Misconceptions With Controlled Release Fertilizers

From Greenhouse Grower

In part five in a series on controlled-release, Pilon and Passchier investigate incorporation rates, temperature and irrigation.

By Paul Pilon and Goris Passchier

Pilon: How do rates vary between incorporation and topdressing?

Passchier: There are two factors involved when speaking about incorporation and topdressing the fertilizer. The first is the potential differences in nutrient availability for the plant when incorporating versus topdressing. The second is the amount of fertilizer that is used when incorporated compared to the topdressing rate.

Let's remember that CRF fertilizers are released by temperature and research shows that different technologies are affected by the temperature changes around the CRF prill that can alter their expected release. Just think about the temperature fluctuations and moisture consistency in the media versus the temperature fluctuation and moisture consistency on top of the media. In general, there is likely to be less temperature fluctuation and more moisture consistency in the pot than on the surface of the pot.

The surface of the media is subject to greater temperature changes between night to day and from cloudy to sunny days. Growers can expect the release of incorporated fertilizers to be more consistent than fertilizers that are topdressed.

Research from Virginia Polytechnic Institute shows temperature fluctuations from 20°C up to 40°C and back down to 20°C in a 20-hour period results in significant differences in release between the CRF technologies. In another experiment, the CRFs were held at 40°C for three days in a flask and then were subjected to constant temperatures for two weeks resulting in a much more consistent release between CRF technologies. From these experiments, we can correlate that logically the temperatures on the surface fluctuates more quickly than in the soil. So depending on the technology, some fertilizers will release more as topdressing than when incorporated.

What is the correlation between topdress and incorporation rates? To answer this, I'll refer to a real life situation. A grower used the same nine-month fertilizer on a crop; some plants were topdressed while others had the CRF incorporated. He followed the recommended rates for each application method on the product's label. The plants that were topdressed were not performing as well as the plants that had CRFs incorporated. It turns out, the recommended topdress rate amounted to half the quantity of fertilizer per pot as applied with incorporation. Even with technology differences, I have never seen a fertilizer that can produce the same crop with the same soil and irrigation practices with only half the fertilizer.

The question then is why is the topdress rate so different than the incorporation rate? Each grower should consider their unique growing conditions, but the answer may have something to do with the above mentioned research. I encourage each grower to become familiar with the rates used for topdressing compared to the rates used for incorporation at their facility. It is important to follow the recommended rates on CRF labels, but growers should understand the differences between these rates and make any necessary modifications.

Pilon: Just considering the role temperature plays on the release of CRFs, the formulation I choose seems critical to a successful fertility program.

Passchier: It requires an understanding of your production practices and how these practices combined with temperature and leaching influence the release and availability of nutrients from these products.

Pilon: Do I choose a formulation based on an average year or do I pick one based on addressing the fertility needs during the adverse or extreme conditions that are likely to occur?

Passchier: Have you ever seen a normal year? Like many aspects of growing, think the process through. Does it make sense or are you just wishing it works? Growers should understand that there is always a possibility for adverse or extreme conditions. No CRF product can release effectively under all circumstances. A good fertility program will have strategies in place to address these extreme conditions. For example, if unseasonably cool weather has reduced the rate of release, growers can conduct soil tests to determine if the nutrient levels are too low and then apply nutrients through foliar feeding or using water soluble fertilizers. Conversely, with excessive temperatures, growers can monitor the crops, and if the salt levels are climbing, they can reduce them by leaching the crop. CRFs do not replace the need for growers to monitor the nutritional status of their crops and make the necessary adjustments on a regular basis.

Pilon: If a fertilizer specifies a six-month release at 70 degrees, what could I expect at 55 or 85 degrees? How would extended periods of these temperatures affect the release/performance of the fertilizer?

Passchier: Most CRF products provide a table, such as the one shown in Table 1, depicting its longevity at various temperatures. Growers should note that each product and formulation will have its own longevity, even when the NPK values are similar.average temperature is higher than it actually is.same rate, it stands to reason that there will likely not be a sufficient quantity of nutrients left in either of these formulations in the long run.

To best determine a product's true longevity, the grower needs to know what the average media temperature is throughout production. As mentioned previously, it is difficult to determine the average media temperature. When measuring the media temperature during the summer months, there are often wide temperature fluctuations that often cause growers to assume their average temperature is higher than it actually is.

The temperature at the prill affects the speed at which the fertilizer goes to a semi-soluble state so it can move through the membrane (coating) or the speed at which the prill cracks and lets the fertilizer out so it becomes available plant food.

The bigger question again is how does the rate of the fertilizer affect the feeding of the plant at the temperature you are growing in? When using 10 pounds of 19-5-8 per cubic yard with media temperatures of 60°F, the CRF may possibly have a nine-month longevity, but the amount of fertilizer available to the plant at this temperature will likely be insufficient for plant growth. Conversely, when using 10 pounds of 19-5-8 at 90°F, the amount of fertilizer being released will likely be excessive.

It is not uncommon for competitive CRF products at various longevities to provide similar results at similar rates. For example, I conducted trials using a competitor's 12- to 14-month product against a five- to six-month formulation with the same amount of product applied per container. In my opinion, both products performed equally well. Achieving equal performance in the short run should cause growers to doubt the longevity of the 12- to 14-month fertilizer and also cause them to question the rate used on the five- to six-month formulation. Logically, if both longevities performed well in the short run at the same rate, it stands to reason that there will likely not be a sufficient quantity of nutrients left in either of these formulations in the long run.

To obtain the best results, growers should utilize formulations that have been developed to fit their geographic area and production temperatures. For example, many growers in the North try to use CRFs developed for production in the South and often do not obtain satisfactory results. CRF formulators can develop blends of fertilizer to match a temperature growing region to help growers optimize the release, longevity and performance for various geographic areas. Unfortunately, when it comes to CRFs, there truly should not be a one-bag-fits-all mentality.

Pilon: What affect do irrigation practices have on the availability of nutrients?

Passchier: The easy answer is everything! Watering practices greatly affect the availability of nutrients to the plant.

Dr. Ted Bilderback at North Carolina State University has done some great research looking at various aspects of irrigation including application timing, methods of delivery, how much to apply and how the architecture of the plant affects the availability of water to the plant. The factors affecting water availability from irrigation practices also affect the availability of nutrients from CRFs. Most importantly, too much irrigation leaches the fertilizer out of the pot and not enough causes the salt levels to build.

As discussed in this series, there are numerous factors and considerations that affect the release of CRFs. Understanding these principles will allow growers to more successfully implement CRFs into production.

Growers should be careful with which formulations they choose, choose an appropriate longevity, use the rates on the products label, have realistic expectations and conduct trials at their facilities to find a formulation that matches their production requirements.