Question from Steve Bell, New Zealand.
The production of Mycorrhiza fungi happens naturally in the soil.
If you have a field, allotment, or garden in which you wish to encourage Mycorrhiza you have to adopt a low till farming or zero till gardening method of cultivation. These work in conjunction with composting. Compost made properly is full of fungi and bacteria and will encourage the Mycorrhiza fungi to develop as well. If you have ploughed or dug the area it can take up to two years to replenish Mycorrhiza (Rodale Institure, USA). Once you have established a crop in this ground then add some intermittent perennial plants. These will preserve the Mycorrhiza when annual plants are harvested, although leaving the roots of harvested plants in the ground is the best policy as they become fresh organic matter for humus formation and won’t disturb the Mycorrhiza fungi.