Misconceptions About Controlled—Release Fertilizers

Delivering and managing nutrition is one of the primary tasks associated with producing greenhouse crops. Most growers deliver nutrients using various water-soluble fertilizers, some growers use controlled-release fertilizers (CRF), while others use a combination of controlled-release and water-soluble fertilizers.

As I travel across the country to visit various growers, I’ve come to recognize that many growers would like to consider using controlled-release fertilizers at their facilities but do not understand these products well enough to implement them into their production systems.

With a shift in irrigation and fertility practices, where growers must control leaching and water runoff from their production sites, the use of controlled release fertilizers will allow growers to produce high-quality crops and manage the nutrients that leave their production sites more effectively.

From my perspective, there is a great need to educate growers of the benefits and risks associated with the use of controlled release technology. To help explain the many technical aspects of these products, I’ve teamed up with Goris Passchier from Northern Star Minerals (a division of Wilbur-Ellis Company) for this series of articles. Goris formulates controlled-release fertilizer formulations for growers based on a number of factors, as we will discuss, and helps growers achieve their nutritional goals. 

Pilon: What is a controlled-release fertilizer?

Passchier: The simple answer is, using various technologies, the water-soluble fertilizers you are accustomed to have been modified to slow the release characteristics of the nutrients.

The long answer is we need to make a definition clarification between what is called slow-release fertilizer (SRF) and controlled-release fertilizer (CRF). There is no official differentiation between slow-release and controlled-release fertilizers. Also, the Association of American Plant Food Control Officials (AAPFCO) uses both in its Official Terms and Definitions (AAPFCO, 1997).

However, we can make a simple differentiation, various products, such as Nitroform, Nutralene, Nitamin, Nfusion, NDemand, Coron, N-Sure and Nitro-30, are reacted urea formaldehyde products and are all considered SRFs. Each one has its own characteristic chain of polymers, which are gradually broken off by microbial activity and then made available for roots to uptake. Relatively speaking, the release characteristic of these products are similar regardless of the particle sizes or if they are in a liquid form. Growers should note that there are no slow release reacted phosphorous or potassium products. Most of these products are used in turf grass management and are not the main source of feed in the greenhouse or nursery trades.

The classification of controlled-release fertilizer should be reserved for encapsulated or coated materials. Just think of a coated M&M candy. You have an outer candy coating and the good stuff inside the coating. The same thing applies to CRFs. The outer coating protects the inner good stuff, called nutrients. Now, somehow we need to get the good stuff available to the plant’s root system in a timely manner so the plant can take it up as a nutrient. 

Pilon: How is fertilizer released?

Passchier: Controlled-release fertilizers that have an outer shell like an M&M candy are released by temperature, but moisture does need to be present. Let’s look at a urea nitrogen granule or prill with a guaranteed analysis of 46-0-0 that has been coated in some form or fashion. This coating protects the nutrients in the prill from releasing all at once.

The first thing that needs to happen is for the solid substance to get into a soluble form so it can get out of the prill. This is where moisture comes in. The moisture in the soil is drawn into the covered coated prill. This happens because the nutrients in the prill are in a salt form and water is drawn to salt. Once the water is drawn into the prill, the salt goes into a semi-soluble form (like the chocolate in an M&M candy that gets all gooey.) The rate the fertilizer becomes more soluble inside the prill is related to the temperature around the prill.

Thinking back to the M&Ms, when they are left in a cold refrigerator, you won’t be able to squeeze any chocolate out of them. If the candy is left on the counter at room temperature, you can squeeze some chocolate out of it. And when the candy is left on the hood of a car on a summer day, you more than likely will have a very gooey mess on your hands when you squeeze the candy, since the temperature made the candy very semi-soluble.

As water has entered the covered prill, pressure builds up on the inside and the ingredients in the prill need to get out. The release of nutrients varies depending on the technology used to formulate the coating. 

Pilon: I see there are several CRF products on the market. Do these products consist of the same technology?

Passchier: The basic principle with most CRF products is the same: taking a soluble form of fertilizer and coating it so you can have "controlled release." The technology of the coating and how that coating affects the release is what makes each CRF product somewhat different.

Some prills may crack open at different times so you get instant mini-releases. This form of release is usually characteristic of sulfur-coated products. Other prills begin to look like they are sweating as the salt moves from the inside through the coating membrane to the outside of the prill. Thicker coatings take longer for water to enter the prill. Consequently, the thicker the coating, the longer it takes nutrients to get out of the prill. This type of release is usually characteristic of polymer-coated products, such as Apex, Diffusion, Multicote and Polyon fertilizers.

Osmocote fertilizers use the words resin and polymers somewhat interchangeably. They are the oldest technology. What makes them different in a practical point is that the Osmocote prill swells in the pot where the other product lines show very little swelling of the prill. Whether this swelling of the prill is good or bad for growing plants should come from the grower’s experience with the products.

Another technology uses a parting agent in the coating, which creates micro-wicks that wick out the fertilizer from the inside to the outside of the prill.

With all CRF technology, it is important to remember that once the fertilizer gets out of the prill, it may only become available to microbes and root hairs, if it comes off the prill. There can be prills laying on the soil surface, moisture can be drawn from the soil and fertilizer can be released, but if it is stuck on the surface of the prill, it cannot be utilized as plant food. Consequently, moisture needs to move over the surface of the prill in order to wash the nutrients off the prill and into the root zone. With all CRF technology, it is important to place the prills where water can move over the prills, especially when using drip or micro-irrigation systems. 

Pilon: How are CRFs products similar and how do they differ?

Passchier: CRF products are similar in that soluble fertilizer is usually coated with either a resin or polymer coating. The difference between these products is how the soluble fertilizer comes out of the covered prill. This may occur by controlled cracking of the outer coating at different times, diffusion of nutrients through the semi-permeable membrane or with a chemical agent inside of the coating that acts as a wick.

With polymer-coated products, besides temperature, the thickness of the coating determines the rate nutrients are released. Combining or blending fertilizers with different thicknesses can be an effective method to predictably deliver nutrients to crops over time. Fertilizer blends consisting of various thicknesses give the fertilizer product a unique blueprint of release based on the temperature where the fertilizer is used. As a grower, these blueprints can be the key to your success or failure.

There are many factors growers must consider when deciding to implement controlled-release fertilizers into their production plans. Future installments of this series will discuss various nutrient sources, address numerous factors affecting the release and longevity of these products and help growers pull this information together allowing them to create appropriate and effective CRF programs.

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