Mycorrhizal fungi

Mycorrhizal fungi are involved in a large number of interactions and processes in soil. Many fungi are part of complex relationships with other soil organisms. Very close relationships between arbuscular mycorrhizal (AM) fungi and plants are widespread; this is where the fungus has part of its structure within the plant root and the hyphae extend widely into the surrounding soil. These relationships are largely mutualistic – in other words, the fungus obtains at least some of its sugars from the plant, while the plant benefits from the efficient uptake of mineral nutrients (or water) by the fungal hyphae.

Effective mycorrhizal associations are also thought to give increased pest and disease resistance to the infected plants.  However, there are circumstances in which the fungus is mildly detrimental, and others where the fungus simply feeds from the plant. About 90% of land plant link up with mycorrhizal fungi.

Only a very few crop species do not form AM fungal associations, such as Brassicae and Chenopodiaceae. Indeed, as the American plant pathologist Stephen Wilhelm has said, “in agricultural field conditions, plants do not, strictly speaking, have roots, they have mycorrhizas”.

Good things for AM fungi –root relationships

  • Rotational cropping with a range of host plants
  • Mixed-species swards with a range of host plants
  • Reduced tillage intensity
  • Regular inputs of organic matter
  • Low levels of soluble phosphorus

Things that aren’t good for AM fungi –root relationships

  • Non-mycorrhizal crops (Brassicae and Chenopodiaceae)
  • Fallow periods
  • Reduced plant species diversity
  • Some modern cultivars
  • Intensive tillage (such as for potatoes and some horticultural crops)

Nitrogen fertilisers, herbicides, pesticides, organic manures and grazing have been shown in some studies to have positive effects on AM fungi and in other studies negative effects are seen.