Fig. 1. Controlled-release fertilizers (CRFs) are incorporated into the substrate prior to filling containers and planting to either some or all of the nutrients for a crop. In this substrate mixing line, the CRF is in the small hopper on the right labeled “1.”
Photo: Christopher J. Currey

How do you approach fertilizing your crops? In response to this question, the number of answers is only limited to the number of people answering the question. Regardless of the fertilizer source used to provide mineral nutrients to your crops, fertilizer delivery falls into one of two approaches: 1) pre-plant; or 2) post-plant.

As the name implies, pre-plant fertilization programs are initiated prior to planting crops. One of the most ubiquitous forms of pre-plant fertilization comes in the form of a “starter charge” in your growing substrate. This starter charge is a small amount of fertilizer that is intended to provide nutrients to the crop immediately following planting for a short period of time. Depending on the length of the production schedule, other fertilizers can be added to provide all or most of the crop needs such as calcium nitrate, potassium nitrate, triple superphosphate and magnesium sulfate for primary and secondary macronutrients, and fritted micronutrients for trace elements, though this approach is used less frequently now. The most widely used fertilizers for pre-plant fertilization are controlled-release fertilizers (CRFs). Typically incorporated into substrate prior to planting (Fig. 1), CRFs meter out nutrients slowly throughout production, and their release rate is primarily governed by substrate temperature.

The biggest benefit of utilizing a pre-plant approach to fertilization is the potential to simplify managing mineral nutrition for your crops. There are a myriad of things you need to manage while growing your crops, and by providing your plants with their mineral nutrients before planting, you can check one more thing off your list. I like to refer to it as “crock pot fertilization”— set it and forget it (of course this is an oversimplification, but hopefully you get the gist). However, there are challenges with pre-plant fertilization as well. Fertilizers are provided at the start of a crop cycle and, as we all know, no two seasons are the same. While more fertilizer can be provided to plants one way or another, you cannot remove fertilizer from the substrate when it is incorporated before planting.

Fig. 2. Some greenhouses have multiple fertilizers, each with their own dedicated injector and irrigation line, while other greenhouses may have a single injector and use multiple stock tanks, each with a different formulation to provide various fertilizers to crops.
Photo: Christopher J. Currey

Post-plant fertilization programs are initiated after plants are transplanted. The primary method of applying fertilizers after crops are planted is using water-soluble fertilizers (WSF). The variety in WSF allows growers to have any number of different formulations at their fingertips, like a painter with a wide palate of colors to choose from. Whether you are looking for a fertilizer to raise or lower pH, one with extra calcium or micronutrients, one for cool or warm weather, there is a good chance there is a WSF for you. Some growers have multiple fertilizer lines, each dedicated to a specific fertilizer (Fig. 2, top). However, since fertilizer injectors are widely used and are flexible, changing between fertilizers using a single injector is easy (Fig. 2, bottom). Controlled-release fertilizers may be applied to crops after planting, but this practice should be discouraged unless absolutely necessary. It is not that CRFs are an inappropriate fertilizer to apply after planting; rather, top-dressing is the only approach to applying CRFs post-planting and is a labor-intensive task that should be avoided when possible to minimize additional costs.

Fig. 2. Some greenhouses have multiple fertilizers, each with their own dedicated injector and irrigation line, while other greenhouses may have a single injector and use multiple stock tanks, each with a different formulation to provide various fertilizers to crops.
Photo: Christopher J. Currey

The primary benefit of post-plant fertilization programs with WSF is flexibility: You can respond to greenhouse conditions and crop growth differently with each application. The drawback is that this more dynamic approach to fertilization requires more diligence as a grower, in that you need to determine what to provide your plants as the greenhouse environment and crop growth changes.

Up until this point, I have approached fertilization as a binomial decision, choosing either pre- or post-plant strategies. However, this is a false dichotomy; you can use a combination of both pre- and post-plant fertilization approaches to keep your crops fertilized and healthy throughout production. For example, a smaller amount of CRF such as one or two pounds could be added to each cubic yard of substrate; then, after planting your crops, a WSF at a low concentration like 50 ppm N could be used. This approach of “splitting the difference” between CRF and WSF has the advantage of providing a base-level amount of fertilizer consistently with the CRF, supplementing with WSF with irrigation when the substrate is dry due to water uptake by plants and/or environmental conditions that are conducive to plant growth such as warm temperatures and bright light.

This “split” approach of using both CRF and WSF can also help accommodate species-specific fertilizer requirements for crops grown in the same environment and irrigated from the same source. Let’s use the example of a greenhouse section that has a mix of crops with low and high nutrient requirements. The heavy-feeding crop could be planted in a substrate with pre-plant additions of CRF and the light feeding crop could be planted in a substrate with no CRF, and both could be irrigated with a solution containing WSF; the CRF and WSF together could provide adequate nutrients to the heavy-feeding crop, while the WSF alone would suffice for the light-feeding crop.

Pre- and post-planting fertilizer programs have their own advantages and disadvantages. Each of the approaches appeals to different growers, and a combination of both can be used during production. Take some time this spring to evaluate if your current approach to fertilization is working for you and your crops, as there may be an opportunity to make changes that can benefit both.

Christopher is an assistant professor of horticulture in the Department of Horticulture at Iowa State University. ccurrey@iastate.edu