by Courtney Llewellyn

Like any product that’s worthwhile, organic growing media has been increasing in quality and popularity over the past few years. Finding the right media for you requires a bit of knowledge.

Many factors may impact a grower’s media selection – the tray size, the crop, the watering method/timing/volume, water quality, the greenhouse environment and nutrient management.

In explaining the medium’s backstory in an eOrganic webinar, Dr. Wenjing Guan, a horticulture specialist at Purdue University, said there’s a lot of variation in organic transplant production to meet producers’ needs. She noted that certified organic production growing media must meet USDA National Organic Program standards, and some of the most common media are perlite, vermiculite, peat and coconut coir. Others contain compost (chicken manure, cow manure, lobster, compost from green wastes and plant-based compost), meals, worm castings, dolomitic lime and calcitic lime, gypsum, potassium sulfate, sand, rock, mycorrhizae, humic substances and biochar. A key attribute of these media is the need to balance aeration, nutrients and water for the plants grown in them – and they all offer different levels of nutrients based on their raw materials.

Some issues have arisen with organic growing media, however. They can need multiple organic amendments with different nutrient combinations, seed germination can be sensitive to salinity, seedling growth may take place in relatively short periods and there is poor understanding of the influence of microbes at transplant and post-transplant growth stages. “Our research can’t answer all these issues, but it can help you better understand the different media,” Guan said.

Dr. Liz Maynard, an assistant professor in Purdue’s Department of Horticulture and Landscape Architecture, looked at plant performance in different media. “With so many media available, it can be difficult to choose which one to use,” she said. “It should be based on chemical, physical and biological results.”

Maynard described a study they undertook in which they tested seven different media on three crops (tomatoes, cucurbits and lettuce) in three Indiana locations, seeded into standard flats. Two of the media included no compost, and therefore were the least expensive (but they had slower emergence and smaller plants as well). Price varies depending on the size of the order and how far it needs to be shipped.

One of the lab tests looked at electrical conductivity (EC), better known as salinity. EC had a strong correlation with plant emergence; a lower EC resulted higher emergence, and vice versa.

The most important thing to consider is germination and emergence, Maynard said. “Ideally, you’ll have high, quick and uniform emergence.”

The researchers observed that media containing compost resulted in larger seedlings than those without. The real test of success, however, is seeing how transplants do out in the fields. Maynard said the tomatoes they measured for four weeks post-transplant showed that larger plants generally flowered and set fruit earlier.

The growing media isn’t everything, though. Organic fertilizers are also necessary. Dr. Petrus Langenhoven, horticulture and hydroponics crop specialist at Purdue, provided some guidance on ways to add nutrients to media – by incorporating them directly into the media, by top dressing and irrigating overhead, or by adding them to the irrigation water. (Nutrients in liquid fertilizer are easily available to plants while solids dissolve or release nutrients slowly.)

The timing of applying fertilizer is critical too. In their experiment, they added fertilizer to media both before planting or as tomato plants emerged and noticed effects on emergence. Generally, they observed delayed emergence when fertilizer was applied before seeding. In most cases, adding fertilizer did not significantly affect total emergence percentage and uniformity of emergence – but when it did, the effect was generally negative (less uniform and less emergence).

“Those without added fertility produced significantly smaller plants,” Langenhoven stated.

Dr. Lori Hoagland, associate professor of horticulture at Purdue, described the importance of media-microbe interactions. “Plants are literally covered inside and out with microorganisms. What exactly are these microbes doing?” she asked. “We know they can do a lot of things. Some can be bad – pathogens that cause disease and food-borne pathogens – but many others can affect metabolism, how they access nutrients and their immune responses.” However, she noted, identifying and managing the “good” microbes is still unclear. Some are vertically transmitted (parent to offspring via seed), while others can be transmitted horizontally (from other plants or even humans) – but what about in growing media?

Growers can inoculate their potting media with a variety of substances for varying results. Arbuscular mycorrhizal fungi can create a symbiotic association between plant roots and certain types of fungi, and they are known for their potential to help plants scavenge for nutrients (especially phosphorus and zinc). These fungi provide other benefits as well, including increased tolerance to water stress and pathogens.

Inoculation with pathogen suppressors, such as Trichoderma harzianum, a fungus also used as a fungicide, can also lead to a potential role in nutrient scavenging. This is a way of providing a biocontrol against pathogens.

Nothing in nature exists in a vacuum, so many other microorganisms are likely to be present as well, Hoagland said. The substrates in potting media and some fertilizers could introduce and support the growth of microbes. These substrates can also indirectly alter microbiomes by influencing root growth and the fluids secreted by plant roots.

What did the Purdue researchers learn from this study? In summary, that tomato transplants are colonized by an abundant and diverse assortment of microbes, regardless of inoculation with specific microorganisms. Potting media and fertilizer do alter the composition of plant microbiomes – and that may lead to an opportunity to establish beneficial microbial communities before transplanting into field environments. Ultimately, proper nutrition is essential for reducing susceptibility to pathogens.

“Know your media before you use it,” Maynard said. “Get the information from the supplier. Is it designed for starting seeds? Will it need additional fertilizer or nutrients? What about moisture levels?” She also suggested testing media when they first arrive at your operation so you’ll know as soon as possible how they will perform.

Growers need to keep in mind that EC is important; fertilizers will enable low-nutrient media to perform well; adequate nutrients can result in larger plants, earlier fruit set and earlier yield, especially in tomatoes, as well as possibly increasing disease resistance; and that media influences the microbiome associated with plant roots.