Mycorrhizal Inoculants: Panacea or Snake Oil?

There is a lot of talk among gardeners today about the importance of mycorrhizal organisms for healthy, weed-free gardens. The assertions go beyond the statement that good soil fungi is important for healthy gardens to the assumption that commercially available mycorrhizal inocula will improve plant health, regardless of the types of plants being grown, geographic location of the planting site, the exact makeup of the inoculant, or the composition of the local mycorrhizae.

I have been hearing these claims for years and have wondered whether there is solid data backing them up. Not only is it unclear to me whether mycorrhizal inoculants will improve growing conditions, I am also concerned about the possibility that the introduction of non-local microorganisms into the environment could lead to unintended, negative consequences.

What are mycorrhizae?
Mycorrhizae are soil fungi that interact with plant roots, sometimes to the benefit of both organisms. One way to understand this is to compare this symbiotic relationship in the plant world with microorganisms, such as Lactobacillus acidophilus, in the animal world, that aid in the absorption of nutrients in the digestive tract. In healthy, undisturbed soils, there is a balance between the types of microorganisms, the plants growing with them, and the conditions in which they are living, such as available water, soil nutrients, biological activity, and so on. Similarly, in the animal world, a healthy gut is teaming with microorganisms suited to help in the digestion of the animal hosting them. The composition of these microorganisms varies with diet, hence, microbes found in the guts of vegetarians and omnivores are not the same.

The word, mycorrhiza, comes from the Greek words myco for fungus, and rhiza meaning root. As one might guess there are many different types of soil microorganisms, including mycorrhizae. In general, mycorrhizae can be divided into two broad categories: ectomycorrhizal fungi (EMF), and arbuscular mycorrhizal fungi (AMF). Again, a little bit of Greek and Latin goes a long way. Ecto from the Greek ektos, means outside, and these fungi surround the plant roots. Arbuscular comes from the Latin word meaning dwarf tree or tree-like. These fungi actually penetrate the cells of roots with small structures that look like branching trees. (2)  AMF are usually found in arid and semiarid regions, while EMF usually occur in wetter, forested lands. (5)

I include this information to illustrate that one mycorrhiza is not the same as another. Not only are there broad categorical differences, there are thousands of different species. Some have very specific plant hosts; others are specific to soil types or locations; while still others are generalists found with many different hosts in many different environments. (5) Furthermore, the ecological study of soils and microorganisms is in its infancy. Soils are complicated and we are, literally and figuratively, just scratching the surface.
Does it help?
Many studies have shed light on how mycorrhizae grow in mutually beneficial relationships with plants, improving nutrient uptake. It is known that soil disturbance – chemical, physical or biological – alters, often destroying, natural mycorrhizae. (3)  Furthermore, pesticides, especially fungicides, and fertilizers, particularly those high in phosphates, are harmful to mycorrhizae. (2) This led to the hope that the introduction of mycorrhizae – either through soil inoculation in the field or the use of plants inoculated with mycorrhizae in containers – would improve growing conditions, especially where soil integrity is compromised.

Mycorrhizal inoculation is promoted for horticultural, agricultural, and restoration applications. Scientific-looking websites display testimonials showing amazing results. Nerdy-looking individuals, with either Dr. before their names or Ph.D. after, answer questions on how their miracle products work. But these websites lack the scientific credibility of academic research and peer-reviewed journal articles. Claims that these products improve growing conditions for desirable plants while discouraging weeds, increase germination and survival of desired seedlings, improve soil properties, including soil structure to increase infiltration and stability, and decrease erosion are made by the people selling the products. (5)

So what is the truth about commercially available mycorrhizal inoculants? Are there any field studies that demonstrate improved outcomes with this treatment? Are there some situations in which it may work, and others in which it doesn’t?

Although I have spoken with many landscapers who use mycorrhizal inocula and assure me that it has made a big difference in the success of new landscapes, the scientific support for these assertions is limited. (2)  In order to determine whether mycorrhizal inoculants are beneficial, the site conditions and specific mycorrhizae must be specified. (6) Let us consider the use of mycorrhizal inoculants in the following growing conditions: residential landscapes, nursery-grown container plants, and revegetation or restoration projects.

