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Soil contains millions of organisms, earthworms for example
Photo: Insectarium de Montréal (Mélanie Riverin)

Soil is more than just particles of rock and organic matter. It also contains water, air and millions of organisms, from algae to bacteria, fungi, sow bugs, nematodes, earthworms, springtails, spiders, snails and mites. All of these beneficial organisms are essential to the soil’s fertility and plant growth: they break organic matter down into nutrients, give the soil a better structure, protect it against insect pests, etc.

How to promote biological activity in the soil

In order to survive in the soil, these organisms need oxygen, moisture, warmth, food and near-neutral pH. You can ensure these conditions by regularly adding organic matter to your soil (mainly in the form of compost or composted manure), using fertilizer that is not directly assimilated by plants, avoiding pesticides that destroy all forms of life, adjusting the pH as necessary and adopting good practices (watering properly, not overworking the soil, mulching, etc.).

Mycorrhizae

Some fungi in the soil form symbiotic associations with plants. The hyphae, or thin filaments, of these fungi attach themselves to the tips of the plants’ roots and extend through the soil. This “mycorrhizal” association is beneficial to both organisms: the plant provides the fungus with nutrients in the form of some of the sugars produced by photosynthesis; in turn, the fungus enables the plant to absorb more water and minerals, protects it from root pathogens and improves the soil structure. Mycorrhizal plants grow faster, root better, are more disease- and drought-tolerant and produce better yields.

Mycorrhizal fungi proliferate in the wild. Unfortunately, they have practically disappeared from our gardens because people tend to overwork the soil, over-fertilize and use pesticides.

How to solve the problem

  • Avoid tilling your soil too deeply. Instead, hoe or turn it over lightly.
  • Cut down on fertilizer use. Use insoluble organic or mineral forms that are not taken up directly by plants.
  • Don’t use pesticides.
  • Increase the amount of organic matter in your soil by adding compost or composted manure. Scratch these amendments into the surface with a hoe or cultivator, because disturbing the soil can destroy the mycorrhizae.
  • Keep the soil pH near neutral by regularly adding small amounts of mineral amendments to the surface.
  • Inoculate your soil with beneficial fungi.

Buying mycorrhizal fungi

You can now buy mycorrhizal fungi from gardening suppliers. These fungi are available in the form of spores, mixed with vermiculite, perlite or peat moss. Specific types are available for vegetable gardens, annuals, perennials, trees, shrubs, bulbs, etc. You can also buy some soil mixtures and types of compost that already contain beneficial fungi.

Tips for using mycorrhizae

  • Some mycorrhizal fungi form associations with only one type of plant. They are said to be “specific” to their hosts. Others are able to colonize many different plants. Ideally, you should choose a mix that is specially designed for the type of plants you intend to grow (eg. trees and shrubs).
  • Plants in the Ericaceae family (rhododendrons, blueberries, cranberries, etc.) grow symbiotically with specific fungi that are not yet available commercially.
  • Some plants do not grow symbiotically with fungi (eg. cabbage, beets, carnations, baby’s breath). A list of such exceptions usually appears on the product label.
  • In order to grow, mycorrhizal fungi must be in direct contact with the plants’ roots. This means that the best time to add them is when seeding, planting or transplanting.
  • Because mycorrhizal fungi are alive, they must be protected from freezing and high temperatures. Ideally, they should be stored in a dry, cool spot (4ºC to 20ºC). Fungi can survive for two years under these conditions.
  • Mycorrhizal fungi grow along with plants’ roots, so you need to add them only once during a plant’s lifespan.
  • The amount of mycorrhizal fungi to be applied depends on the extent of the plants’ root system. Check the product label for recommended dosages.

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