Talking Temperature

Pilon: How does temperature affect release?

Passchier: As we discussed previously (see the July 2007 issue), controlled release fertilizers can be compared to M&M candy since the good stuff is inside of the coating of each of these products. As the temperature increases so does the solubility of the candy and fertilizer on the inside. The main difference is the inside of the coated prill of the fertilizer contains some form of salt. Moisture needs to be present around the coated fertilizer prill in order for moisture to be drawn inside allowing the solid fertilizer salt to turn into a semi-soluble state.

As the temperature increases, so does the solubility and the pressure inside the prill. Consequently, with higher temperatures and increased internal pressure, it requires less time for the fertilizer to come out of the prill and be available for plant uptake. The trick to formulating and using CRFs is to have the release rate from the coating meet the feeding rate of your plant. This sounds pretty simple in theory, but is not that simple, considering growers are faced with various factors, such as media type, fertility requirements of various crops and the source and quantity of water applied, that may affect the availability and performance of these products. 

Pilon: In regard to temperature, do I monitor soil temperature or air temperature? If temperature is the main factor affecting release, why is this concept so misunderstood by our industry?

Passchier: Many growers feel it is important to match the fertilizer release to the temperature of the soil. I often ask growers, "What value is there to monitoring the soil or air temperature?" If the soil is too hot, how are you going to cool it? If the air temperature is too hot, how will you cool it? Instead of monitoring things we cannot control, I feel it is more important for growers to have a general understanding of their ambient growing and soil temperatures in relationship to the type of CRF being used.

I have seen some growers make decisions regarding the type of fertilizer they purchase for various container sizes based on measurements of soil temperatures taken from a single-sized container. This can be problematic since it is not uncommon for a small, one-gallon container to reach up to 104°F (40°C) on a sunny day, while a large 25-gallon tree container at the same nursery may only get up to 77°F (25°C). Growers tend to be conservative and do not want to risk burning their crops with high production temperatures. Using this example, the grower above would likely pick the fertilizer that will release the correct amount at 104°F to reduce the potential of burning the crops with high salts from excessive release under high production temperatures. This thinking may work well for one-gallon containers, but more than likely the crops in 25-gallon containers will exhibit signs of nutrient deficiencies.

The concept of fertilizer release and temperature is often misunderstood and consequently CRFs can be improperly chosen and used by commercial growers. Perhaps this misunderstanding originates from within our own industry since most technical sheets display release or longevity based on a continuous temperature. For example, a given fertilizer might provide four months of release at 70°F. This longevity is more than likely based on a laboratory test of putting fertilizer is a sand column or in flasks of distilled water at a constant temperature. In reality, when used in a greenhouse or nursery setting, the rate of fertilizer release can be very different than stated on a tech sheet. There is definitely a difference of release pattern when comparing the temperature fluctuations of a potted crop compared to testing procedures using constant temperatures such as in distilled water or using a sand column. 

Pilon: What is longevity or the release pattern?

Passchier: Longevity refers to the duration of time a fertilizer is designed to release at specific growing temperatures. The release pattern refers to the release curve of nutrients at a given time in the longevity. For example, there could be two fertilizers that release 2 pounds of nitrogen per cubic yard of growing mix over a six month period of time, but one formulation may release 0.40 pounds of nitrogen per cubic yard in the first month and the other formula may only release 0.10 pound of nitrogen per cubic yard.

Success does not come easy in understanding controlled-release fertilizer language. It has always been hard for me to understand when a technical sheet states the longevity at 60°F is 10 to 12 months and at 90° the longevity is seven to eight months, but the recommended rate of 10 pounds per cubic yard to incorporate never changes. As I travel across the country, growers commonly ask for a fertilizer by its longevity. For example, a grower might request longevity of nine months, yet in reality depending on temperature, the acceptable release (sufficient for proper plant growth) of a nine month formulation is often around six months.