Residential Gardens
Some landscapers use inoculants when planting natives in typical garden sites. The sites may have heavily disturbed soils, such as sites that are cleared with machinery and graded, or areas that are cleared of lawn by physical or chemical means. Other landscape projects, though, occur in minimally disturbed beds, such as when a few new plants serve as replacements or additions to a mature landscape. Sites with minimal soil disturbance, where plants are growing well, are unlikely to benefit from mycorrhizal inoculants, and adding them may even disrupt healthy microbial soil activity. (2, 5)

It is known, though, that disturbance has a damaging effect on mycorrhizae, so applying inoculants of mycorrhizae in disturbed sites may be helpful, though again, there is not strong scientific support for this. (2, 5, 6) As mentioned above, the complexity of the biological relationships found in soils cannot be overstated. It is possible that the path to healthy soil is not the addition of mycorrhizae, but rather the cessation of practices that hinder their survival, such as the use of pesticides, fertilizers, irrigation, and soil tilling. (2)

Commercial Nurseries
Mycorrhizal inoculants are also used in nursery situations. It would seem that this application would be most beneficial for plants that are propagated and grown in sterile potting media. However, it is known that fertilizers, especially those containing phosphate, and fungicides, both commonly used in nursery operations, damage or kill mycorrhizae. If the plants are being fertilized and treated with pesticides, it is likely the mycorrhizae are also being destroyed.

Another common use of inoculants in nursery container plants is to produce plants with arbuscular mycorrhizal fungi (AMF) living within their roots. In this case, nursery practices would be very different. Still, it is a big jump to assume that microorganisms that typically occupy the region bridging the plant roots and the soil can survive when the plant is transplanted. This, in fact, represents a major disturbance, and it has already been shown that disturbance is detrimental to mycorrhizae. However, the experiments of Vogelsang, et al, 2004, (6) suggest some benefit to native container-grown plants inoculated with AMF, though the plants in this study were being grown in a field situation, not in a nursery production operation.

Revegetation or Restoration Projects
Revegetation projects using locally-appropriate native plants are often overrun with weeds. These disturbed sites, usually lacking soil micoorganisms, are also subject to erosion. (5, 6) Knowing that soils with proper mycorrhizal fungi are more stable and less susceptible to invasive weeds makes mycorrhizal inoculation a very appealing practice. Though there is strong scientific data supporting the importance of healthy soils with appropriate microbial activity and balance, the best way to get there is not yet clear. (2)

Soil disturbance has a detrimental effect on soil microbes, but the evidence for the benefits of introducing commercially produced fungi is not strong. (2, 5, 7) The fungi in these products are usually foreign, though most are not identified. (3, 5)  Furthermore, there are, at most, a handful of species in commercial inoculants. A 2004 study by Vogelsang, et al  suggests that inoculating disturbed soil with appropriate AM fungi may in fact improve revegetation efforts and reduce erosion; however, the evidence from their own tests was not especially strong. (6) However, a three-year field study (7) showed that the native inoculum cultured from a nearby donor site outperformed a commercially available product in reducing soil erosion.

Caltrans (California Department of Transportation) is responsible for landscaping and maintaining areas with highly disturbed soils. The control of weeds and erosion is very costly, as is water for irrigation. The department has been interested in determining best practices for growing native plants in these areas. In 2011 Caltrans published a “targeted review of the available literature regarding mycorrhiza to ascertain:

1. positive or negative effects on revegetation from the addition of local or non-local mycorrhizal fungal inocula, and
2. whether a compelling case exists now for the routine addition of commercially available mycorrhizal fungi spores as part of any Standard Specification for roadside revegetation projects conducted by Caltrans.”

According to the review, “Severely altered soils lack a viable population of mycorrhizal fungi (Moorman et Reeves 1979; Jasper et al 1989). Even salvaged and re-applied topsoil, a routine practice in land rehabilitation, suffers some degradational effects with decreases in mycorrhizal populations (Rives et al 1980; Gould et Liberta 1981; Miller et al 1985), reduced growth of arbuscular mycorrhizal fungi (Visser et al 1984b), dramatic shifts in the microbial community structure (Visser et al 1984a), and general declines in soil quality and nutrient cycling capacity (Vogelsang et Bever 2009).” (5)

Even in the case of typical Caltrans projects in which soils are severely damaged, the review of scientific literature on the use of mycorrhizal inoculation for revegetation concluded that there was not strong positive evidence for its use. (5) Rather, its efficacy probably depends on things like host plant species, fungal species, and soil conditions.

