May 1995

"You Must Be Present to"

Understanding the Slow Release Component

of Slow Release Fertilizers

David M. Kopec, Extension Turfgrass Specialist

In the March issue, the types of fertilizer carriers for nitrogen containing fertilizers was discussed. It included quickly available sources and the slowly available sources. The slowly available types include the slowly soluble types (IBDU, Ureaform products) and the slow release forms (sulphur coated urea, known as SCU and the newer plastic polymer coated products). This months article explains how to determine and understand how much nitrogen is available in the slowly soluble and the slow release types mentioned above. This way you will be better able to select materials and make proper application rates.

The slowly soluble and slow release types are synthesized from urea. There are in fact three percentages (classes) of nitrogen solubility classes. These include the terms:

(1) CWSN - cold water soluble nitrogen, (2) HWIN - hold water soluble nitrogen, and (3) CWIN - cold water insoluble nitrogen.

At a temperature of 77F, it is the CWSN fraction which turfgrasses can absorb. This includes uptake of unreacted urea, and small size chains of methylene urea. Long chains of methylene ureas must be changed by soil bacteria before the turf can use them. Since this microbial activity is temperature dependent, the degree of solubilization is increased as the temperature increases. Thus more of the fertilizer is available during the warmer parts of the year. Therefore, fertilizers applied in the cooler fall and winter (of products like urea formaldehyde) may show limited response until the soil warms up which favors breakdown of the CWIN position of the fertilizer. You can still apply UF in the winter to save time, but it will just be available later.

The activity index AI is a measure of how much of the CWIN that is soluble in hot water.

AI = CWIN - HWIN X 100


The higher the AI value, the more rapidly the nitrogen is solubilized with soil moisture. A UF carrier with a minimum AI of 40% will have sufficient nitrogen solubilization within one year of the application. Also, the urea:formaldehyde ratio is another value which helps understand the solubility of UF type fertilizers. A ratio of 1.3:1.0 (of urea to formaldehyde) has 33% CWSN and 67% CWIN. A fertilizer with a UF ratio of 1.9:1.0, has a CWSN of 67%, and CWIN of 33%. So, the more urea, the greater the (cold water) solubility.

For understanding the release factors for IBDU, note that the CWSN-CWIN-HWIN that is used for UF-type fertilizers does not work here. Solubility of IBDU is only slightly effected by temperature. It is primarily dependent on particle size and moisture of the soil. The dryer the soil, the slower the release (usually not so in turf). The smaller the particle, the faster the dissolution rate for IBDU. Therefore a mixture of IBDU pellet sizes would stagger the release rate.

A common question asked about slow release fertilizers is when to apply them and when you would expect results. Depending on the CWSN percentage in the UF-type fertilizers, late winter spring applications may or may not provide a rapid turfgrass fertilizer response. Treatment with IBDU in late winter may have a greater response then the UF, since IBDU is less temperature dependent, and is more affected by moisture for its release. If however these are applied in the field in the fall, the IBDU should have a good response. The UF will be slower, due to its temperature dependency.

Finally, for the slow release product sulphur coated urea (SCU), the seven day dissolution rate (SDDR) is the index used to measure speed of release for this product.

The SCCR is the percent of the total nitrogen that passes through pore species in the sulphur coating, when the fertilizer is put in water at 95F for seven days. Most SCU fertilizers have a SDDR of 30%. Therefore, initial turfgrass response following an application of SCU is moderate.

In review, slowly available nitrogen carriers include products which include slow initial but long term turfgrass response. The temperature dependency for release is low for IBDU and SCU, while the temperature dependency is high temperatures for UF and natural organic products. All these products generally have a low burn potential, low leaching potential, and slow steady even growth of the turf, given conditions described above.

For those of you who missed the March 1995 article, the quick reference guide for nitrogen carriers is included again on the back of this page.
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


Slowly Soluble

IBDU 31 Mod Low Low Moderate Moderate

Ureaformaldehyde 38 Low Low Very Low Mod to Long

Slow Release

Sulfur Coated Urea 22-38 Low Low Moderate Moderate

Polymer Coated Urea 10-20 Low Low Low Moderate

Natural Organics

Sewage Sludge 6 Very Low Very Low Very Low Long

Other Natural Products 3-10 Very Low Very Low Very Low Long 

Return to Turfgrass Research
[Cooperative Extension] [AgInfo] [UAInfo]

University of Arizona Cooperative Extension
4341 E. Broadway Road
Phoenix AZ 85040-8807
602-470-8086 FAX: 602-470-8092

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.

All contents copyright © 2004. Arizona Board of Regents.