Quick and Easy Guide: On-farm AM fungus inoculum production

Following is the crib-notes recipe for producing beneficial AM fungus inoculum on-farm. For more details on the when, why and how, read A Complete How-To.


The following list will produce 16 “Grow Bags” of inoculum, enough to make 200 or 400 ft3 of inoculated greenhouse potting media depending on the dilution ratio (1:9 or 1:19) of inoculum:potting media used in the final step.

Bahiagrass seed (sources for bahiagrass seed can be found on the internet, for example: http://agriseek.com/market/p/Pensacola-Bahiagrass-Seed.htm)

Conical plastic pots  (we use RLC-4 Pine Cells from Stuewe and Sons, Corvallis, OR 97333)

Coarse sand such as swimming pool filter sand (240 in3 for 80 seedlings)

Ground cover fabric (16 Grow Bags fit on a 1.2 m x 3.6 m or 4 ft x 12 ft section)

16-7 gallon “Grow Bags” (one source is Worm’s Way, Bloomington, IN 47404)

4-4 ft3 bags of vermiculite

4 ft3 of compost

In order to maximize mycorrhizal proliferation and colonization of the host plant, the inoculum bags should be setup outside as soon as possible after the last frost. Some work is necessary before this date. The finished inoculum will be ready for use the following spring.

4 months before the predicted last frost:

Germinate bahiagrass seeds (or other host seeds) in vermiculite or seed starter.

Order any needed materials.

3 months before the predicted last frost:

Transplant bahiagrass seedlings into conical plastic pots filled with 1:3 soil:sand mixture (volume basis). In order to avoid introducing pathogens, we suggest using sterilized field soil. Another option is to use soil from a natural area of the farm or from a field that has not been used within the past 2 years to grow the crop that will be inoculated.

As soon as possible after the last frost:

Set up the inoculum production area by covering an area with the ground cover fabric. This will provide a clean, open area that makes maintenance easy. It will prevent weeds from growing around the bags and contaminating the inoculum with weed seed.

Set up the grow bags:

Mix compost and vermiculite in chosen dilution. A basic recommendation for yard clippings compost from municipal facilities is a 1:4 compost:vermiculite ratio (volume basis).

Fill bags ¾ full with mixture. Roll the lip of the bag down to just above the level of the mix.

Add 100 cm3of field soil as the “inoculum starter” and mix well.

Pool 4-5 soil samples taken from the surface to 10cm (4 in) deep. Sieve out roots and rocks.

To avoid introducing pathogens and to obtain a diverse sample of AM fungi, take samples from a natural area of the farm or from a field that has not been used within the past 2 years to grow the crop that will be inoculated.

Transplant 5 bahiagrass or host plant seedlings into each bag.

During the growing season:

Weed and water the bags as needed. The mycorrhizae will proliferate as the plants grow.

Frost will kill the bahiagrass and the mycorrhizae will overwinter naturally outdoors in the bags.

The following spring:

Harvest the inoculum:

To keep the inoculum clean, cut off the dead bahiagrass leaves and discard.

Shake the compost and vermiculite mix from the root ball into a bin. This mix will contain the mycorrhizal spores and hyphae.

Cut the roots into short segments (less than 1cm or ½ inch) and mix into bin. The roots contain the mycorrhizal vesicles.

Mix the inoculum into your potting medium:

Use a 1:9 inoculum:media mix (volume basis) for flats with cells of 50 cm3 or smaller. For larger cells a 1:19 mixture should be sufficient.

Amend your greenhouse fertilization regime to avoid P-sufficient plants that will resist colonization:

Conventional farmers: Try to achieve a P addition of 3 ppm or less for no more than three fertilizer applications per week. Apply P-free solutions at other times if necessary.

Organic farmers: If your potting medium requires additional fertilization, use a low P source. If your potting medium contains all the nutrients needed during the greenhouse culture phase, no modifications are recommended at this time.

Adapted from David Douds’ “Checklist of steps for the on-farm production of arbuscular mycorrhizal [AM] fungus inoculum”
in his eOrganic article  On-farm Production and Utilization of AM Fungus Inoculum.
This material is based upon work supported by the U.S. Department of Agriculture, Agricultural Research Service, under Specific Cooperative Agreement Number 58-1935-5-524. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

4 Responses to “Quick and Easy Guide: On-farm AM fungus inoculum production”

  1. Corazon Cook

    I would like to teach the farmers in the Philippines how to use mycorrhiza. Vermiculite is one of the materials needed for the inoculum production. Can it be substituted with coconut coir because vermiculite might be difficult to obtain.

    How do you measure the success of the inoculum production? Do you check the mycorrhiza production under a powerful microscope?


  2. amanda

    Dr. David Douds did look at alternatives for vermiculite and says that although they didn’t use coir, he thinks it could be used.

    Dr. Douds also included the following regarding measuring success of propogation:

    Researchers evaluate the success of the inoculum production process by isolating and counting the spores of AM fungi produced in the compost and vermiculite mixture. This requires a dissecting microscope, sieves, and a centrifuge. Also, roots can be observed for colonization by the fungi. This requires a variety of chemicals and the same dissecting microscope.

    However, the method has always been successful in our hands, so one can safely assume a propagules density of approx. 25 per cm3.

  3. Ton Lagerway


    Have become very much interested in Mycorrhizae and intend to grow some in an “old disfunctional” freezer.
    I can get hold of Bahia grass seeds in Australia to sow in the sub-tropical climate prevailing in the Gold Coast hinterland!

    I would like to utilise the inoculated soil produced by the grass as the soil used in the punnets for the production of new seedlings for planting out.

    Would the mycorrhizal propagules remain viable in the harvested & stockpile as (inoculated) compost for the next 1/2 year when stored in plastic bags under the house where the temperature remains moderately cool even during our hot summers?

    If so, this would (could) allow for the production of a crop of “Bahia Grass inoculated compost” every 1/2 year or so.

    Would inter-planting additional species of plants amongst the Bahia Grass allow/promote the maintenance of a diverse range of Mycorrhizal species with which I intend to inoculate the first batch?

    Another thought:

  4. Nick Rose

    I live in San Mateo, California and have a about 2600sqft of land that has lawn, roses fruit trees and a small vegetable area. The house was built in 1947 and no major disturbance to the soil has occurred since then except for a new 1000sft back lawn that was put in around 2013. I have not used any chemical fertilizers, herbicides since 2013, no pesticides have been used on the site for probably decades. I know the only true way to find out if I have mycorrhizae in the soil is by having it tested for. But with that said is it possible that mycorrhizae have re-populated the soil. On the NRCS websoil survey shows the area is Urban land-Orthents, reclaimed complex. The land use to be tidal flats but the area was filled with around 5ft of fill.


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