Fungi are organisms with a cell nucleus. They can be unicellular or multicellular. Mushrooms are undoubtedly the most familiar type of fungi. However, they are only a small group within the overall fungal kingdom. Of an estimated 1.5 million species, some 100,000 fungi have been described. A square metre of soil under grassland can easily contain several kilometres of fungal filaments (hyphae). When there is a lack of food, drought or other unfavourable conditions, fungi become inactive. Spores can sometimes survive in the ground for decades. Important groups are: mycorrhiza, plant-pathogenic fungi (e.g. Fusarium sp.) and oomycetes/plant parasites (Pythium, Phytophthora).

The main functions of fungi are:

  • to break down (complex) organic compounds (cellulose, lignin)
  • to create stable aggregates
  • to excrete acids, making some nutrients more available to the plant
  • to increase uptake of phosphate and water (mycorrhizae)
  • to promote disease resistance

Fungi are eaten by a large number of “grazers” higher up the food web.