AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Winter 1996, No. 6
Nitrogen fertilizers used on turf may be classed as quick release or slow release. Common quick release sources include ammonium sulfate, urea, and ammonium nitrate. These sources tend to cause flushes of growth that last for only a few weeks. Therefore, the most effective way to apply quick release nitrogen fertilizers is by "spoon feeding" in frequent applications. Slow release nitrogen...SRN...sources provide a way to avoid the "boom and bust" growth cycles encountered with the more soluble sources.
There are many slow release sources of nitrogen commonly used on turf. An understanding of the mechanisms of release of nitrogen from these sources and the factors that affect that release is essential when selecting the best source for a specific situation. Slow release nitrogen fertilizers are characterized by an extended response period, low burn potential, and a relatively high cost per unit of nitrogen when compared with the quick release sources. Following is an overview of common slow release nitrogen sources and their characteristics.
Coated Nitrogen Sources
Sulfur Coated Urea (SCU): Release of nitrogen from SCU results from the movement of water through tiny cracks and pinholes in the sulfur coating. Water penetrates the coating and dissolves the urea core, resulting in release. Coating integrity, thickness and uniformity are important in determining the value of this source.
Polymer Coated Urea (PCU): This source consists of a urea granule encapsulated by a polymer coating. Water diffuses through the coating and the urea core begins to dissolve. The dissolved urea then diffuses out through the membrane. Temperature affects the diffusion controlled release of nitrogen from PCU. One manufacturer reports a doubling of release rate with every 18°F temperature increase. Thus, nitrogen release from PCU in the summer will be more rapid than in the fall or spring.
Sulfur and Polymer Coated Urea: The outer polymer coating is added to SCU to protect the integrity of the granule and to slow the movement of water into the urea core. This material provides the cost advantage of SCU and the improved release characteristics of PCU.
Synthetic Organic Nitrogen Sources
Isobutylidenediurea (IBDU): The release of nitrogen from IBDU is controlled primarily by hydrolysis, or reaction with water. This source is especially effective in cool seasons since temperature is not a critical factor affecting nitrogen release.
Urea-formaldehydes (UF): These materials consist of organic molecules of varying size and solubility. The release of nitrogen from UF sources is controlled by microbial breakdown. Nitrogen release from UF will be more rapid in warm months than in cool since high temperatures accelerate microbial activity.
Natural Organic Nitrogen Sources
There are many other nitrogen sources available for use on turf. These include sewage sludge and animal manures. Nitrogen is released from these materials by soil microorganisms. Factors such as temperature, soil pH, and moisture influence the rate of nitrogen release from the non-synthetic organic sources.