Taken from Hortidaily article, July 23, 2014:

Should you reuse your greenhouse media?

While reusing greenhouse media makes economic sense, disease and changes to physical properties are just two of the issues that need to be considered.

“Greenhouse vegetable culture in Alberta has changed substantially over the last 30 years,” says Dustin Morton, commercial horticulture specialist, Alberta Ag-Info Centre, Stettler. “Currently, it is not uncommon for growers to use the same media once, twice or even three times but there are potential risks.”

“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,” says Morton. “Much research has been done on this topic, as reuse is much more prevalent in Europe, with media sometimes being used for five or six 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, notes Morton.

“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 that had built up over subsequent croppings.”

Another concern is the mechanical properties of the substrate. “Over numerous cropping cycles, organic based media such as sawdust and coco coir begin to break down,” explains Morton. “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.

A common theme amongst all these media is the possibility of changes to electrical conductivity 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 vital to ensuring a healthy crop,” adds Morton. “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 trialling it in your operation will ensure that you make an educated choice that helps your operation’s bottom line.”

For more information:
Alberta Agriculture and Rural Development, AG-Info Centre 
Dustin Morton
310-FARM (3276)
www.1.agric.gov.ab.ca