Does it hurt?
The question, though, must go beyond whether it helps. Could the introduction of commercially-produced mycorrhizae damage the environment?

The 2011 scientific review determined that the origin of mycorrhizal fungi found in commercial products is rarely specified. In most cases, it not possible to determine the origin of inoculum derived from large-scale production in commercial labs, though it is likely that most are not native to California (with the exception of AM120 California Coastal Suite and AM120 Desert Suite, Reforestation Technologies, Appendix B). (5)

We have made this mistake before, blanketing burned forests and mountains with non-native seed hoping to promote revegetation and reduce erosion faster than would occur without human intervention. (4) The unintended consequences are still being studied. Do we now introduce yet another non-native organism into the environment with little understanding of the ultimate consequences of this act? Not only could the non-native microorganisms change relationships between native soil organisms, plants and animals, they also pose the risk of introducing pathogens.

What should be done?
In many cases, though not all, patience is an appropriate strategy. My home garden has few weeds in areas that have had several years with minimal disturbance. Each time I remove turf-grass – and I do this by digging it out – I can expect one to two years of weeding. As time passes, and the native plants become established, summer irrigation becomes unnecessary, and few weeds make an appearance. Necessary weeding is done carefully to minimize disturbance. Soil cultivation, hoeing, and the use of fertilizers and pesticides are practices that have been banned from my garden altogether.

Another recommended horticultural practice for the control of weeds is the spreading of a thick layer of organic mulch. Sheet mulching is a variant of this in which layers of cardboard or newspaper underlie the mulch to further keep sunlight from reaching the weeds. Over time the organic material decomposes, resulting in loamy, biologically active soil. Although a discussion of mulch for weed control is beyond this post, it should be said that organic mulch is not appropriate for all gardens. For example, gardens with desert plants are likely to suffer from overly loamy soil, and these gardens may actually have more weeds from the addition of organics to the soil. On the other hand, if you are planning a vegetable garden with plants that are heavy feeders, this may be just the ticket.

One last example
For the past six years I have been doing battle with weeds in a local nature park. The park was built on a degraded, three-acre site with disturbed soil and an abundance of weeds. The soil was scraped and graded in preparation for planting native plants. Within months the weeds were back, in force. I have found that the method described by Joan Bradley in her classic book, Bringing Back the Bush – The Bradley Method of Bush Regeneration, is, in fact, the most successful. (1)

Herbicides, applied by city workers, resulted in a massive return of weeds, instead of natives. This would be expected given the relationship between weedy plants and mycorrhizae. Weedy plants do not have strong mycorrhizal relationships. (3) When herbicides are applied, mycorrhizae are destroyed, creating the perfect environment for the re-emergence of weeds, and re-emerge they did. Weed removal through hoeing also led to more weeds. In fact the only places in the park where weeds are on the decline are less disturbed areas where native plants are taking hold. Now I spend most of my time carefully weeding around native plants, as suggested in the Bradley Method.  Native seedlings are appearing in all areas of the park where older natives are growing.

Sagebrush surrounded by weedy mustard.

Mustards removed and encelia and sagebrush liberated.

Tiny sagebrush seedling rescued in the process.

There is one area, though, that is especially problematic. It is a slope that is infested with Maltese star thistle or tocalote (Centaurea melitensis). Since we have our hands full weeding other parts of the park, I long for an easy solution for ridding this area of this especially noxious weed. We find tocalote popping up in other parts of the park, so I know it is important to reduce the spread of its seeds. We decided this year to try to mow the slope using a scythe to remove the tocalote before it bloomed and went to seed. Unfortunately, this clever weed produces some seeds early and low on the plant, probably eluding the scythe. Digging it out will just disturb the soil further, and there is a hefty seed bank full of weed seeds. We have considered mulch but it is a slope and I fear it will just wash away with the first rains. Solarization may help but, once again, it will do nothing to help with the development of healthy mycorrhizae in the soil, in fact, it does just the opposite. If hydroseeding with an AMF inoculant were shown to be effective, I would be happy to try it, but as yet I am unconvinced.

