Controlled-release fertilizers (CRFs) offer container nursery growers a convenient and labor-efficient method of crop fertilization. Although CRFs have been around for approximately 40 years -- since the initial development and introduction of the Osmocote fertilizer brand from Scotts --much confusion still exists about what CRFs are, how they work and how to select and use them for optimum results.
Here are answers to the most commonly asked questions about CRFs and suggestions of some best-management practices to help growers better use these fertilizers in their operations.
CRFs are water-soluble prills or granules coated with a polymer overcoat designed to impart a predictable nutrient release over a defined, extended period of time. A grower using CRFs can apply a single dose of fertilizer at the beginning of a crop production cycle and achieve safe, continuous feeding of plants over an entire growing season. By using CRFs, growers can save considerable time and labor compared with soluble fertilizer applications. Furthermore, CRFs can help growers to produce consistently high-quality crops while also minimizing nutrient waste and runoff -- especially in container nursery settings. Many factors impact actual CRF performance, including product selection, application rate, application method and timing.
CRF product/rate selection
There are a variety of CRF products in the marketplace with different coating technologies, but in most cases, nutrient release from the fertilizer prill to the root zone is driven by the temperature within the plant container. Higher temperatures generally result in quicker nutrient release; lower temperatures result in slower nutrient release. Generally, CRFs begin to work when soil temperatures exceed 40°F, with nutrient release accelerating as soil temperatures climb.
CRF product bag labels list longevity ratings based on a standard and constant lab benchmark (e.g., 70°F) to give growers some guidance, but observed longevity (how long the product will actually sustain the crop with nutrients) in any given nursery will depend on the actual container temperatures during the growing season. A product rated as having eight to nine months of longevity at a constant benchmark temperature may only last five to six months in a warm climate or could last 10 to 12 months in a cooler climate. By understanding what these longevity guidelines mean, one can select the best CRF product for a specific growing season, cropping cycle and intended use.
When selecting CRFs for best performance consider these variables:
* Nutrient content of the product. Study the label on the bag. Look at the NPK ratio. Examine the breakdown of nitrogen sources. Does the product include essential elements such as calcium, magnesium, sulfur and micronutrients? What raw material sources are used? CRF product quality can affect crop quality.
* Homogenous nature of product. Is it blended, or are all particles the same? Depending on the crop and container size, blended products can lead to inconsistent fertilizer application and uneven nutrient distribution per pot, as raw material components can segregate in the bag or during application.
* The amount of coated product in the bag. Are all the nutrients in a CRF form or only some? Will all nutrients be available throughout the crop cycle or is a significant portion of the fertilizer in soluble, quickly available form? Fertilizer manufacturers are required to state how much of the total product, including nutrients, is CRF (see the Slow Release Statement on the label). The amount of coated product should impact how you use the product and at what rate.
* The compatibility of plant nutrient needs with the CRF product. If these items are incompatible, plant quality will suffer and nutrients can be wasted. Some CRF products can provide different patterns of nutrient release within a growing season. CRF products should synchronize plant nutrient release with plant demand; otherwise, crop quality suffers. For example, if you are growing a plant that only exhibits one early season flush, and if the nutrient release from the CRF treatment is delayed until after that flush has initiated, you may miss the boat and the whole season’s growth may be affected adversely.
The use of other type of fertilizers in the program, like water-solubles, should be accounted for when applying CRFs.
* Growing media composition. Tighter mixes should have lower rates. Is green bark being used that might tie up nutrients? If so, higher rates might be needed.
* Any special nutritional requirements of the plants.
* Appropriateness of the rate for the crops and intended application. Label rates on a fertilizer bag are generic suggestions to cover a wide range of situations. These suggestions should only serve as starting points for growers. Fertilizer companies may base these values on in-house research or published data, but myriad environmental factors, irrigation water quality, productions practices and crop needs will impact the success of your fertilizer program. Only you can determine the best rate for your operation with well-designed production trials that use controls.
* The effect of increased rates on product longevity. As the dose per pot increases, the point in time at which the CRF cannot sustain the crop is delayed and the feeding longevity is extended.
Application methods
CRFs can be applied in a number of ways. They can be incorporated or blended into the growing media at planting time. They can be top-dressed on the growing media surface or dibbled or sub-dressed into the root zone. Application will depend on grower objectives, available equipment and timing.
Often, incorporation is used when plants are canned up, and this is the most labor-efficient application method. Incorporation can be used with simple pad mixing as well as with more sophisticated soil mix lines; however, the key objective of incorporation is to ensure consistent fertilizer distribution from pot to pot. Include prill counts in your quality-control program to ensure uniform mixing. Mix should not be stored for an extended time after incorporation since the CRF begins to release at this point (especially if soil piles heat up), and high-soluble salts and fertilizer waste will result if piles are not used quickly.
Displacement is another issue many growers ignore when up-potting to larger container sizes. An example would be when moving plants from 3-gallon to 5-gallon containers. While the new soil mix might have the appropriate incorporation rate, the new containers will only receive 2 gallons of fresh media; the rest of the container volume will be displaced by the 3-gallon rootball. In this case, plants will not receive enough fertilizer to sustain the crop. Higher incorporation rates need to be used to compensate for the volume displacement of the old rootball.
Topdressing is used when fertilizer is applied for a second growing season. Product can be spooned on or broadcast depending on the operation, and automated dispensing equipment is available as well. Spillage of top-dressed CRF is an issue when plant containers tip over. Solutions can include switching to a tacky material that won’t spill under these conditions or applying fertilizer into the root zone via dibble, sub-dress or tablets. While these other application methods can be more labor-intensive and riskier from a soluble salts concentration standpoint (depending on rate and product type), they can result in better crops, fertilizer efficiency and reduced environmental impact.
Consult with your fertilizer expert to consider your options, and as always with CRF -- trial, trial, trial to make sure your product selection and use are fully optimized.
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- Fred C. Hulme
Fred C. Hulme is director of technical services, the Scotts Co., (413) 549-8411; www.scotts.com.
March 2008
