Peter Thurman asks if mycorrhizal inoculants are of any use
In the natural environment, mycorrhizal fungi are associated with the roots of most woody plants. They live in a symbiotic or mutually beneficial relationship with plants, by wrapping around or penetrating roots and then proliferating through the soil beyond the host’s roots range. This effectively extends the root system volume of the host by hundreds of times. These fungi release enzymes that help to break down nutrients into forms more easily used by the plant.
Many researchers and arborists consider that, while this group of fungi is important – perhaps even vital – in natural conditions, there is a significant risk that they are often depleted by cultivation, chemical use and spillage, compaction and topsoil erosion.
The uniqueness of mycorrhizal fungi lies in their ability to readily absorb elements such as phosphates, manganese, copper and zinc dissolved in water, thus making them available for use by the host plant. Given this important symbiotic association, it is natural to think that it would be beneficial to add more mycorrhizal fungi to the soil. However, some researchers have found that you must inoculate the plant’s roots with mycorrhizal fungi; you cannot inoculate the soil.
You can now buy mycorrhizal fungi inoculants in packets when you purchase a tree, shrub or rose bush. Many of these mixes are sold as host specific, but this may not be true, because the inoculant usually consists of only two or three types of fungi. There are hundreds, if not thousands, of different kinds of mycorrhizal fungi in the soil where a tree or shrub may be planted, and some of these fungi are specific to certain types of plants, especially when it comes to trees.
The commercial ‘manufacturer’ hopes that, with a large number of spores, perhaps some will be responsive. Conversely, other products are sold as a do-all, fix-all cocktail, suggesting that it is suitable for a very wide range of species. Both claims appear to be based on hope rather than fact.
There is no doubt that mycorrhizae are very important, if not vital, to plants growing in the natural environment, but can they be successfully ‘dropped in’ to manmade landscapes? The trouble is, we don’t know. Do they, will they, ‘work’ in such circumstances and situations – whatever the mix being offered?
Put simply, inoculants are commercial products that lack scientific trials and proof of effectiveness. There are thousands of scientific papers on mycorrhizae and commercial inoculants but most have been written by people with a vested interest in the product – such as scientists who work for the commercial manufacturer.
Furthermore, my colleague Len Phillips and I failed to find any papers on successful, well-designed trials that have been peer reviewed and replicated by independent scholars or organisations that would not benefit financially from positive results.
It is difficult to determine if an inoculant is of high quality. Some manufacturers count ‘propagules’ instead of ‘spores.’ Propagules can include root fragments and other inert materials, so the spore count might actually be much lower. Also, these specific species are almost certainly from an area that is foreign to the soil where a tree or shrub is to be planted. It is even possible that these foreign species will inhibit the growth of the native species, but there is often no way of knowing the species in the product, because the information on the packaging can be vague or confusing.
There is also no way to know if the product contains active fungi. They could all be dead. Manufactured inoculants are heat sensitive. If the container is left in a building or a delivery vehicle too long, the sustained high temperatures kill the fungi. All the positive research appears to have been done in a climate-controlled site, and for enhancing crop production and containerised nursery stock.
While it is well known that mycorrhizal fungi are a vital part of native ecosystems, it is not known for example, if they will survive in a tree pit in a city street. This is likely to be the case in many urban situations and highly managed landscapes such as garden lawns, public parks and manicured golf courses. If not dead on arrival, mycorrhizal fungi may decline or be killed by such diverse things as fertiliser applications, chemical spillage, soil compaction, excess animal urine or low humus soil conditions.
When applied at the time of planting young trees, or as a soil inoculant around mature trees, the trees may or may not be in the best physiological condition to exploit any benefits offered. Creating a good environment around a new or stressed tree by mulching may attract local mycorrhizal fungi spores into the humus, and so commercial inoculants would be a waste of time and money.
So, mycorrhizal fungi inoculants may not be useful in a ‘manmade’ and unnatural environment. Horticulturists, arboriculturists and landscapers should take this into consideration when specifying the use of inoculants when planting trees and other woody plants in artificial situations. A shovel-full of soil dug from around the roots of a nearby healthy specimen or similar species may be just as good, if not better than an inoculant.
Perhaps creating a mycorrhizal-friendly environment by mulching the surface, adding rich humus and creating non-compacted, unpolluted soil conditions, the spores may just turn up, survive and happily proliferate naturally. That is certainly what they are finding at RBG Kew after some experimentation (Kirkham, 2016) and in the streets of Stockholm (Embren, 2014).
We would be the first to endorse these products if convinced by peer-reviewed or bone fide scientific proof that they ‘work’. In the meantime, we worry they are nothing but snake oil.