Tocalote Hill. Tocalote was mowed with a scythe, mustard still abundant. The trees are Engelmann oaks planted as part of a city tree-planting grant.

I do not know the best way to tackle the tocalote weed problem but I suspect it is to employ the strategy I suggested above, patience. I think that we need to continue encouraging the growth of native plants throughout the park by carefully weeding around them. As we continue to expand the areas where the weeds are somewhat controlled, then we can start planting natives on Tocalote Hill. California buckwheat (Eriogonum fasciculatum), black sage (Salvia mellifera), and sagebrush (Artemisia californica) would be appropriate native plants for the area. The important thing is to stay focused so that we do not lose ground. Weed control is a long process, and what goes on in the soil, at the microscopic level, is as important as what we see above ground.

The weeds aren’t so bad in these areas where the native coastal sage scrub plants have taken hold.

Black sage, encelia, sagebrush and coast live oak.

The coastal sage scrub is marching out – conquering the weeds!

Conclusion
So the answer to the question, Are mycorrhizal inoculants a panacea or just snake oil?, is probably, both. As yet, there is little evidence that today’s commercially available mycorrhizal inoculants provide benefit, but knowing the importance of healthy mycorrhizae in soil for weed control, erosion control, and the growth of stable, healthy native habitat, we can hope that new methods of improving the microbial condition of soils, possibly including the use of inoculants, will be developed.

References
1.    Bradley, Joan. 2002. Bringing Back the Bush: The Bradley Method of Bush Regeneration. Australia: Reed New Holland.

2.    Chalker-Scott, Linda. 2009. Mycorrhizae: So, What the Heck Are They, Anyway? MasterGardener Newsletter.
http://www.puyallup.wsu.edu/~linda%20chalker-scott/Horticultural%20Myths_files/Myths/Mycorrhizae.pdf
Less technical article summarizing scientific evidence on value of mycorrhizal inoculation in horticulture.

3.    Chaudhary, Bala and Margot Griswold. 2001. Mycorrhizal Fungi – A Restoration Practitioner’s Point of View. Published in the Spring 2001 Ecesis, Newsletter of the California Society for Ecological Restoration – SERCAL
http://www.newfieldsrestoration.com/PDFs/Mycorrhizal_Fungi_Ecesis.pdf
Brief review of methods used in Southern California coastal sage scrub restoration and revegetation with emphasis on mycorrhizal treatments.

4.    Keeler-Wolf, Todd. 1995. Post-Fire Emergency Seeding and Conservation In Southern California Shrublands. In Brushfires in California Wildlands: Ecology and Resource Management Edited by J.E. Keeley and T. Scott. 1995. International Association of Wildland Fire, Fairfield, WA,
http://www.dfg.ca.gov/biogeodata/vegcamp/pdfs/PostFireEmergencySeeding.pdf

5.    Roadside Erosion Control and Management Study. 2011.  Mycorrhiza: Issues relevant to roadside revegetation.
http://www.dot.ca.gov/hq/LandArch/research/docs/final_RECMS_Mycorrhiza_Review_Hallock.pdf
Scientific literature review, 1970-2010, to determine whether there is a scientific basis for the routine addition of commercially available mycorrhizae part of Standard Specifications for roadside revegetation projects conducted by Caltrans.

6.    Vogelsang, Keith, James Bever, Margo Griswold, Peggy Schultz. 2004. The Use of Mycorrhizal Fungi in Erosion Control Applications.
http://www.dot.ca.gov/hq/LandArch/research/docs/final_report_65A070.pdf
Final report on experimental determination of impact of mycorrhizal inoculants on growing native plants in disturbed soils.

7.    Vogelsang, Keith. 2010.  The Use of Native Plants and Mycorrhizal Fungi for Slope Stabilization and Topsoil Management.
http://www.dot.ca.gov/hq/LandArch/research/docs/65A0180_Final%20Report_Mycorrhizal.pdf
Final report on comparison mycorrhizal inoculae from commercial sources and native inoculum cultured from nearby donor site.

8.    California Department of Transportation Website, Soils Research
http://www.dot.ca.gov/hq/LandArch/research/soils.htm