How to Interpret Soil Foodweb Assays

Step 5

Step Five:
Mycorrhizal fungi are needed by some plants, absolutely critical for other plants, and are probably detrimental for other plants. You need to know what kind of plant you have, but in general, very early successional plant species, such as many (weeds, brassicas, mustards and kale crops do not require mycorrhizal fungal and may be harmed by mycorrhizal fungi. Annual vegetables, flowers, grasses and row crops or broadacre crops need vesicular-arbuscular mycorrhizal fungi. Most evergreen plants require ectomycorrhizal fungi, and blueberry and ericoid plants require ericoid mycorrhizal fungi.

The percentage of the root system that must be colonized has not been fully established in the mycorrhizal literature, mostly because determining benefit is relative. Mycorrhizal fungi can protect the roots from disease organisms, through simple spatial interference, by improving nutrient uptake, and by producing glomulin and other metabolites that inhibit disease. Stress in plants can be reduced because the mycorrhizal fungi can solubilize mineral nutrients from plant not-available forms to plant available forms, and translocate those nutrients to the root system in exchange for sugars provided by the plant.

Given that mycorrhizal fungi can influence so many aspects of plant growth, and documenting all these benefits is usually extremely expensive and difficult, they have not been documented. Therefore, probably the best that can be done is to say that perhaps as low as 12% colonization might be documented to be beneficial (work by Moore and Reeves in the mid-1990’s), but more likely a minimum level of 40% colonization is required, as suggested by Mosse, and St. John in various publications and comments.

Early researchers found colonization as high as 80% in root systems, but most likely because they did not differentiate false-arbuscular and vesicular structures produced by disease-causing fungi from true VAM structures. Thus, colonization is rarely as high as 80% is not commonly found now that we recognize these non-mycorrhizal forms.

In the last 10 years, some researchers have suggested that some mycorrhizal fungi do not produce vesicules under all conditions, and so VA mycorrhizal fungi should be called arbuscular mycorrhizal fungi, not vesicular-arbuscular mycorrhizal fungi. Just be aware that sometimes, people say VAM, sometimes AM. Whatever.

1. If the plant does not require mycorrhizal colonization, there probably is no reason to assess the roots for mycorrhizal colonization. Although the Allens showed that one way for certain plants to exclude non-mycorrhizal plants from a community was to make sure the mycorrhizal fungi were present, because the mycorrhizal fungi pulled nutrients from the non-mycorrhizal plants. This is a probable mechanism for mycorrhizal crop plants being able to outcompete weeds and earlier successional plant species.
2. When mycorrhizal colonization is low, or less than the desired range, given that the desired plant requires VAM or ectomycorrhizal colonization or ericoid mycorrhizal fungi, then check how low the colonization is.
   1. If less than perhaps 10 to 15%, then addition of mycorrhizal spores would be a good idea. If it is an annual plant, placing VAM spores near or on the seed or seed pieces is the simplest way to get the roots colonized as soon as the roots area produced.
      1. With permanent turf, adding VAM spores into the compost mixed into the aeration cores gets the VAM spores into the root system without destroying the turf.
      2. With perennial plants, verti-mulching and adding the VAM or ecto- spores into the compost mixed in the vertimulch is the simplest way to get the spores next to the root system. In cases where we have added inoculum in this fashion, roots have gone from 0% colonization to 25 to 30% within a year, and to 50 to 60% in two years, with addition of humic acids through the season to help the mycorrhizal fungi grow rapidly (see next section)
   2. If colonization is between 15% and 40%, then all that is needed is additional fungal foods to help the mycorrhizal fungi improve plant growth, reduce plant stress, and improve root protection.
      1. There is a dose response relationship to humic acids additions. Typically addition of 2 to 4 pounds of dry product, or 1 to 2 gallons of liquid product per acre are adequate to improve fungal growth. But, if there are toxic chemical residues to overcome, additional humics of fulvics may be needed. It is best to check periodically to see that colonization is improving as desired.
      2. Be aware that that most humic acid products contain 10 to 12% humic acids. If the product you are considering is less expensive, please check the concentration of humic acid. Half the concentration of the humic acid means they can drop the price, but your fungi get less benefit.
      3. Check colonization periodically to make sure the fungi are growing and colonization is increasing. Weather can cause problems with colonization, and severe drought, floods, burns, compaction causing by over-grazing, heavy machinery, herds of people walking on the lawns or turf can reduce colonization. If that happens, additional applications of fungal foods will be needed to help resuscitate the damage. Fungi are just like any other organism. If they are harmed, they need care to recover. Triage for fungi includes adding foods they love (humic acid is like chocolate to a choc-a-holic, but they’ll also accept any woody, wide C:N ratio fungal food), and putting on a mulch or litter layer on the soil surface.
   3. If colonization is above 40%, then the plants are getting the help they need from the fungi. Periodically check to make sure nothing has harmed them.
   4. What if colonization seems too high? This is extremely rare, but does happen, and seems to be associated with the fungi taking more than their fair share of the plant’s resources. Stop applying fungal foods. Consider helping the bacteria compete with the fungi for a bit.

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