Fungi must be present to perform their functions of competing with the more difficult disease-causing organisms, retaining nutrients especially micronutrients like calcium, and making macro aggregates which form air passageways and hallways to allow air and water to move through the soil. This is a critical step in improving soil structure, but cannot occur without the first step of good bacterial biomass.
The “correct” density of fungal biomass, or amount of fungal
activity, is just now begun to be understood based on observation in different
soils, climates and plant species. Seasonal variations and the requirements
of different plants appear to be the most important factors.
When total fungal biomass is too low, fungi must be added back into the soil using either compost, compost tea. Alternatively, these fungi might be found in healthy soil, especially the humus layer of a healthy forest. But be careful not to destroy that resource by removing too much, or disturbing too much.
When total fungal biomass is high, most of the time this means improved ability to perform fungal functions, but if the balance between total bacteria and total fungi becomes inappropriate for the plant species, then the balance needs to be restored. However, you don’t kill off fungi if they are higher than the desired ratio, you improve bacterial biomass instead.
On rare occasions, total bacteria may compete with fungi for food resources, and in this case, reducing bacterial foods may be a good idea, to allow the fungi to have a chance to grow. High total fungal biomass, combined with too low active fungal biomass may indicate a fungal disease outbreak in progress. This can be confirmed by examining the roots for necrosis, galls, or other signs of fungal disease.
Beneficial fungi require aerobic conditions and if oxygen falls below 5 — 6 mg oxygen per liter, then the beneficial fungi may not survive. Anaerobic bacteria attack and consume fungi in these low oxygen conditions. Disease-causing fungi are benefited by anaerobic conditions, either because they no longer have competition from the beneficial's, or because they require anaerobic conditions for best growth. In either case, anaerobic conditions select for and allow the disease-causing organisms to “win” in the fight for plant tissues.
Just like any other creature, fungi require food. Feed the beneficial fungi, if fungal activity is too low. Sloughed root cells and dead plant tissue often supply the more complex carbon substrates that fungi require, such as cellulose, cutins, lipopolysaccharides, complex protein-sugar-carbohydrate, and lignins. Fungi are good at condensing organic matter into ever more complex forms, such as fulvic to humic acids.
Fungi need N, P, K, Ca, and all the other nutrients as well, and obtain those from organic matter as well as from inorganic sources. Many species of fungi can solubilize mineral elements from the mineral components of soil, but no one species effectively solubilizes ALL minerals. A diversity of species is needed to obtain all nutrients.
Often soil tests will indicate that some nutrient is in low supply, but merely by adding the appropriate bacterial or fungal species, these organisms will convert plant unavailable nutrients into plant available forms.
Diversity is the key, as well as feeding that diverse set of species so they will perform their functions.
Both bacteria and fungi are important in holding nutrients in the soil when they would otherwise leach into deeper soil layers, and into ground water. The importance of microbes in forming soil structure and preventing erosion is well-known, but in order to hold the nutrients in soil, bacteria and fungi must turn them into biomass, which is not-leachable as long as the glues and strands that the fungi and bacteria use to hold themselves on any surface are not destroyed.
If activity is low, then fungal foods need to be added to increase growth rates and improve numbers. A diversity of foods needs to be added, and thus dead leaf material is a much better choice than purified cellulose. Fish hydrolysate also adds bacterial foods, and N and other micronutrients. Wood, sawdust, bark, paper and cardboard can be used as well, but diversity is key.
If activity is higher than the desired, then try to balance the ratios of the organisms by improving the organism group that is too low.
If active fungal biomass is low, but total fungal biomass is high, this is a good indicator that disease is either rampant, or about to be rampant. Add BENEFICIAL fungal foods and build soil structure as rapidly as possible to compete with the disease, and protect the plant roots from the disease.
In rare instances, it may be because some environmental disturbance occurred that put the majority of the fungi to sleep, but did not kill them.
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.
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 out compete weeds and earlier succession plant species.
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.
If less than perhaps 10% — 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.
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.
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% — 30% within 1 year, and to 50% — 60% in 2 years, with addition of humic acids through the season to help the mycorrhizal fungi grow rapidly
If colonization is between 15% — 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.
There is a dose response relationship to humic acids additions. Typically addition of 2 — 4 pounds of dry product, or 1 — 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.
Be aware that that most humic acid products contain 10% — 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.
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.
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.
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.
Read also: Flagellates, Amoebae, Ciliates