Controlled Release vs Slow Release Fertilizer Products in Vegetable Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist;

There has been considerable work on controlled release fertilizer over the years and many of the different technologies have shown potential for use with vegetable crops. Controlled released fertilizer is most useful with nutrients that are subject to leaching losses, particularly nitrogen.

Controlled released fertilizers should not be confused with slow release fertilizers. With slow released fertilizers, release pattern over time is not easily predicted and may be affected by moisture, temperature, and microbial activity. Historically slow released fertilizers have included organic sources that require decomposition and mineralization such as manures, composts, waste products from plant or animal sources, and plant residues.

There are also slow release mineral based fertilizers such as magnesium ammonium phosphate (an N, P and Mg source), clinoptilolite (a natural Zeolite that can be reacted with NH4 which is held tightly and reacts as a slow release N source), rock phosphate (a P source), greensand (a K source), limestone (Ca and Mg source), glass frits (fritted trace elements in a fused glass form), and elemental sulfur. These materials are released upon weathering and dissolution (sulfur is converted to sulfate by microbial action).

An older technology that produces slow release N fertilizer is when urea is combined with an aldehyde. These fertilizers are in liquid or dry forms and include UreaForm (UF), Methylene Urea (MU), Triazone, and IBDU. Longevity depends on length of chemical chain and microbial activity needed to release the N. An exception is IBDU where N is released by hydrolysis at a slow rate and granule size controls longevity. Liquid forms of these products are very useful for foliar fertilization as they have much less injury potential than salt based N sources.

Another older slow release fertilizer technology is sulfur coated urea. This technology was developed at the TVA and commercialized in the 70s. Liquid sulfur coats urea prill then hardens (prills are often then coated with wax. Breakdown and release of the N in sulfur coated urea is by both physical and microbial action and the size of coat determines the release rate.

In contrast to slow release fertilizers, controlled released fertilizers have a predictable release pattern over time that is commonly temperature based. Controlled release fertilizers that are currently used are based on diffusion coatings (polymer and resin coated products). These coatings include thermoset resins, where a fertilizer prill is surrounded by a hardened shell from resins added in multiple layers (such as Original Osmocote), thermoplastics where the prill surrounded by plastic shell with additives to create pores or wicks (such as Nutricote); and reactive layer coatings where a thin polyurethane shell is produced when 2 chemicals react as they are sprayed on the fertilizer prill (such as ESN).

Polymer coatings can be used on most fertilizers and are common in the nursery and greenhouse industries with complete fertilizer products applied to potted plants. Coated product technologies have advanced over the years to give more precise release properties. However, release will still be dependent on the type of coating, the thickness of the coating, as well as temperature and moisture. Controlled release fertilizers are commonly rated as to how long they take to release nutrients in days (70 day, 90 day, 120 day formulations for example). They can also be mixed with a small amount of regular soluble fertilizer to give an initial nutrient charge.

Coast of reactive layer coated urea has decreased over the last decade and this provides an economical opportunity to provide controlled released nitrogen to vegetable crops. These fertilizers increase plant nitrogen-use efficiency by reducing N applied. Use of these products also eliminates need to sidedress or fertigate, giving fuel and time savings. From an environmental perspective there is reduced nitrate contamination from leaching due to the release pattern of the coated fertilizer.

Controlled released fertilizer research in Delaware on vegetable crops has shown equivalent yields to conventional fertilizer in multiple application with the controlled release fertilizer placed before planting (thus eliminating applications). In some cases reduced rates were needed and there was reduced N leaching. Controlled released fertilizers were tested on squash, melons, watermelons, tomatoes, peppers, and strawberries with good results.

Research on the use of ESN polymer coated urea by Cornell on Long Island showed that reduced rates of N could be used on potatoes and sweet corn with equivalent yields.  Recommendations from these studies were not to use straight controlled release but to blend with conventional N sources to provide 75-80% of the total N as controlled release. Overall they were able to reduce total N rates/acre by up to 20% in these vegetable crops.

Because controlled released N fertilizers are more expensive, for the economics to work out they need to be used at 10-25% reduced rates. Research has shown that this is possible on many vegetable crops.