The Use and Storage of Controlled Release Fertilizers in Nursery Production

Controlled release fertilizers (CRF) are granular fertilizers that have been coated with sulfur or a polymer. These products have been developed to allow a slow release of nutrients into the root zone throughout crop development.  In the following article we will describe several characteristics of CRFs that should be understood and various cultural practices that should be implemented to optimize nutrient use and minimize nutrient leaching when CRFs are used as part of a fertilization program.

How Controlled Release Fertilizers (CRFs) Work 

Nutrients are released out of coated fertilizer prills through osmosis at a rate that is positively correlated with increased temperature.  The release rates are associated with the type and/or thickness of the coating on the fertilizer (prill) granule. Different manufacturers estimate the nutrient release rates of their products under different temperature regimes. For example, some manufacturers test release rates of their products at a constant temperature of 70F, while other manufacturers test release rates at a constant temperature of 82F. Most manufacturers test at a constant temperature between 70-82F. If temperatures go above these thresholds, nutrient release rates are faster and product longevity (length of time some available fertilizer remains in the coated granule) decreases. Likewise, if temperatures are below these thresholds, the nutrient release rates slow down, but the longevity of the products is longer.

CRF Products Available - Longevities

Controlled release fertilizers are available with different longevities. Most manufacturers have products with 3, 6, 9, or 12-month release periods. Products with different longevities are manufactured by blending fertilizer prills of different coating thicknesses: the thinner coated prills release nutrients first and then the thicker coated prills release nutrients later. Shorter term crops would probably require three-month release products and longer-term crops such as woody perennials would require fertilizer products with a 12 month release period. Keep in mind, that if the nursery production facility is near the cooler coast, fertilizer release rates will be slower and the CRF will last longer. Likewise, for hot inland climates, CRFs may release quickly and last for a shorter duration.

Cultural Practices

A. Application Methods

The CRFs can be added to containers via four methods: 1) Topical application; 2) Blended into media; 3) Dibble; and 4) Layered. The primary focus of the application method should be to optimize fertilizer release from the prills in proximity to actively growing roots. Depending on the method used, certain Best Management Practices (BMPs) need to be followed. 

 

1. Topical applications (Figure 1) are done by applying the CRFs to the surface of the containers either by hand or by a dispenser. With this cultural practice, the fertilizer is released from the prills and the nutrients will move down into the root zone from the container surface.
   
   BMP 1: If a fertilizer spreader, rather than a scoop or dispenser, is used to broadcast the prills, either have the containers can-tight and/or sweep up any prills that did not land in the containers.  Otherwise, much of the fertilizer will be lost and nutrients will be released into runoff water.

2. Blending into media (Figure 2)  can be done, especially if a medium is custom blended.  In this process, the fertilizer prills are uniformly mixed into the media. Fertilizer is released throughout the medium.  There are a couple of BMPs that should be considered with this application technique. 
   
   BMP 2:  Minimize CRF prills’ impact with mixers and blades, since cracking of the coatings will result in a rapid release of fertilizer.  Cracked prills will shorten the longevity of the product and increase the likelihood of nutrient leaching, especially during the beginning of production cycle. 
   BMP 3: Prepare only enough media that will be used immediately.  Piles of media will heat quickly and result in an accelerated release of nutrients from the CRF prills.  This may result in crop burn from excess fertilizer salts released into the media and nutrient leaching from the medium when irrigation begins.  

 

3. Dibble applications (Figure 3)  are done by adding a premeasured amount of CRF into holes augured into containers during the canning process.  The plant liner or plug is then placed into the hole.   If automated, this process can save time, money and increase crop uniformity.

Caution: Do not use this method for non-coated granular fertilizers, as quick nutrient release will burn roots.

 

 

 

4. Layer applications (Figure 4) are done by adding media to a container part way, then spreading the CRF to the container, adding the plug or liner of the plant, and adding media to the rest of the container.  This process is practiced in humid climates and is done to prevent nitrogen volatilization, which can especially occur when CRFs are spread on the surface of containers.

B. Irrigation Programs

Even though CRFs offer slow release of fertilizers and can mitigate nutrient runoff, improper irrigation practices can leach nutrients past the root zone, increasing the possibility of nutrient runoff.  Therefore, the following BMPs should be practiced; otherwise, the benefits of using CRFs are lost.

BMP 4:  Do not over irrigate, since excess leaching of nutrients out of the root zone will occur as nutrients are released from the prills.

BMP 5: Be aware of the size of the root ball relative to the container size, as this will dictate how much of the container volume is being utilized for nutrient uptake.

BMP 6:  If using drip irrigation, be sure that the volume of the container is uniformly watered, otherwise only a portion of the container volume will be utilized for root growth and nutrient uptake.

BMP 7:  If the substrate being used has a tendency to become hydrophobic (repels water), be sure that the media is never allowed to dry out, otherwise rewetting is difficult, if not impossible.

BMP 8:  If the substrate is hydrophobic, consider another substrate or utilize a surfactant to increase wettability of the substrate.

BMP 10:  Consider pulse irrigation programs to improve uniform wetting  of the media, which simultaneously increases watering amount, while not increasing leaching out of the container.  The pulse irrigation works, by first providing water that premoistens the medium.   Subsequent irrigation cycles later provide easy, uniform absorption of water from the already  premoistened substrate.

C.  Fertilizer and Substrate Storage

Even though the fertilizer in CRFs is coated in a polymer or sulfur coating, improper storage can still result in fertilizer loss.  Likewise, if CRFs are blended into the media, the product should be used immediately or the substrate should be stored in a cool location.

BMP 11: Store all fertilizers in a cool dry location.  Wet or humid environments will result in caking of granular fertilizers.  CRFs will have volatilization of the fertilizer out of the prills, which can be determined if the prills are found to be hollow. 

BMP 12: If any type of fertilizer, granular or CRFs, is blended into media, do not store the media since granular fertilizers will solubilize and CRFs will start releasing nutrients.  These processes are accelerated as the temperature of the medium increases.