Contrast this to the original growing medium – soil – where we find a different situation altogether. Apart from the “inert” materials (minerals), soil contains a whole bunch of living organisms:
- Bacteria, which do numerous jobs including fixing nitrogen, changing ammonia into nitrite and subsequently nitrate, converting nitrogen into nitrous oxide, decomposing organic matter including cellulose, and giving soil its “earthy” smell that we are all familiar with.
- Fungi, which act as decomposers, colonize plant roots (mycorrhizae), and are pathogens/parasites.
- Soil protozoa, single-celled animals that feed mostly on bacteria and in so doing release nitrogen as ammonia, thereby making nitrogen available to plants.
- Soil nematodes, which feed on plants and algae, bacteria and fungi . . . even on other nematodes. Like soil protozoa, this process releases ammonia. Nematodes are also a good source of food for higher organisms, such as soil insects.
- Soil arthropods, which act as shredders, predators, herbivores, fungal feeders, burrowers, pests and pest controllers.
- Earthworms. These are our friends that, among other things, enable air to move through the soil profile, create drainage, eat pathogens and provide valuable sources of nutrients for plants. Worms are the most prominent group of soil invertebrates based on biomass.
We can add some of these organisms back into our growing media: for instance, commercial inoculants of mycorrhizae have been readily available for many years. But as we strive for more control over our growing media, we typically take away the significant contributions that each of these groups of the “soil foodweb” brings to the table.
Composts have been the focus of considerable interest for their disease-suppressive activities.
A decade ago, with a project funded by the B.C. Greenhouse Growers Association, Dr. Zamir Punja at SFU looked at the use of composts to control two important root pathogens of greenhouse cucumbers, including Fusarium root and stem rot (“RSR,” caused by F. oxysporum radicis-cucumerinum). “Three composts (greenhouse waste, windrow dairy solids, vermicompost dairy solids) were evaluated for their ability to reduce RSR. All three composts reduced RSR to some degree, with greenhouse waste compost reducing mortality significantly by 50 per cent. Autoclaved compost had no effect on disease suppression, suggesting that microbial antagonism was involved. Under semi-commercial greenhouse propagation trials conducted three times over two years, PreStop WP, PlantShield, Mycostop and windrow composted dairy solids reduced the severity of RSR in two out of three trials.”2
LOOKING AT COMPOSTS FOR DISEASE SUPPRESSION
■ According to Punja, “both general and specific mechanisms of disease suppression are involved, and include microbial antagonism, nutrient release, induced host resistance, and abiotic inhibitory factors.” He further added that “. . . composts added at seeding time to rockwool medium reduced Fusarium infection, through microbial antagonism.”
Sadly, since Punja published this work, there seems to have been little uptake of his ideas by industry. Clearly, we are unlikely to go back to soil for growing most greenhouse crops. But perhaps we can consider the significant benefits of balanced soil micro flora and fauna in growing media, and the significant potential benefits of the “soil foodweb” organisms.
- Elaine Ingram, “Soil Biology,” USDA Natural Resources Conservation Service.
- Punja, Rose, and Yip, (2002), “Biological Control Agents and Composts Suppress Fusarium and Pythium Root Rots on Greenhouse Cucumbers.”