Organic Potting Soil Considerations

Organic Agriculture Centre of Canada

Finding the perfect replicable “soil” for containerized production has been an Achilles heel for many in the organic greenhouse community.  The Canada Organic Standards are explicit that hydroponics and aeroponics are prohibited and “soil” is required. The standard defines soil as “A mixture of minerals, organic matter and living organisms”.

Some research suggests there might be some benefit to using specific materials as the base materials for a compost when striving for particular effect. For example when Allium white rot is a threat onion waste based compost might be of some benefit.

Mainstream producers are also interested in finding cost effective, eco-friendly growing media. Their usual alternatives are rockwool (heated rock spun into fibrous slabs), and peat. Both are fertigated with soluble nutrients and have performed well over the years, but both have their detractors.

Peat must be replaced every production cycle and the industry norm is to replace rockwool every three years. Rockwool is unacceptable in organic systems as it does not meet the “soil” requirement. It also does not breakdown, cannot be recycled, and thus is expensive to dispose off.  Peat is acceptable by organic standards, but on its own as a growing medium does not meet the “soil” requirement either. A bigger issue is the sustainability of the peat moss resource.

The Canadian Sphagnum Peat Moss Association states that less than 1/10th of a percent of peat bogs are harvested annually; and that they are left to rejuvenate before they are cropped again. Critics agree that bogs do recover to a certain degree but they also say there is irreparable damaged done. They further point out that no matter how good the peat harvesting methodology sounds, bacterial activity is stimulated contributing to greenhouse gases.

It surely makes sense to find eco-smart alternatives. Fortunately research is demonstrating that compost is the solution:  it is a renewable resource that should be locally available, it tends to be cheaper than other alternatives, it is as a source of plant nutrients and depending on the compost profile it can suppress various root diseases, and in some cases stimulates growth by the action of Plant Growth Promoting Rhizobacterium (PGPR).

Common root pathogens that can be “managed” with the addition of compost include wilt caused by various strains of Fusarium oxysporum , bacterial wilt of tomato caused by Clavibacter michiganensis subsp. michiganensis, damping off caused by Rhizoctonia solani or Pythium ultimum, root rot by Phytophthora cinnamomi, Cylindrocladium spathiphylli, or Pythium ultimum. Pathogenic nematode populations such as Meloidogyne javanica can also be suppressed by quality compost.  

How it works depends on the pathogen. For example, when the population of beneficial micro-organisms is greater in number Pythium and Phytophthora can’t compete to infect plants roots. Rhizoctonia, a colonizer of fresh organic matter, will not survive in extremely well composted material riddled with Trichoderma harzianum.There are commercial Trichoderma inoculating products available.

Poor quality compost does not have the same pathogen suppression capacity as good quality compost. The ingredient profile makes the difference. For example, hardwood barks have the best disease suppressive properties but can only make up 15% of a potting mix by volume.  Composted pine bark can make up 65-100% by volume. Composted yard waste should be restricted to 15-25% by volume and composted animal manures no more than 15% by volume. If peat moss is the practical option use only the long light fibrous kind to decrease the chance of compaction. The final potting mix must be analysed for physical properties of air space and water retention. Air capacity must be above 20% for most crops and above 25% for crops sensitive to root rot.

It is also essential to monitor and turn compost piles properly during the heating phase and to have a sufficient curing period. A successful curing phase allows sufficient time for the compost pile to be recolonized with beneficial species such as Bacillus, Flavobacterium, Streptomyces, Gliocladium catenulatum, Pseudomonas putida, Trichoderma atroviride and harzianum. It is also critical to keep the outside of the pile moist as the beneficial microorganisms fall out of the air onto the pile to begin the recolonization process.Moist compost piles on the edge of forests recolonize faster than piles further away.

High quality compost can be a cost effective and eco-friendly potting mix alternative to peat moss or rockwool. If well cured it has the potential to suppress various root pathogens.

This article was written by Rochelle Eisen on behalf of the OACC with funding provided by Canada’s Organic Science Cluster (a part of the Canadian Agri-Science Clusters Initiative of Agriculture and Agri-Food Canada's Growing Forward Policy Framework).  The Organic Science Cluster is a collaborative effort led jointly by the OACC, the Organic Federation of Canada and industry partners. For more information: or 902-893-7256.

Posted May 2011