. How do you decide which is the right product for your needs?
A fluid fertilizer is formulated and packaged as a liquid. This includes fertilizers that are clear liquids (solutions) or liquids that contain suspended solids (suspension fertilizers). Turf managers use fertilizers packaged and sold as fluids less frequently than solids (granules), although such products are more common in agricultural applications. In fact, fluid fertilizers account for about 40 percent of total U.S. fertilizer sales. Examples of fluid fertilizers include anhydrous ammonia (which is actually transported as a fluid and injected into soil in gaseous form), nitrogen (N) solutions (usually made from a mixture of urea and ammonium nitrate), ammonium polyphosphate and triazones.
Solid fertilizers are dry particles that manufacturers size between an upper and lower limit of screen sizes. They may be finely crushed, granular, crystalline, powder or processed into uniform prills. These fertilizers by themselves usually are water-soluble for quick release but often are coated as controlled-release products. Controlled-release products also are called slow-release, slow-acting, metered-release or controlled-availability fertilizers.
Soluble materials Water-soluble fertilizers are rapidly available for turf growth. Examples of common water-soluble turf products include ammonium nitrate (34-0-0), potassium nitrate (13-0-44), ammonium sulfate (21-0-0), potassium sulfate (0-0-50) and urea (45-0-0). Some water-soluble fertilizers are homogeneous products (every particle has the same composition). These have a uniform appearance and are made from blends of raw fertilizer materials such as superphosphate, ammonium solutions, monoammonium phosphate (MAP), diammonium phosphate (DAP), urea, potassium chloride or potassium sulfate. (Not all phosphate fertilizers are completely water-soluble.) Fertilizer bags always list which raw materials the manufacturer used in the specific fertilizer grade in the bag.
Other solid fertilizers are non-homogeneous blends (you can see the individual granules of different fertilizer materials), where the manufacturer simply has mixed particles together to produce a desired overall composition. Non-homogeneous products may not spread as uniformly as homogeneous products, especially if the particles are different sizes. Some products are a mix of soluble and slow-release fertilizers-the bag should list the percentage of each in the product.
*Pros and cons of granular application. Growth and color effects from the application of water-soluble fertilizers are comparatively short-lived, so you need to apply these materials relatively frequently during the growth season, perhaps as often as every 4 to 6 weeks. The rule of thumb is never to exceed 1 pound of N per 1,000 square feet with any single soluble-fertilizer application.
Water-soluble fertilizers produce rapid greening, have a low cost per unit of nutrient, are easy to apply and are readily available from a wide range of dealers. The rapid greening from these fertilizers is due to N, and perhaps sulfur or iron in the fertilizer as well. These products are usually easy to handle and do not take expensive equipment or intensive training to ensure correct application. Regular application of these products may also offer a business bonus-your clients see you at their site frequently.
On the negative side, some solid products can burn foliage if you don't quickly water them into the turf. In addition, they may require repeated applications to ensure continued turf quality and may cause spurts of turf growth, which can present mowing and other management problems. Application of high rates of a water-soluble material (which most experts do not recommend) also can present potential environmental hazards. Although easy to apply, care is necessary with granular fertilizers-we have all seen the all-too-visible effects of an incorrectly calibrated spreader or incorrect application by an inattentive employee!
*Pros and cons of liquid application. Applicators often apply solid water-soluble fertilizers as liquids. They mix the dry fertilizer with water to form a solution and apply it to the turf as a liquid or foliar feed. Liquid application of fertilizer uses a high spray volume (3 to 6 gallons per 1,000 square feet) to move nutrients to the soil, a common application method for many commercial lawn-care companies. Foliar feeding uses a lower spray volume to apply a small amount of fertilizer (for example, iron commonly is applied this way) directly to the foliage, providing rapid uptake of nutrients and quick correction of a nutrient deficiency. Typically, applicators use foliar feeding to supply a small amount of a deficient nutrient or as part of a fungicide application, not to supply all the needed fertilizer for turf growth.
