October 1994 - Volume I, Issue 10

Easing the Roller Coaster Ride With Slow Release Fertilizer

David M. Kopec, Extension Turfgrass Specialist

There are basically two broad categories of turfgrass fertilizers. 1) Quick release synthetic organic fertilizers and 2) slow release fertilizers. Quick release synthetic organic fertilizers include products such as ammonium nitrate, ammonium sulphate, potassium phosphate, potassium nitrate and calcium nitrate. The general properties of these quick release fertilizers include; high water solubility (quick release), relative high burn potential at excessive rate applications (high salt index), low cost per unit of fertilizer nutrient and readily available for plant use. Mis-applications of these type of fertilizers can lead to excessive growth of shoot systems in turf (increased mowing frequency), potential burning of the turf and possible leaching of the fertilizer. The "quick release" aspect of the synthetic organic fertilizers is one of the reasons multiple applications are scheduled throughout the active growth periods. The use of multiple fertilizer applications is acceptable, however, some alternatives are available which make sound agronomic sense.

Alternatives include the slow release fertilizers. Many people today associate the terms "slow-release" and "organic" or "natural" as synonymous. As you know, they are not.

The following deals with the processes of how slow release fertilizers are made, some of their release properties, and conditions necessary for their successful use.

Slow release fertilizers include products in which the nutrients contained within the product are either slowly soluble, slowly released, or held in a natural organic form (which require mineralization and nitrification in the soil).

Slowly soluble nitrogen forms include IBDU and the methylene ureas. Here the nitrogen is contained and released by either hydrolysis or by microbial activity in the soil. Both hydrolysis and microbial activity control the rate of how fast the nitrogen is available to the turf. Other slowly soluble forms include sulfur coated urea and plastic resin coated fertilizers. The layer of sulfur and the size of the pores in the coating developed during the prilling process determines the solubility of the urea inside. Once water enters the sulfur shell, the urea is dissolved and the nitrogen slowly leaks out. Plastic coated fertilizers have been developed in the last 5 years. The break down of the plastic film determines the rate of fertilizer availability. Although all slow release nitrogen fertilizers have the same goal of limiting the speed of nitrogen to turf, the way they become available differs.

IBDU is an excellent example of a slow release fertilizer in which the mechanism is low water solubility. It is not dependent on soil microbial activity. This is why IBDU is available in colder soils and can be used effectively as a slow release fertilizer for cool season turfs. IBDU applied in the early fall in our state (where Kentucky bluegrass, perennial ryegrass and fescue are grown) benefit both shoot and root growth. These grasses form new roots in all but the coldest parts of the winter. The slow release fertilizers favor root growth over shoot growth at this time (in cool weather).

Slow release nitrogen fertilizers that depend on microbial activity (and warmer soil temperatures) include urea formaldehyde type products. these include methylene ureas and the natural organic sewage sludge and processed manure products.

Urea-form fertilizers are combinations of various methylene-urea polymers such as methylene diurea, dimethylene triurea, trimethylene tetraurea and so on. The longer the polymer, the longer the residual (slower the release).

The last slow release category includes the natural organics, like activated sewage sludge (Milorganite), and processed turkey manure (Sustane). Other organic products can include wheat germ, soya, poultry manure, seed meals, other manures or tank residues. Some include inoculated mixtures of "favorable" bacteria. These products are slow (to very slow) in their nitrogen release and are expensive in cost per unit of nitrogen but do offer low burn and low leaching potential. These types of fertilizers are dependent on soil temperature and biological activity for conversion to ammonium and nitrate forms.

These products offer some flexibility in the application of nitrogen fertilizers for turf especially 1) when fertilizers are applied to large turf areas once or twice a year 2) when labor costs are high and 3) when mowing is accomplished at infrequent intervals (based on labor schedules rather than plant growth responses).

Characteristics of Slow Release Fertilizers

% Leaching Burn Low Temp. Residual

Source N Potential Potential Response Effect

Organic - natural
Activated sewage 6 Very low Very low Very low Long
Manures 3-10 Very low Very low Very low Long
Other natural 3-10 Very low Very low Very low Long

Organic - natural
Urea 45-46 Moderate High Rapid Short
Urea solutions 12-14 Moderate High Rapid Short
Sulfur coated urea 14-38 Low Low Moderate Moderate
Resin coated urea 24-35 Low Low Moderate Long
Isobutylidene diurea 30-31 Mod. low Low Moderate Moderate
Methylene ureas and 38 Low Low Low Mod. long
ureaformaldehyde to long

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