Q&A On Controlled-Release

From Greenhouse Grower

In part four in a series on controlled-release fertilizers, Pilon and Passchier examine the differences between indoor and outdoor production.

By Paul Pilon and Goris Passchier

Pilon: What are the methods controlled-release fertilizers (CRF) are applied?

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

Topdressing entails placing a predetermined amount of fertilizer on top of the growing medium of each container. Generally, topdressing is reserved for second-season crops, but there are some growers who topdress crops just after planting new liners. Care should be taken with this practice to make sure you have a first-season fertilizer and not a second-season topdress fertilizer.

The first method I was exposed to 15 years ago was the "big hand" method. The grower/owner would show the hired help how much fertilizer to put in their hands and then place it on the surface of the container. Needless to say, the size of the hand seemed to make an optical illusion of what was the correct amount. There were very inconsistent amounts applied using this method, even when only one person was making the application.

The spoon method entails using plastic spoons with pre-measured (numbered) quantities depicting how many grams each spoon size contains. This method works pretty well with smaller containers, but becomes less effective on larger containers where multiple spoons per pot are necessary and it is easy for applicators to lose count.

For large containers, many growers use PVC pipes capped at one end that have been cut to hold a specific amount of fertilizer for the desired container size. The PVC pipe is filled with fertilizer and poured over the top of the container. This method ensures the delivery of the proper amount of nutrients without the worry of improper applications due to miscounting when small measuring devices are used.

There have been a number of portable and backpack applicators engineered over the years to help make topdressing more efficient and practical for growers. Some of the most popular applicators include the original Fertil Pak by Great Western Bag Company of St. Louis, Mo., Select-a-Feed and Perfect-a-Feed dispensers available from a number of distributors, the Fertil Dispenser from Simeoni Tecnogreen out of Italy (www.simeonitecnogreen.com) distributed by Agrinomix of Oberlin, Ohio, (www.agrinomix.com) and the GreenElf applicator out of Australia (www.greenelfworks.com).

Some growers use the drop spreader method for topdressing their crops. This method is usually performed over young, small plants or over-wintered crops that are in pot-to-pot space configurations. Several growers have retrofitted various types of drop spreaders or used a Nursery Special Spreader with high clearance from the Gandy Company (www.gandy.net/nursery_sprdr.php3).

Pilon: When producing plants inside greenhouses and in outside production sites, how do I pick an effective rate to use?

Passchier: Understanding CRFs and determining the optimal rates to use is very challenging for most growers.

Many growers find the easiest method is to use the medium rate recommended on the fertilizer's technical sheet. This strategy gets them through most growing conditions, but may lead to excessive or inadequate fertility under certain circumstances.

From my observations, greenhouse growers are most familiar with using parts per million (ppm) of Nitrogen when referring to fertility rates while outdoor producers seem most comfortable with rates of CRFs expressed in pounds of Nitrogen per cubic yard. Each type of grower can produce awesome crops, but each has different cultural practices and their rates of fertility are highly affected by differing cultural practices.

Greenhouse growers tend to utilize growing mixes with a high percentage of peat moss, which holds more water. Their irrigation practices are well managed, reducing the amount of leaching compared to many outside growers. They often have had mixed experiences with CRFs in the past. Why do many greenhouse growers obtain less than favorable results? The answer is simple and painful: the nutrient release was less than or greater than the grower was able to control.

From my observations, the most successful growers are using approximately 1 pound of elemental Nitrogen incorporated per cubic yard. They typically use a NPK ratio of 2:1:2 or 3:1:1, such as the ratios found in Diffusion 17-6-17 GH and Scott's 15-9-12. At 1 pound of elemental Nitrogen per cubic yard, growers need to incorporate 6 pounds of Diffusion 17-6-17 GH or 6.8 pounds of Scott's 15-9-12 per cubic yard. At these rates, many crops will perform very well in greenhouse situations and it still leaves room for tweaking the crop with their water-soluble sources. If the same crops were produced in outside ranges, growers would likely increase the rate up to 1.25 to 1.5 pounds of Nitrogen per cubic yard.

Why the difference in rate between indoor and outdoor production? As stated earlier, temperature determines the release and leaching determines what is left to feed the plant. In the greenhouse, there is often greater control of temperature and leaching than in outside facilities. Generally, outside ranges have greater fluctuations in temperatures and are irrigated more frequently or subject to heavy rainfalls. These temperature fluctuations often result in a lower average temperature, which reduces the release from CRFs. Using slightly higher rates will help compensate for these reduced temperatures. With more irrigation and leaching, it requires more volume of nutrients in the container to keep up with the amount of nutrients that are leached out.

Additionally, the growing mixes used for outside production typically contain a high percentage of bark and little peat moss compared to greenhouse blends. Consequently, these substrates are more open and porous. When a substrate is more open, there is a tendency to hold less water and leach more. With a higher leach fraction, more nutrients are required for sufficient plant growth. If growers could reduce the amount of leaching that occurs, they could reduce the rate of fertilizers needed. Leaching less may sound easy, but it takes exceptional water management to keep the nutrients available to the plant in outdoor sites. I estimate that only 20 out of 100 growers manage their water to the maximum ability. As an industry, we have many opportunities with new innovations and techniques to manage our water and nutrients better.

In the final installment in this series, we will continue our discussion of how the formulation, application method and irrigation practice affect the release of controlled release fertilizers.