Mycorrhizal fungi

Mycorrhizae are beneficial fungi growing in close partnership with plant roots. A helpful simplification is to think of the fungi as extensions of plant root systems. The fungi gather additional supplies of water and nutrients

To grow well, plants need a wide range of nutrients in various amounts, depending on the individual plant and its stage of growth. The three key nutrients usually taken up from the soil are nitrogen, phosphorus and potassium. Plants also need carbon, oxygen and hydrogen to make carbohydrates — their main energy source. Carbon and oxygen come from the air as carbon dioxide, while hydrogen is drawn from water absorbed by the roots. Other important soil nutrients include magnesium, calcium and sulphur. Most garden soils contain enough of these nutrients to support healthy plant growth.

– particularly phosphorous – and transfer them to the plant. The plants then feed the fungus with sugars made through photosynthesis

Photosynthesis is the process by which plants make their own food, harnessing the energy in sunlight to fuse water (absorbed from the soil) and carbon dioxide (absorbed from the air) to create simple sugars.

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Mycorrhizal fungi grow as long, branching microscopic strands (hyphae) which weave among fine roots (Hartig net) and extend into the soil to form an invisible interconnected network (mycelium). These cells are highly sensitive and adaptable, and can:

• absorb water and nutrients
  

  To grow well, plants need a wide range of nutrients in various amounts, depending on the individual plant and its stage of growth. The three key nutrients usually taken up from the soil are nitrogen, phosphorus and potassium. Plants also need carbon, oxygen and hydrogen to make carbohydrates — their main energy source. Carbon and oxygen come from the air as carbon dioxide, while hydrogen is drawn from water absorbed by the roots. Other important soil nutrients include magnesium, calcium and sulphur. Most garden soils contain enough of these nutrients to support healthy plant growth.

• alter unavailable forms of nutrients to make them usable by plants
• detect and respond to cues in the environment or from other life forms, like grazing wildlife or pathogens
• send resources and signals back and forth across its network and on to its plant partner. 

A single plant can support multiple mycorrhizal partners, and these can change over the course of its life.  A single mycorrhiza has the potential to connect the root systems of separate plants such that resources and signals can pass across the network, a phenomenon popularly known as the “wood wide web”. The long, winding fungal cells also bind soils together, providing invisible, moisture-retentive scaffolding. As a result, soils rich in mycorrhizal fungi resists compaction and temperature fluctuations, helping the soil web of life to flourish there.

There are two main classes of mycorrhizal fungi, based on how they connect to plant roots.

• Arbuscular Mycorrhizal Fungi (AMF) form partnerships with most plants and grow inside root cells. This type reproduces when the root cells die naturally and the fungus creates masses of microscopic spores underground in the rootzone.
• Ectomycorrhizal Fungi (ECM) partner with a selection of trees or woody plants (including oak, beech, birch, pine, hazel), growing around and between root cells, sheathing the fine root tips so these can be seen with the naked eye. This type reproduces by forming fruiting bodies, such as mushrooms, that appear aboveground and spread spores on the wind.

A third class grows both in and around root cells, known as ectendomycorrhizae, but far fewer plants have this type of partnership.

Further types of mycorrhizal fungi are specialist partners of certain plants, particularly for orchids and ericaceous

Plants that thrive in acid or ericaceous soil, which has a pH of less than 7 (neutral). You can measure your soil pH using a simple testing kit. Ericaceous or acid-loving plants include rhododendrons, camellias, heathers and blueberries.

plants.

The relationship between plants and mycorrhizal fungi is mutually supportive. Each partner collects and transfers resources, provides protection, and sends signals to the other. Mycorrhizae can offer protection against diseases and grazing wildlife, and help plants adapt more rapidly to stressful environmental conditions like drought or excessive sunlight.

Some mycorrhizae, particularly those partnering with orchids, contribute to the decay process of soil organic matter and supply organic compounds to the plant partners.

The life cycles of the mycorrhizal fungus and its plant partner are often linked in nature, where the wildlife that move plant seeds to new places also transfer mycorrhizal spores that then help the seedling survive after germination

Germination is the process by which a seed develops into a new plant. It starts with the seed coat absorbing water, expanding and splitting, which allows oxygen and water into the seed so its food reserves can be broken down and used by the embryo for growth. The first root (called the radicle) pushes into the soil to anchor the new plant in place and start absorbing water and nutrients. The first shoot (called the plumule) then grows upwards, out of the soil, towards the light.

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Certain activities harm mycorrhizal fungi directly or suppress mycorrhizal partnerships from developing.

• Thorough cultivation breaks up the connections of the fungal network and forces the fungi to regrow, costing them energy and resources.
• Regular soil disturbance means slower-growing fungi can never establish, and reduces the diversity of fungi that can survive there.
• Adding fertilisers, whether from synthetic or organic sources, deter plants from forming or maintaining their mycorrhizal partnerships, because they have their nutritional demands met that the fungi would otherwise fulfil.
• Use of fungicides harms beneficial mycorrhizal fungi as well as the intended targets and can be counterproductive as plants and soil with depleted fungal communities are more prone to pathogen attacks.

Mycorrhizal fungi spores are abundant in the air and in soil, and plants will partner with mycorrhizae naturally over time.

Gardens have a huge diversity of plant species and cultivars growing in them, and the mycorrhizae that are best suited to the conditions of a garden and the plants growing in it will find their own way there through slow and sustainable gardening practices, such as choosing plants that suit the conditions of your garden, digging less extensively or less often, and letting garden waste rot down on top of the soil.

Mycorrhizal biofertilizers are products intended to contain living mycorrhizal spores or fragments which can form partnerships with newly planted plants. They are often sold as a powder or pellet and can contain other microbes and ingredients. Such products rely on mycorrhizal fungi which can partner with a broad range of plants (generalists), and often contain a mixture of species.

When soil conditions are lacking in microbial diversity, it could benefit plants to introduce them to mycorrhizal partners from biofertiliser products.

For mycorrhizae biofertilisers to be effective, they need to persist within the plant roots, so adding the fungi in direct contact with roots will give them the best chance of successfully establishing there. Introducing fungi to younger plants also increases the likelihood that the fungi in the product will partner with the plant, as older root systems may have developed a complex microbiome around their roots already. Planting younger plants is generally good practice.

The market has expanded rapidly in recent years and companies vary widely in the quality of their products and production methods. The market is largely unregulated in the UK, so consumers have few assurances if the formulation, quality or claims made about these products are appropriate.

Recent scientific reviews have highlighted the risks associated with poor-quality products, including contamination with pathogens, spreading non- native

A native plant is one that originated or arrived naturally in a particular place without human involvement. In the British Isles, native plants are those that were here during the last ice age or have arrived unaided since.

species that could harm native fungal communities, containing undisclosed fertilisers, or the fungi simply may not have survived the production and retail process.

Do your due diligence when buying a new product to find a trustworthy supplier that only uses native microbes to safeguard soil microbes from unwanted pressure from non-native invasive

Invasive plants are those that grow vigorously, spread rapidly and can out-compete other plants. Native, non-native and cultivated plants can all be invasive.

species. The RHS endorses Empathy Rootgrow, one of the longest established companies in the sector, that has high standards of quality assurance, and only includes fungi of native provenance in its products.