Truffle hunters love them, podiatrists hate them, mycologists are fascinated, and gardeners may have mixed feelings.
It is estimated that there are over 1.5 million species of fungi on earth. Of these, only around 100,000 have been identified so far. We have long known and used mushrooms, truffles and yeasts in the kitchen and brewery. Just over 100 years ago, fungi came to the fore in healthcare, as they are used to make antibiotics.
There’s a downside to fungi too. Mould in between the bathroom tiles is never welcome. Gardeners and farmers battle mildews and fungal attacks on their crops. The discovery of powdery mildew on your favourite rose bush or pumpkin vine is understandably frustrating for the home gardener.
But these are the annoying relatives in a family of solid useful citizens who toil away relentlessly to improve your soil, and thus your entire garden. Fungi are critical in the decomposition of organic material, breaking materials down into their basic components. And then they go further and aid the plants in re-absorbing those nutrients.
It’s only recently that we have begun to study mycorrhizae, those fungi with a symbiotic relationship with plant roots.
Arbuscular Mycorrhizal Fungi
Different fungi have different roles in the soil. Some are involved in various processes of decomposition, while others make nutrients available to plants. Mycorrhizal fungi – and specifically arbuscular mycorrhizae – are those fungi that both benefit soil structure and help plants to absorb water and nutrients (N, P, K etc). In return, they take carbohydrates manufactured by the plants to support their own survival.
To fully understand arbuscular mycorrhizal fungi it’s useful to know a little about the meaning of the terms used in its name. ‘Arbuscular’ refers to the tree-like branching of the fungi’s mycelia that forms a network of fine strands (or hyphae). The term ‘mycorrhizal’ can be broken down into two parts: ‘myco’, which refers to the fungus itself, and ‘rhiza’, which means root. ‘Mycorrhizal’ means that the fungi has a symbiotic relationship with plant roots; a relationship that benefits both the fungi and the plant.
Improving Soil Structure and Preventing Erosion
In 1996 Sara Wright and colleagues described arbuscular mycorrhizal fungi secreting a material referred to as glomalin. This was a highly stable protein-based material which appeared to both hold soil particles together and to increase the storage of carbon and nitrogen in the soil. This material, in combination with the widespread networks of mycorrhizal mycelia, appears to significantly improve soil structure and can help to prevent erosion. So this is one easy to understand benefit of fungi. But the truly fascinating and intriguing aspect of arbuscular mycorrhizae is in their dynamic and complex relationship with plants and their root systems.
Making Nutrients and Water Available to Plants
The hyphae of arbuscular mycorrhizae can penetrate between the surface cells of a plant’s roots, joining the two organisms and allowing a two-way flow of nutrients and chemicals without harming either the fungi or the plant. While plant roots can go some distance (metres) in search of water and necessary nutrients, fungi will venture much further afield. What’s more, fungi can quickly respond to changes in the environment, giving the host plant a surprising adaptability to changing conditions, and a far greater reach than its own roots provide.
Not all plants have these relationships (for example, canola and other brassicas, beets and spinach do not), and if these are grown in the same soil continuously, the levels of arbuscular mycorrhizae will decline.
Encouraging Beneficial Fungi in Your Soil
Rotating crops and planting green manures which form a symbiotic relationship with arbuscular mycorrhizae will maintain viable fungal populations in your soil. Arbuscular mycorrhizae appear to mainly reproduce via spores, which can remain viable in the soil for 5 years, so digging over your garden bed may break up existing fungal networks but likely won’t prevent the recolonisation of crops which benefit from these fungi.
Do Fungi and Host Plants Communicate?
Fascinating research has begun on this complex relationship between fungi and plants, with indications showing the relationship also involves ‘communications’ about pest attacks and moving nutrients from areas of plenty to areas of scarcity. For example, the hyphae of arbuscular mycorrhizae are known to spread more densely into areas with low phosphorus: the host plants, when stressed with low phosphorus conditions, will release an exudate from their roots which promotes the growth of the fungal hyphae. These hyphae can greatly increase the uptake of phosphorus, benefiting the plant, which in turn photosynthesises and makes carbohydrates and other carbons available to the fungi. A true symbiotic relationship.
We have barely touched the surface of what fungi are capable of, but we do know that they can be of great benefit in the garden. So don’t slosh around the fungicide carelessly and without thought.
Remember – most fungi are your friend.
References
First image: www.freepik.com
Arbuscular Mycorrhizas – SA | Fact Sheets | soilquality.org.au
Merlin Sheldrake. Entangled Life: how Fungi Make Our Worlds, Change Our Minds and Shape Our Futures. Published by Penguin. 28 September 2021.
Wright et al., 1996; Wright and Upadhyaya, 1996
Related Articles:
Soil Contamination in Home Gardens
While growing your own food can be incredibly rewarding, it’s important to be mindful of the soil quality in your garden. Soil contamination is an…
Cover Crops 101: The Key to Building Soil Health
Cover crops, also known as green manure, are plants grown primarily to benefit the soil rather than for harvest. These crops play a crucial role in…