Nitrogen: finding the form that fits
The right choice depends on understanding your needs and knowing which products will meet them.
Fertilizers can be confusing. Advertisements frequently tout nitrogen (N) fertilizer as the "slowest release," "the quickest greenup" or "the most available." Add technical terms such as methylene urea, ureaformaldehyde and controlled-release polymer, and learning about nitrogen fertilizers starts to seem like Chemistry 101. Although fertilizers can be a complex topic, some basic information about how they work will help you determine what N sources are right for your operation.
Turf managers who carefully tailor their program to meet their turf's needs throughout the year typically incorporate several products into their operation. Although the variety of available products can be bewildering, most have their place. Your goal is to determine which ones will work for your operation.
Nitrogen sources are often divided into two major groups: water-soluble and slowly available (see table, page 18). Water-soluble N sources contain nitrate, ammonium (or both) or urea. These "quick-release" compounds are rapidly available for plant use (urea must first undergo hydrolysis, which converts it to ammonium). An important point to remember is that the only forms of nitrogen plants use in significant quantities are nitrate (NO subscript 3 superscript -) and ammonium (NH subscript 4 superscript +). If the nitrogen is in some other form, it must be converted to nitrate or ammonium before the plant can use it.
Slow-release sources fall into one of three broad categories:
- Organic N, which must be converted to nitrate and ammonium by microbes.
- Nitrate or ammonium contained by some type of a physical barrier.
- Urea that has been chemically reacted in a way that makes it more slowly available.
We often separate slow-release N sources into two groups; organic slow-release N and synthetic organic slow-release N. (Synthetic organic N fertilizers are "organic" because their chemical structures contain carbon (C), which is the defining characteristic of "organic" compounds. This can be confusing because people often associate the term "organic" with "natural.")
Water-soluble N sources Sources of water-soluble N include ammonium nitrate [34-0-0; NH subscript 4NO subscript 3], urea [46-0-0; CO(NH subscript 2) subscript 2], potassium nitrate [13-0-44; KNO subscript 3] and ammonium sulfate [21-0-0; (NH subscript 4) subscript 2SO subscript 4]. These are all inorganic compounds except urea. However, unlike most other organic types, urea is soluble enough to be considered a quick-release product.
Water-soluble sources are useful in several ways. Soluble fertilizers provide quick turf greenup, which may be important in early spring. Also, you can use water-soluble sources when you want to encourage turf growth, such as during grow-in or to help heal divots or tears on athletic fields. Superintendents who "spoon-feed" their greens typically do so with soluble N sources.
Always apply water-soluble sources at lower rates (0.5 to 1 pound of N per 1,000 square feet) and water them in when possible. This helps avoid the turf burn that can occur with heavier rates of soluble products.
Organic slow-release sources Organic slow-release N sources generally contain some type of waste material. Sometimes the material is composted to help reduce odors, or the material may be dried and granulated to improve handling and spreading characteristics. Common fertilizer waste materials include sewage sludge, poultry litter, meat-processing waste and other animal by-products such as fish or feather meal. Much of the N in such fertilizers is organic N in the form of relatively complex chemical compounds and is not available for plant uptake until microbes have converted it into nitrate and ammonium.
Soil temperature greatly influences microbial activity. In cold soils, little activity will occur - an organic N fertilizer applied during winter in the northern United States will just sit there with essentially no N available for plant use until the soil warms. By contrast, fresh poultry litter applied to turf during hot weather is quickly available as most of the organic N is rapidly converted to nitrate and ammonium.
Some relatively new N fertilizers on the market are blends of organic wastes, such as fish meal, feather meal or poultry litter, and a water-soluble inorganic N such as ammonium sulfate. Such a product would produce a rapid greening response from the inorganic N and an extended response from the organic N. These "hybrid" materials can still burn turf if you apply them at high rates, and the labels usually have a warning to that effect. Fertilizer labels will always tell you the percent water-soluble and water-insoluble (the slow-release part) N contained in the product (see sample label, at left).
Synthetic organic slow-release nitrogen sources Manufacturers produce synthetic organic slow-release N sources in one of two ways. Many products use a physical barrier (coating) around the fertilizer granule to slow release a water-soluble N material. Manufacturers are now able to tightly control the characteristics of the coatings. This allows them to determine N-release patterns with great precision. Other N products are the result of chemical reactions involving urea that create organic compounds with less water solubility and therefore slower N release. Soil temperature and moisture largely determine the release rate of the N from these products.
- SCU. The oldest coated N fertilizer is sulfur-coated urea, or SCU (32-0-0). Introduced decades ago, it still is a common product. SCU is made by spraying molten sulfur onto urea granules. Release of N from the sulfur-coated urea granule depends on the time it takes water and microorganisms to break down the sulfur coating. The thicker the coating, the slower the release rate. Release will be faster in warm, wet soil - conditions that favor microbial activity.
One problem with SCU is that the coating process creates larger granules, which are easily crushed or picked up by mowers. Newer micro-prill technologies have helped solve this problem, and SCU products remain a viable slow-release N source for turf.
- PCU. Polymer-coated-urea (PCU) products have fast become a major part of the slow-release N market. These products work by allowing urea to gradually diffuse through the polymer membrane at a rate that, depending on the exact technology, may vary according to temperature, moisture or coating thickness. These products provide a precise N-release rate, and some can even deliver N for an entire growing season. They are, however, relatively expensive. Some fertilizer blends contain a mixture of water-soluble and polymer-coated products to reduce cost. Again, a quick look at the label will give the percent composition of each in the product.
- Reacted urea. Slow-release fertilizers created by chemical reactions all start as urea. The most common product currently on the market in the turfgrass industry is ureaformaldehyde (UF), formed by reacting urea and formaldehyde. Ureaformaldehyde is also known as methylene urea (MU), and this is the term that fertilizer advertisements typically use.
MU consists of chains of urea and carbon-hydrogen groups. Release gradually occurs as microorganisms break the chains into urea, which is available for plant use. This means that MUs are like organic N sources. If there is no microbial activity, little release will occur. The release patterns of MU products are controlled by the length of the chains - the shorter the chain, the quicker the release. Short-chain MUs are frequently marketed as liquid slow-release materials. Because a variety of names are given to MUs [for instance, the slowest-releasing (longest-chain) MUs are often called ureaform], you should consult the fertilizer label to determine the percent N that is water-insoluble.
- Triazones and IBDU. The two other general groups of slow-release N fertilizers are triazones and isobutylidene diurea (IBDU). A combination of urea and isobutyraldehyde, IBDU does not depend on soil microorganisms for release but is broken down by water into urea. The rate of urea release from IBDU varies with particle size, temperature and moisture. The smaller the particle, the faster the release. The higher the temperature, the faster the release.
Triazones are cyclic compounds that contain ammonia. These are sold as a liquid slow-release nitrogen material.
The materials I discussed above account for nearly all available turf (and ornamental) fertilizers. As you can see, the basic categories aren't really that numerous or difficult to understand. With variations in nutrient ratios, coating types, type and proportion of slow-release N and other characteristics, you can see how the number of possible (and actual) products can become so large.
So how do you pull all this information into a coherent plan for selecting a fertilizer? By carefully defining your needs and then determining which fertilizers best fit them. The needs of a golf-course superintendent will differ from those of a park manager, whose needs will differ from those of a lawn-care operator. Many variables can be important - labor, cost of materials, method of application, aesthetics and agronomic goals. Thus, it's impossible to say that any particular product is best. Many commercial products use a combination of slow- and quick-release sources to provide the benefits of each type. Likewise, turf managers often use several different products to help achieve their overall management objectives.
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