A water trough or electric cement mixers can help growers amend small batches substrates.
Photo: Christopher J. Currey

As we head toward the spring growing season, greenhouses are again filled with the diversity of species that characterizes annual and perennial bedding plants. And, with this diversity, we are reminded of the limitations we face in the greenhouse. Looking at the greenhouse environment, we have to pick a single air temperature for an entire greenhouse. Or if we are lighting, all the plants under lights will be subject to the same photoperiod. The same goes for crop culture. While the variety of commercially available products is staggering, and the possibilities when mixing your own endless, many facilities use one or two substrates and fertilizers. When you are limited in substrates and fertilizer selection, look at amending your substrates to take a one-crop-fits-all product and tailoring it before planting. This can save you time during production as you tend to the different needs of each crop. Substrate amendments can be used to improve macro and micronutrient fertilization, as well as modify substrate moisture and irrigation requirements.

One of the biggest ways annuals and perennials vary is in their feeding requirements from “light” (New Guinea impatiens) to “heavy” (i.e. vegetative geraniums). While this can be accommodated by applying stronger concentrations of water-soluble fertilizer, greenhouses can be limited in the number of fertilizer injectors or lines available. Adjustable injectors can provide different fertilizer concentrations from the same stock solution, but the amount of adjusting can become prohibitive, depending on the amount of variety and their spatial arrangement in the greenhouse. Controlled-release fertilizers (CRFs) are one option that can be used to simplify your fertilizer program. While the idea of using CRFs as the sole nutrient source does not appeal to everyone, CRFs can fill a supplemental role in a nutrition program. By adding between one and three pounds of CRF per cubic yard of substrate, depending on the fertilizer analysis and crop needs, you can provide extra nutrients for specific crops within a greenhouse with single or limited WSF fertilizer options.

Crops also vary in their micronutrient requirements. Several species do not require special accommodations, and these are placed in the “general” group. Some species either have a greater requirement for micronutrients or are not efficient at taking them up, and these crops are in the “micronutrient-inefficient” or “petunia/calibrachoa” group. Alternatively, some species are very efficient at taking up micronutrients, and these are “micronutrient-efficient” or “geranium/marigold” plants. This can be managed in one of two ways: 1) the availability of micronutrients; or 2) the concentration of micronutrients. For the “petunia/calibrachoa” group, they are often grown at a lower pH (5.4 to 5.8) to increase the availability of micronutrients in the root zone. However, supplementing with higher concentrations of micronutrients increases the amount of nutrients available to plants even at less-than-ideal pH levels. Conversely, the “geranium/marigold” group is usually grown at a higher pH to decrease the availability of micronutrients in the root zone. Reducing micronutrient concentrations in fertilizers also accommodates these plants. Amendments can help accommodate these differences even when there is only a single substrate or fertilizer. For the “geranium/marigold” group, incorporating some additional limestone in the substrate can raise the pH, limiting the availability of micronutrients. Alternatively, for the “petunia/calibrachoa” group, controlled or slow-release micronutrient fertilizers can be added to substrates. While the pH may still be between 5.8 and 6.2, the increased micronutrients should meet the plants’ needs.

Finally, you can amend your substrate to accommodate moisture requirements. The diversity among crops can create a challenge to meet the exact moisture requirements of each crop and, while a careful eye (or well-calibrated sensor) is the best way to manage irrigation, we can alter the physical properties of substrate to accommodate different crops. For those species that prefer to be grown on the dry side, loosen up the mix by adding additional perlite. The looser mix will provide more drainage and allow the substrate to dry down more quickly. Alternatively, for those crops that grow better with more moisture, add some additional ground sphagnum moss or vermiculite. This will help retain moisture for a longer period of time between irrigations for those water-loving crops.

We have enough constraints on production during our spring, but these amendments can be used to try and simplify your spring production just a bit. The more you can try to accommodate your crops prior to planting, the simpler it will be to grow across your entire range with a simplified approach. And the good news is you won’t need a complete mixing line to make your customized substrates. Something as simple as a watering trough (Fig. 1) or electric cement mixer can be all you need to amend small batches of substrate for big results.

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