Growing Media For Organic Greenhouses

The demand for organic food continues to rise in Canada. Given our long winters and year-round consumer demand for fresh, locally-grown vegetables, there is increasing interest in organic green- house produce. However, as of 2019 only six percent of Canadian greenhouses were certified organic. 

Transitioning to organic production may seem daunting for conventional green- house operators, particularly those growing hydroponically and relying on synthetic fertilizers and pesticides. Because the foundation of organic agriculture is soil, hydroponics is not permitted and there are strict requirements for the composition and minimum amount of growing media each plant can access. Fortunately, over the last three Organic Science Clusters (OSC), Canadian researchers have been investigating how to create an ideal growing media and provide a balanced supply of nutrients to greenhouse crops. 

According to the 2020 Canadian Organic Standards, the growing media for container crops and other greenhouse crops (as de- fined in Section 7.5) must: 

• “Contain a mineral (sand, silt or clay, excluding perlite and vermiculite) and biological fraction, which contribute to the physical soil structure;” 
• “Be composed of at least 10 percent by volume of compost (exception: seedling/ starter mixes may contain less than 10 percent compost if needed to ensure adequate germination/rooting);” and; 
• “Contain at least two percent by dry weight or volume (whichever unit of mea- sure is appropriate) of minerals (sand, silt or clay, excluding perlite and vermiculite) at the start of a production cycle.” 

A successful organic greenhouse is a finely tuned system where operators, with the help of soil life, synchronize the release of nutrients from soil amendments with nutrient requirements of the crops. This is particularly difficult for seedlings. As Dr. Dorais states, “the amount of fertilizer that should be added to the medium has to be low enough so the salinity does not cause damage to the developing root system but high enough to sustain plant growth up to transplanting.” 

Salinity, which is measured through electrical conductivity (EC) readings, can inhibit germination and stunt seedlings. A high proportion of compost or other organic fertilizers can lead to high salinity. Salinity is less of a problem when compost is based on plant materials rather than manure. 

One approach is to provide slow-release nutrient sources, such as compost, in growing media and supplement through topdressing and liquid nutrients. Gravel’s team found that in raised bed containers, organic tomatoes grown in a peat-based medium had yields as great as non-organic tomatoes grown on a coir-based medium, but only in the second year the medium was used. In the first year, the plants may have suffered from a lower release of nutrients due to less microbial activity. In the second year, the microbial community was more established. 

Introducing microorganisms to the grow- ing media can help transform nutrients into forms plants can use. For example, Dr. Gravel found that Trichoderma harzianum (Rootshield®) stimulated biological activity. In strawberries, applications of T. harzianum led to higher levels of the polyphenols and anthocyanins (phytochemicals linked with flavour and human health benefits). 

Université Laval’s Pierre-Paul Dion found that different organic fertilizers had differ- ent effects on soil life, which affected how quickly nutrients were released. Alfalfa meal provided slow, long-term N release. In contrast, blood meal and feather meal provided quick, short-term N. Shrimp meal and pelleted poultry manure provided “a better-balanced organic fertilization sup- porting diversified microbial communities and reducing the need for mineral inputs to sustain plant nutritional requirements other than N.” However, half of the nitrogen in the pelleted poultry manure was released quickly; this could damage plants if the application rate is too high. 

Supplying nutrients is one issue; keeping nutrients in the soil until needed is another. This is where biochar can play a role. Var- ious OSC activities have studied how bio- char can retain nutrients (thereby reducing leaching) and stimulate soil life. Adding 15 percent biochar (by volume) to the growing media led to increased yields of sweet peppers and tomatoes. Dr. Vicky Lévesque explains that the improved growth was due to an increase in available nitrogen and phosphorus, and that the biochar led to greater establishment of plant-beneficial bacteria. Efficacy depends on what mate- rials and temperatures were used to create the biochar. 

OSC studies are giving operators more tools to optimize organic greenhouse production. Future OSC research may bring even more needed insights. This can en- able more conventional operators to make the transition to organic production and help meet the ever-growing demand for local, organic food.