There are a few people in the country that understand how to tweak a fertilizer’s formulation to meet the growers needs and expectations. Diffusion fertilizers are different from most other CRF products with formulations designed for a specific temperature area. For example, if a grower requests a formulation with 2 pounds of nitrogen per cubic yard to release in a five-month time period, the formulation engineered for a production site in Alabama would be different from a formulation designed for a facility in Minnesota since the production temperatures and the rate of plant development are very different. Growers should really look at their specific production temperatures when determining which CRF product or formulation will provide the best results. 

Pilon: It seems various fertilizer labels use different temperatures to determine the longevity of a formulation. What do I need to know?

Passchier: As stated above, the longevity on a fertilizer’s label or technical sheet is most likely determined using laboratory testing at a constant temperature. Depending on where the testing occurred, the longevity is commonly determined using 70˚F, 76˚F or 86˚F. There is currently no commercially accepted standard temperature that all fertilizers use to determine longevity, so read the labels carefully. Growers need to use the longevity as a guideline and not as an absolute, since temperatures fluctuate daily within a pot and temperature extremes from year to year are likely to occur. It is important to pick a formulation that works under normal (average) conditions but will provide adequate results at slightly less than average production temperatures and offer safety to crops when production temperatures rise above normal during the production cycle. 

Pilon: What are the methods CRFs are applied?

Passchier: The two most common methods of applying controlled release fertilizers in greenhouses and nurseries is to incorporate them into the growing mixes prior to planting (incorporation) or placing the fertilizer on top of the potting substrate (topdressing).

Incorporation implies that a predetermined amount of fertilizer is evenly distributed within the growing medium prior to planting. There are several methods used commercially to incorporate fertilizers into the growing medium.

The cement mixer method entails blending the correct amount of fertilizer with the corresponding amount of potting substrate using a cement mixer. The growing mix and fertilizer are turned for several minutes until the CRF is uniformly incorporated.

A few growers incorporate fertilizers into their growing mixes using a manure spreader. With this method, a predetermined amount of media is placed into the spreader, the appropriate amount of fertilizer is added over the media and more growing mix is added. When the manure spreader is turned on, it slings the contents into a wall or deflected into a pile using custom guards. When completed, the result is a good blended media and fertilizer combination.

The cement skid steer front loader method entails placing the appropriate quantity of growing mix on a cement pad, adding the appropriate fertilizer on top of the pile, and turning the pile with a front loader until the pile is uniformly mixed. Growers should note that some CRF coatings could be compromised due to the abrasion of the cement pad when mixing and sporadic or unpredictable release of nutrients is likely to occur.

Many growers with specialized potting equipment install fertilizer hoppers over the belt that deliver soil to the potting lines. In addition to adding controlled-release fertilizers into the growing mix, several growers add additional nutrients such as dolomitic limestone or micronutrients using a separate hopper for each nutrient being added. When calibrated and functioning properly, these hoppers provide growers with a reliable and effective method of incorporating nutrients into the growing media.

These are examples of some of the most common methods of incorporating fertilizers into the growing mixes; other innovative methods are commonly practiced. Regardless of the method used to incorporate CRFs, it is important to monitor and verify that the proper quantities have been incorporated. It is not uncommon to have too little or too much fertilizer in the soil or to have inconsistent distribution of the prills.

An easy check system is to have a clear, one-gallon container with the correct amount of fertilizer and media in it and do a visual check as often as you think it is necessary. Another very tedious and time consuming method is to do a prill count per quantity of fertilizer per media amount. There may be as many as 20,000 to 30,000 prills per pound of fertilizer, and that is an awful lot of counting. Fortunately, there are some laboratories (www.mmilabs.com and www.qal.us) that are set up to perform prill counts for growers.

In the next issue, we will continue our discussion of application techniques, the role of temperature on the release of CRFs and why growers should consider their irrigation and leaching practices when developing a reliable CRF program.

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