Benefits from using soluble solids as liquid fertilizers include the ability to apply nutrients through irrigation (fertigation), possible use as a carrier for post-emergence herbicides and flexibility of application as a foliar feed. Liquid application of a soluble-solid fertilizer can reduce the risk of foliar burn, provide even coverage and allow simultaneous application of fertilizers and pesticides. You can apply liquid fertilizers at low rates on a frequent basis to spoon-feed turf, promoting even greening and consistent growth. Application of small amounts of fertilizer at regular intervals can prevent over-application, lessening environmental risk.
Negatives may include the cost of new or specialized application equipment and the issues of handling a heavy, bulky, liquid material. Plus, it can be difficult to apply higher rates of nutrients in an appropriate spray volume to avoid burning turf-frequent application becomes the key. However, the need for frequent application can be a problem, especially if labor is in short supply.
Controlled-release materials Although we often think of controlled-release fertilizers as our newest types of fertilizer materials, they are actually our oldest. Slow-release organic fertilizers such as manures have been around for thousands of years, and they remained the only source of crop nutrients until the 1850s. Developments in fertilizer technology in the 1950s and '60s led to the N-fertilizer materials ureaformaldehyde (or methylene urea or UF-all similar products), isobutylidene diurea (IBDU) and sulfur-coated urea (SCU). Soil microbes, soil temperature and the exact chemical ratios in the fertilizer control release of N from UF and related materials. Particle size and soil moisture control release of N from IBDU. Release of N from SCU varies with particle size, moisture and the thickness of the sulfur coating. These materials are still in common use today, and a close look at the label of many granular fertilizer blends reveals that UF or IBDU is the fertilizer that contributes to the slow-release nature of the product.
New technologies have led to the development of resin or polymer-coated fertilizers. Marketed by several companies, these products are usually a coated potassium nitrate, urea or potassium sulfate. Nutrient-release rates depend on factors such as moisture and temperature (depending on the product) and vary with the composition and the thickness of the coating. These fertilizers tend to be uniform in granule size and provide a controlled release of turf nutrients. They are an excellent choice in high-value turf or when frequent application of soluble N is not an option.
On the positive side, use of slow-release fertilizers provides controlled release of nutrients, which creates a long-term, consistent turfgrass response. Because you don't need frequent applications, you save labor. In addition, these products usually have a low burn potential. On the negative side, they do not tend to provide rapid greenup (though some manufacturers mix in some rapidly available N to offset this), and their cost per unit of nutrient is higher than that of soluble sources.
Liquid slow-release products also are available, such as the triazones. These products combine the advantages of using a liquid (such as low burn potential and tank-mixing) with the benefits of a slow-release source of N. However, like all liquid applications, they require the appropriate equipment and the ability to store and handle liquids.
Making the final selection As this discussion shows, all fertilizers have both advantages and disadvantages. The appropriate type for your operation depends on several factors. Consider the following in making your choice of fluid or solid fertilizers: *Do you have the labor to make the frequent applications that soluble liquid or solid products require? *Do you have the equipment (or the budget to buy the equipment) to apply liquid fertilizers in the appropriate spray volume? *Calculate the cost per unit of nutrient; on large expanses of low-maintenance turf, soluble sources may be your cheapest and most effective source of fertilizer. *Is your turf high-value? Consider slow-release products or light, low-rate applications of liquids in times of stress to avoid burning. *Do you need quick greenup or elimination of a visible iron, sulfur or N deficiency? Foliar liquid application may be the best solution. *Think about the environment. Slow-release and properly timed applications of fertilizers can help protect surrounding water supplies. This is important in areas prone to heavy rains or large amounts of surface runoff, especially near wetlands or other environmentally sensitive areas. *Know your goals and needs. Consider soluble sources when you want to encourage quick turf responses and greenup, and slow-release sources to promote consistent, even growth. *Finally, don't pay for complete (N, P and K) fertilizer sources if you don't need them-test your soil for P, K and lime recommendations.
Dr. Beth Guertal is associate professor of soil fertility at Auburn University (Auburn, Ala).
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