David M. Kopec, Extension Turfgrass Specialist
Nitrogen is the single most overall important nutritional element for controlling turfgrass growth and performance. It is also the limiting element in soils when compared to other macro-elements or secondary elements (which include phosphorous, potassium, calcium, magnesium, etc.). Responsible fertilization includes knowing the nutritional requirements of the grass and being able to supply the turf with those requirements at the proper time in the proper amounts (relatively speaking).
This is where the source of nitrogen (form or carrier) is important. If you do not know the form of nitrogen you are applying, you may not get the anticipated response, waste fertilizer, or cause an undesirable condition. Undesirable conditions can include fertilizer burn, too much growth (scalping) and fertilizer imbalances.
Today's superintendent has many forms of nitrogen fertilizers to select from and they can be used in programs to help provide good overall turf maintenance (grow in overseeding cold weather color retention, transition, regrowth after aerification, dethatching, tee and greens transition, summer bermuda maintenance and bentgrass maintenance). My experience on those items above will be covered in an upcoming article.
Basically, there are two forms of nitrogen classification, which is based on availability of the nitrogen for the turf to use it. It is important to realize that in the soil, total nitrogen can take various forms and that turfgrass plants use the nitrate form in the greatest amounts (NO3-), followed by the ammonium form (NH4+). Under naturally occurring conditions in healthy soils, the ammonium form (NH4+) is converted to nitrate (NO3-) by soil bacteria. Once the initial conversion starts, it is completed rather quickly. Note that "quick release" merely means that the elemental form of the nitrogen component itself is readily dissolved in water and not all quick release forms have "nitrate" (NO3-) in them. Urea nitrogen contains neither of these, but the urea -N- is readily available, because it is readily soluble and dissolves immediately in water.
Slowly available forms (or slow release fertilizers) provide a release of nitrogen over a longer period of time. The cause of the "slow release" is caused by any of a number of mechanisms of the fertilizer. One way this occurs is the form of the nitrogen itself. For example, the natural organic fertilizers have the bulk of their nitrogen in the form of proteins (waste products) of composted or by-product materials. This is similar to the "old compost heap" you became familiar with as a child. The proteins in these products when incorporated into an irrigated soil change to ammonium and then to nitrate. the overall conversion of the proteins to ammonium is a relatively slow process which occurs over time. Thus "slow release". Other mechanisms for slow release include chemical man-made compounds which contain nitrogen that must be converted in the soil to the ammonium and nitrate forms to be usable by the turf. The other way slow release can be achieved is by providing a physical and/or chemical barrier in place for slowing down the release of the nitrogen inside the fertilizer prill. Sulphur coated urea is one example. Both the particle size of the prill and the size of the pores (cracks in the prill) affect the rate of release, and thus control the rate of availability. Larger pore sizes and smaller prill size increase the release, while larger prill size and smaller pore sized (cracks) slow down the release.
Even newer, is the coating of fertilizer pellets with plastic. The plastic thickness (and breakdown of it) and type of plastic used controls the rate of release. There are even products that contain both a plastic coating on top of sulphur coating.
Slow release fertilizers have simple special considerations you need to make before deciding on their use. Large amounts of product can be applied at one application, since some fractional proportion of the total nitrogen will be available at any one time. The potential advantage is even growth, with less numbers of field applications. There is nothing to say that slow release forms cannot be supplemental with an initial or supplemental "quick release application". In other words, one does not exclude the other. Listed below are general characteristics of nitrogen carriers. This is one of the important first steps in what you need to know about successful and responsible nitrogen management for turf.
Characteristics of Nitrogen Carriers
Fertilizer % N Leaching Burn Low Temp. Residual
Source Content Potential Potential Response Effect
Ammonium Nitrate 33-34 High High Rapid Short
Calcium Nitrate 16 High High Rapid Short
Ammonium Sulfate 21 Moderate High Moderate Short
Ammonium Phosphate 27 Moderate High Moderate Short
Urea 45-46 Moderate High Rapid Short
IBDU 31 Mod Low Low Moderate Moderate
Ureaformaldehyde 38 Low Low Very Low Mod to Long
Sulfur Coated Urea 22-38 Low Low Moderate Moderate
Polymer Coated Urea 10-20 Low Low Low Moderate
Sewage Sludge 6 Very Low Very Low Very Low Long
Other Natural Products 3-10 Very Low Very Low Very Low Long
Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona. The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.