Mycorrhizal fungi are so important to plants that all but 2 plant families are known to form associations with them. Consequently they are prolific in the environment.
However, in some situations their populations are severely depressed or completely lacking, such as in compacted or water-logged soil, “manufactured” topsoil, peat based growing media and soil repeatedly treated with pesticides or synthetic fertilizers.
Even tilling will sever the fungal strands and repress fungal populations.
Mycorrhizal fungi can be re-introduced by incorporating them into the top few inches of the soil. A few handfuls of forest or natural grassland soil will do, but the spores can also be purchased.
Compost is not a source of mycorrhizal fungi because they can only live in association with the roots of living plants and do not multiply in compost.
In cases where a new garden is planted in imported “manufactured” topsoil it is highly advisable to add mycorrhizal fungi to the planting holes. Plant propagators do not commonly add these fungi to their growing media, nor are they guaranteed to be present in the imported soil.
Without these important allies plants are unable to access the water-insoluble phosphorus in the soil, and are also much more prone to drought stress.
We go much deeper into this topic in our online course, which you can discover here: