Controlled-release fertilizers 101

Departments - Production Pointers

There are several key factors for growers to consider before using a CRF.

A CRF applied at different rates and applied on different ornamental plants
Photo courtesy of chrisTOPHER J. currey

Providing adequate mineral nutrientS to greenhouse crops is essential to produce healthy, marketable plants that will perform well for consumers. However, managing fertilizers is just one of many tasks growers have to deal with every day.

Water-soluble fertilizer (WSF) is ubiquitous in greenhouse crop production, and for good reasons. Yet controlled-release fertilizer (CRF) can be a great option, either as an alternative to WSF or as a complement in production.

This month’s Production Pointers will cover some of the important considerations for anyone considering CRFs for their crops.

Controlled vs. slow

First, the focus of this article is on controlled-release fertilizer — not slow-release fertilizer (SRF). It is common to hear these names used interchangeably, but they are indeed different. While both CRF and SRF release fertilizer over time, CRF utilizes polymer coatings, as well as additional chemical treatments in some instances, to release nutrients in a controlled fashion. Alternatively, SRFs rely on the breakdown and degradation of their coating to release nutrients.

As with any fertilizer, the analysis states the percentages, by weight, of mineral nutrients in the CRF. Just like with WSF, there are a variety of different complete fertilizers available to choose from, with nitrogen:phosphorous:potassium (N:P:K) ratios ranging from the standard 1:1:1 to those formulations with less P or with greater K. Formulations containing no micronutrients, as well as those with a complete suite of micronutrients, are also available. Be sure to check that what you are purchasing will meet your needs.

Nutrient release from CRFs can vary in time and pattern. First, CRFs are rated in the duration in time of nutrient release — typically in either months or days. There are a wide variety of release durations, from as short as two months up to one year, and they should be matched with the crop they are going to be used for. The release durations stated are based on a specific temperature, and it is important to keep in mind release will be slower or faster at cooler or warmer temperatures, respectively.

Formulation differences

In addition to the total release period, there are specific formulations that provide a quicker — or slower — start of release to further accommodate grower needs.

When it comes to prill size for CRFs, one size does not necessarily fit all. In addition to the standard — or “regular” — sized fertilizer prills, manufacturers are producing CRFs in smaller sizes. Pound-for-pound, these micro prills have the same amount of fertilizer as larger prills when analyses are the same. However, the smaller prill has advantages compared to the larger prills in certain situations.

Most importantly, micro prills are well-suited for use in small container sizes to help uniformity in fertilizer distribution across containers. Take, for example, a flat of bedding plants in 1204 packs. Using the same amount of fertilizer, by weight, micro prills would result in a more even distribution of fertilizer across the 48 cells in the flat compared to larger, standard prill sizes.

When it comes to CRFs, it can be useful to “think outside the pot”. We are often considering how CRFs can be used to provide mineral nutrients to containerized crops during production. Yet, there are also opportunities to fertilize plants once they have left the greenhouse.

Although we would like to believe consumers fertilizer their plants, I don’t think any of us are under the illusion this is the reality. When CRFs are used to produce plants in the greenhouse, this can potentially provide additional nutrients to plants after they have been purchased by consumers. For example, if a CRF with a 3-4 month release rate is used for producing herbaceous annual bedding plants in 4-inch containers, the length of the production time in the greenhouse is less than the CRF release period, and essential nutrients will continue to be available for consumers.

Consider a blended approach

Using CRFs is also not an all-or-none proposition. Consider combining CRFs in combination with WSF. Using a low rate of CRF, such as incorporating 1.5 to 3 pounds of CRF per cubic yard of substrate, along with low concentrations of WSF allow you to take advantage of the positive attributes of both CRF and WSF, providing plants with consistent baseline nutrition while still maintaining flexibility in fertilizer programs.

Or, along with WSF, use CRFs to provide more nutrients to those heavy feeding plants in your annual bedding plants program.

Controlled-release fertilizer technology has come a long way, and the CRFs available on the market today provide opportunities to simplify production. Remember that CRFs can also be used as a tool to cater to the needs of specific crops in a diverse greenhouse, and potentially provide benefits to consumers.

Christopher is an associate professor of horticulture in the Department of Horticulture at Iowa State University.