Culturing Arbuscular Mycorrhizal Fungi Inoculum for Your Market Garden

As part of the Farming with Fungi project we are adopting and trialling a method of horticulture that aims to enhance support soil and plant health by focusing on the activity of Arbuscular mycorrhizal fungi (AMF). That is to say that the primary management objective is the provisioning of the hyphosphere, the area of interaction between plant roots and AMF. Part of this approach utilises a simple method of culturing AMF to use in seed and potting mixture to ensure the relationship between plant and fungi is established early in the development of the plant. In this article we will explain this process of making your own AMF inoculum and how it can be applied in a market garden..

What is Arbuscular mycorrhizal fungi (AMF)?

Arbuscular mycorrhizal fungi, sometimes referred to as endomycorrhizal fungi, or vesicular arbuscular mycorrhizal fungi, (AMF for short!) are the most important mycorrhizae in agricultural ecosystems due to the fact that they form a mutualistic symbiosis with the majority of crop plants. Known as “obligate symbionts,” AM fungi must associate with plant roots to survive; it is this association that begins a mutually beneficial relationship between the fungi and the plant. In return for sugars from a plant, the long, thread-like structures of fungi, the hyphae, act as an extension of a plant’s root system and increase a plant’s access to immobile nutrients including phosphorus (P), zinc and copper. While plant root hairs extend 1-2 mm into the soil, the mycorrhiza’s hyphae explore a greater volume of soil and can extend up to 15 cm from the plant’s roots. The relationship between mycorrhizae and crop plants often enhances plant growth and yield, but even when no growth enhancement occurs, the majority of P uptake can be attributed to mycorrhizae.

Mycorrhizae have also been credited with increasing a plant’s disease resistance, improving a plant’s ability to grow under drought conditions, and improving soil structure.

Creating Your AMF Inoculum

There are various mycorrhizal inoculum products on the market they come in the form of powders, granules and liquid. Some of these products list indicate what species they contain including the amount of propagules per gram, some contain ecto-mycorrhizal spores that have no use in the application for annual plants. Because of the challenges in isolating AMF spores these products, especially the better, more appropriate ones, are often expensive. It also might be the case that using these strains of AMF that have been cultured from other localities might not, initially at least, be as beneficial as the local, indigenous AMF already adapted to your locality.

Therefore, this technique of making your own indigenous AMF inoculum is by far cheaper than buying a commercial product and utilises the adaptation of your local AMF strains. Originally pioneered by the Roledale Institute in 2010. It is a simple process of growing grass in containers, the detail comes in the soil mixture used, the substrate, but as you will see with a few simple steps and a little patience this method can be easily incorporated into your soil fertility practice.

Here’s a step-by-step guide to get you started:

1. Sourcing Spores and Propagules: Obtain AMF spores by collecting soil samples from areas with healthy vegetation. Grassy edges of nearby woodland and forests, or well-established organic gardens are good places to start. Make sure to collect from multiple locations to ensure a diverse AMF population and choose flora that you know readily sustains associations with AMF.
2. Culturing Host Plants: Select host plants that readily form symbiotic relationships with AMF. We are using sorghum sudan grass. As it has been shown to form symbiosis with a wider range of AMF species. It is also a sub tropical plant and will  be sure to die in the winter, this stimulates more sporulation of the AMF.
3. Start your Host Plants in Vermiculite: Plants that form AMF associations produce phytohormones that stimulate the AMF, exuded through the roots they are chemical signals that call for the AMF. When germinating your seeds in vermiculite the plant quickly uses the latent nutrition in the seed and begins to produce these phytohormones. In short you create AMF hungry seedlings. Use cell trays, flats, and aim to have 4 plants per container (in step 6 below)
4. Inoculation of Host Plant: You can now transplant your hungry seedlings into a medium of sand and your inoculum, the soil and root fragments of your indigenous AMF you collected earlier. Using larger cells or pts (we use nursery trays) allow the seedling to grow for another 2-3 weeks.
5. Preparing your soil medium (Substrate): Understanding that there is a greater AMF association in nutrient depleted soils, the soil substrate needs to be low in nutrients, in particular P. The original study showed that a soil mixture of 1:4 compost to vermiculite performed best. This is a lot of vermiculite, something that is expensive and has an industrial legacy. We are experimenting with biochar, we will cover this in another article, but be aware that biochar needs to be “charged” to some extent. If used directly it will absorb the already little amount and nutrients in the compost. You should aim to have at least 20l of substrate so use approximately 25l containers (used strong plastic bags (feed bags) will suffice).
6. Symbiotic Growth: Once the risk of frost has passed, transplant your inoculated grass seedlings into the containers. You will now allow the host plants to grow throughout the year providing them with appropriate care (weeding and watering). The AMF will colonise their root systems during this period. In the winter cut the grass back to the roots to ensure it dies off, if using a hardy grass like rye you may need to take extra measures such as drenching the roots and covering with a cardboard mulch. We are experimenting using sorghum sudan grass as it has been shown to assoicate with many differnet species of AMF.
7. Applying Your Inoculum: In the following spring the root bound mass in the containers should be full of AMF propagules. To apply this indigenous AMF inoculum simply break up the root bound mass of soil medium and add to your seed and potting mixtures at a ratio of 1:40 and 1:20 respectively.

Benefits:

Using your indigenous AMF inoculum can offer several benefits for your market garden, the research and evidence is now extensive, we are gathering links to papers and article in our research section of this website but the following is breif overview:

1. Improved Nutrient Uptake: AMF-enhanced plants can access nutrients more effectively, leading to healthier growth and improved yields.
2. Water Efficiency: AMF can enhance the water-holding capacity of soil, reducing water stress on plants.
3. Reduced Fertiliser Dependency: With enhanced nutrient absorption, you may be able to reduce fertiliser application.
4. Increase Carbon in Soil: AMF exude a carbon rich compound known as Glomalin, rich in carbon it can remain in the soil for many years.
5. Environmental Resilience: AMF contributes to soil health and ecosystem balance, aligning with sustainable farming practices.

Considerations:

It is important to note that the process of creating AMF inoculum requires careful attention to avoid contamination. Additionally, while using AMF can bring significant benefits, it’s just one component of holistic soil and plant health management. Learning more about AMF can give us a greater insight into the symbiotic relarionships in a healthy thriving ecosystem, provisioning the hyphoshere is an approach that seeks to enhance these biological interactions

In Conclusion: Partnering with Fungi for a Thriving Market Garden

Making your own AMF inoculum is a simple activity that has the potential to increase crop yields while also connecting you with the intricate web of fungal life within the soil. By incorporating AMF into your garden, you’re tapping into the remarkable power of nature’s alliances to enhance your plants’ vitality and ultimately create a more abundant and resilient agro-ecosystem.