Across the United States, a number of greenhouse operations have started growing some of their containerized plants organically, particularly vegetable and herb transplants. This has resulted in an overall 83 percent increase in organic production in greenhouses and nurseries since 2004.

For growers who are considering growing organic, there are many important production and marketing challenges to consider. We surveyed current organic growers in Maine to determine what they considered the greatest production challenges. All growers were, not surprisingly, concerned about managing insects organically in greenhouses. But, the other major concern was how do you utilize organic substrates and fertilizers for consistent and quick plant growth? In this article, we describe how to approach organic substrate and fertility management.

If you decide to produce organic containerized plants, the substrates and fertilizers you utilize must be acceptable within the USDA-certified organic program. One common way to determine if a product is certified organic is to check with an external group called OMRI (Organic Materials Review Institute). OMRI reviews and lists almost all organic products, making it easy to search for certified organic fertilizers and products in one location. However, not all companies pay to have their materials included on this list. So if something is listed as a “USDA-certified organic material” and is not on the list, you are typically okay. Simply check with your certifying agent to ensure it is an allowable product.

Organic Substrates

Almost all the components commonly used in greenhouse substrates are accepted for use in organic production. Even if they are not labeled “certified organic” or are listed as “OMRI approved,” they are accepted as organic due to the nature in which they were produced or harvested. However, some additives in conventional potting mixes, such as synthetically produced fertilizers and wetting agents, are not allowable in organic production. Materials that are always acceptable to use in organic production include: peat, perlite and vermiculite. If you make your own mix in-house out of these ingredients, you will only need to ensure that the fertility you add to the mix is organic.

Another component to consider incorporating into organic substrates is compost. It has a high-water and nutrient-holding capacity. Compost typically contains nutrients, and it will act as a starter fertilizer in the substrate. Some growers have found that using compost improves root growth. One thing to consider with compost is that it retains a lot of water in a pot.

To make sure your mix doesn’t become oversaturated with water, incorporate no more than 30 to 35 percent compost. Including perlite or rice hulls in a compost-based mix will improve drainage substantially. One compost-based substrate we’ve successfully used incorporates peat, perlite or vermiculite, and compost at a 1:1:1 ratio.

Despite the benefits of using compost, it’s important to choose a compost carefully before incorporating it into your substrate. Some composts we’ve tested have a high concentration of non-nutrient salts (higher than 4 mS/cm). We’ve grown basil and marigolds in a variety of compost-based substrates and found that growth was acceptable in only half of those substrates. This could have been due to high salt concentration, low nutrient concentration or lack of maturity in the compost. Before choosing a compost, make sure you test the salt concentration, and grow at least a few plants in the final substrate to ensure that plant performance is optimum.

Because many substrate components are considered organic, both Sun-Gro and Fafard are currently making organic potting mixes. Both of these are similar to their conventional substrates. For example, Fafard Organic contains bark, peat, perlite and vermiculite, similar to Fafard 3B. Its organic mix contains an organic wetting agent and starter fertilizer.

Managing Fertility

Organic fertilizers come from naturally occurring minerals or are derived from plant or animal matter. Some examples of organic-mined materials and the nutrients they supply include: dolomitic limestone (calcium and magnesium), rock phosphate (phosphorus) and greensand (potassium). Some animal- and plant-based organic fertilizers include: alfalfa meal, blood meal, fish emulsion or hydrolysate, seed extracts, poultry litter, seaweed extract and manure composted according to National Organic Program (NOP) guidelines.

Your options for supplying organic fertility are 1) incorporating it into the substrate before transplanting, 2) top-dressing during crop growth and 3) adding liquid organic fertilizers with the irrigation water. While conventional greenhouse growers provide most of their fertilizers in the irrigation water (i.e. constant liquid feed), several of the organic fertilizer components noted above cannot be readily dissolved or suspended in water. Because of this, many vegetable and herb transplant growers incorporate fertilizers into the substrate prior to transplanting. One benefit is that several components can be combined in an attempt to supply all required nutrients.

For example, at Cornell University, we tested some substrates for tomato seedling growth. The base mix contained 70 percent peat, 30 percent perlite and dolomitic limestone. After six weeks, we found plant growth was acceptable in treatments containing conventional fertilizer or 10 percent vermicompost. The largest plants received a combination of vermicompost and blood meal. It should be noted that vermicompost can have high salt levels – a rate of 10 percent by volume worked well for tomato but may be too high for salt sensitive seedlings.

One downfall of relying on substrate-incorporated fertilizers is plants will eventually deplete nutrients. A given mix might be suitable for producing plants over a short cropping cycle, such as four to six weeks, but may not supply enough nutrients for extended growth. Top-dressing with a fertilizer source can supply additional nutrients, but because these are not directly incorporated into the root-zone they may not supply readily available nutrients.

Another option to supply additional fertility is to use liquid-formulated products. Some ingredients include fish emulsion, kelp extract, seed extract and finely ground blood meal suspended in water. Similar to substrate-incorporated fertilizers, some commercial liquid products combine multiple ingredients to develop a balanced source of macro and micronutrients.

One challenge with organic fertilizers is not all the nutrients are provided in a plant-available form. That is, nutrient release depends on naturally occurring microorganisms breaking down complex organic matter into mineral ions that plant roots can absorb. For example, when a conventional fertilizer is used, the entire amount of applied nitrogen is readily available and plants will quickly “green up” following fertilizer applications. However, with organic fertilizer sources, you will need to look at the product label for the percentage of ammonium, nitrate and urea. Plant roots readily absorb these. Any additional nitrogen in the fertilizer should be thought of as “slow release.”

Monitoring Fertility

An additional challenge in using organic fertilizers is that it is more difficult to monitor substrate fertility. In conventional production, general fertility level is monitored by measuring soluble salts with electrical conductivity meters. In conventional fertilizers, all supplied nutrients are dissolved in water as mineral ions. Because some organic substrates are high in non-fertilizer salts, EC monitoring may not directly indicate the amount of available nutrient ions. Therefore, EC guideline charts developed for conventional fertilizers are not directly applicable. Periodic monitoring is still a useful tool – in particular, trends over time can be tracked. When EC values decline over time, this indicates fertilizer ions are being consumed by plants faster than they are supplied. The monitoring of root-zone pH is also useful as it directly affects solubility of nutrient ions.

Another practical approach to monitoring fertility is visually inspecting your crop. Record notes weekly on your plants’ appearance. Have plants grown any larger in the past week? Stunted development, sparse branching and poor leaf growth may indicate that fertilizer has run out. Symptoms of low fertility often manifest first as nitrogen or phosphorus deficiency. Nitrogen deficiency appears as uniform yellowing of lower leaves (Figure 2) whereas phosphorus deficiency appears as purpling of lower leaves (Figure 4). Damage from high salts can cause poor germination or weak growth. In addition, burning of lower leaf edges can occur (Figure 3).

Organic production promises to continue to be a growing sector of the greenhouse industry. While managing substrates and fertilizers organically requires a different approach, it is possible to grow healthy, marketable plants using organic fertilizers. Keep in mind, that as one would do with any new product, be sure to test organic fertilizers and substrates on a small number of plants before using them on the entire crop.