Greenhouse Media Re-Use Dustin Morton

Commercial Horticulture Specialist

Greenhouse vegetable culture in Alberta has changed substantially over the last 30 years. From growing in soil and straw bales to sawdust, coconut coir and biochar, the media available has evolved substantially and crop failures have declined accordingly. The early years of greenhouse vegetable production in Alberta are an excellent example of this. Nearly all crops were grown in ground beds (the soil of the greenhouse) and repeated cropping saw entire crops lost to Fusarium wilt and Pythium root rot. The switch to soilless techniques allowed growers to change out their media and reduce this risk but at an economic cost. Currently, it is not uncommon for growers to use the same media once, twice or even three times but there are potential risks. Disease and changes to physical properties are just two of the issues that need to be considered when reusing media to ensure a healthy crop.

Conventional wisdom tells us that whenever we crop something repeatedly, we get a build-up of disease and pests that are partial to that crop. Much research has been done on this topic, as reuse is much more prevalent in Europe, with media sometimes being used for 5 or 6 different crops. Contrary to prevailing wisdom, many of these studies have shown little incidence of increased disease or a negative effect on yield.

Numerous papers and anecdotal evidence use the concept of biodiversity to explain this and the ability of different media to support it. For example, in one paper, rockwool was found to be more conducive to Pythium because of decreased microbial activity and increased water holding capacity. Conversely, some compost-altered mediums were found to foster beneficial bacteria and fungi which suppressed pathogenic microbes. The plant benefited indirectly from the microbial community which had built up over subsequent croppings.

Of further concern to greenhouse growers are the mechanical properties of the substrate. Over numerous cropping cycles, organic based media, such as sawdust and coco coir, begin to break down. This decomposition often means decreased aeration, as small particles wash through the profile and the medium compacts. Ultimately, this compaction means decreased oxygen in the root zone, resulting in unhealthy, stressed plants.

While not immune to compaction, inert media such as rockwool and perlite, have a tendency to retain their original characteristics through multiple croppings. Certain brands of rockwool slabs and bricks are now being designed for multiple uses and perlite has been shown to be quite easy to restore nearly to its original condition.

In fact, in the case of tomatoes grown in perlite, one study actually saw an increase in production, despite multiple uses. In this study, the medium was reconditioned either by stirring or sifting and then subjected to hot water sterilization. Researchers surmised the increase in production came from the salt removal by the water and the baseline level of nutrients that was established in the medium, which can take substantial time in perlite.

A common theme amongst all these media is the possibility of changes to Electrical Conductivity (EC) and pH. Months of irrigation and fertilization can significantly change these properties, making them less hospitable for subsequent crops. Furthermore, chemicals released by decaying root and microbial material may have detrimental allelopathic effects on the plants in production.

As always, monitoring your crop, leachate and medium is imperative to ensuring a healthy crop. Always ensure that you’ve weighed and are comfortable with the risks associated with re-using your media, regardless of the economic benefits. Talking to other growers, research and trialing it in your operation will ensure that you make an educated choice to contribute to your operations bottom line.