For the last 8 years, The Rodale Institute has partnered with the USDA-ARS to pioneer an On-Farm AM Fungus Inoculum Production System; a system which would make the economic and environmental benefits of mycorrhizae available to more farmers. Over the years, our trials have focused on developing a low-cost procedure and identifying features to maximize propagule production. Here we summarize what we’ve learned from those years and share everything you need to know to start your own on-farm system.
Arbuscular mycorrhizal (AM) fungi are the most important mycorrhizae in agricultural ecosystems due to the fact that they colonize 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 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.
While some standard agricultural practices including frequent tillage and heavy phosphorus fertilization negatively impact mycorrhizae, many sustainable farming practices can be used to bolster native mycorrhizal fungus populations. Even soils that have been intensively managed for an extended period of time contain populations of mycorrhizae that can be augmented by using cover crops, developing a diverse crop rotation, and growing crops that form a symbiosis with AM fungi.
Inoculation with mycorrhizal fungi offers another opportunity to take advantage of the benefits of mycorrhizae. Mycorrhizal spores, pieces of colonized crop roots, and viable mycorrhizal hyphae function as active propagules of AM fungi that can be used as inoculum to “infect” other plants with AM fungi.
Under certain circumstances, using inoculum may be preferable to using management practices to bolster AM fungi populations. Previous fungicide use and other extreme cases may require inoculation to reintroduce AM fungi to the severely degraded soil microbial community. In less severe circumstances, inoculum may be used to produce pre-colonized seedlings that can take advantage of mycorrhizae’s benefits from their first day in the field. Past research shows that this competitive advantage can have a positive impact on yields. In a 2008 study, strawberry plants inoculated with AM fungi prior to outplanting produced 17% more fruit than uninoculated controls (Douds et al. 2008). One study with peppers observed a 14 to 23% yield increase from inoculated plants grown in compost-amended soil and a 34% increase one year from inoculated plants treated with chemical fertilizers (Douds and Reider 2003). Yield increases have also been observed for a variety of other crops including tomatoes, potatoes, onion, peanuts, watermelon, garlic and celery.
While commercially produced inoculum is available, it comes at a cost to farmers. The price of commercial inoculum reflects the costs of current production methods including greenhouse or lab space as well as the labor and time associated with isolating AM fungi from the original medium and/or mixing the spores with a carrier substrate. These costs, as well as shipping and handling, are all passed on to the farmer (Douds 2010).
Mycorrhizal fungi are invaluable partners for plants, forming symbiotic relationships with roots that enhance growth, nutrient uptake, and overall resilience. By increasing the mycorrhizal fungi in your garden soil, you can help your plants thrive! In this guide, I’ll explain what mycorrhizal fungi are, why they are so beneficial, and give tips on growing mycorrhizal fungi levels through organic gardening practices.
What Are Mycorrhizal Fungi?
Mycorrhizal fungi form mutually beneficial relationships with plant roots. The word “mycorrhiza” literally means “fungus root” in Greek. The fungi colonize the root cells of host plants, receiving carbohydrates in return for increasing the plant’s access to water, nutrients, and soil minerals.
There are several types of mycorrhizal fungi including
- Arbuscular mycorrhizal fungi (AMF) – the most common; they penetrate root cells and form arbuscules for nutrient exchange
- Ectomycorrhizal fungi – form a sheath around root tips, common with trees
- Ericoid mycorrhizal fungi – associate with heathers and heaths
- Orchid mycorrhizal fungi – provide nutrients to orchids with very small seeds
The Benefits of Mycorrhizal Fungi
Mycorrhizal fungi increase the effective surface area of roots, providing improved:
- Nutrient absorption – especially immobile nutrients like phosphorus, zinc, and copper
- Drought tolerance and disease resistance
- Soil aggregation and structure
- Resistance to soil toxins and pollutants
With mycorrhizal fungi on their roots, plants can decrease dependence on chemical fertilizers and increase overall sustainability and yields.
How Mycorrhizal Fungi Help Plants
Mycorrhizal fungi improve plant growth through:
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Increased Nutrient Absorption – The fungal hyphae explore and access more soil and minerals.
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Improved Water Uptake – The extra-radical hyphae bridge gaps between soil particles, improving absorption.
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Disease Protection – Acting as secondary roots, the fungi limit direct root exposure to pathogens. They may also induce systemic resistance.
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Improved Soil Structure – Fungal hyphae and the glycoprotein glomalin contribute to stable soil aggregates.
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Pollutant Protection – Mycorrhizal fungi can metabolize certain pollutants and reduce toxicity to plants.
Ways to Increase Mycorrhizal Fungi
Here are some methods to increase mycorrhizal fungi in garden soil:
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Avoid excessive tillage – Repeated tilling damages fragile fungal networks.
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Grow mycorrhizal host plants – Grow tomatoes, peppers, clover and more. Avoid brassicas.
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Apply compost – Contains fungi spores and mycelium to spread to plant roots.
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Top dress with organic matter – Aged manure, leaf litter, leaf mold harbor mycorrhizal fungi.
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Interplant with nurse crops – Clover roots harbor fungi to colonize nearby plants.
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Inoculate with commercial mycorrhizae – Add concentrated inoculants when planting.
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Reduce soil disturbance – Limit tillage and use cover cropping for healthy fungal networks.
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Eliminate synthetic fertilizers/fungicides – These harm beneficial fungi, prioritize organic practices.
Making DIY Mycorrhizal Fungi Inoculant
Make homemade inoculant by:
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Collecting soil samples from healthy, undisturbed areas containing abundant mycorrhizal fungi.
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Mixing soil samples with vermiculite, perlite, peat moss or compost to increase volume at a 1:3/4 ratio.
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Planting mustard, millet or sorghum seeds in the inoculant blend and allowing 4-6 weeks growth until established.
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Cutting shoots, chopping colonized roots into the inoculant medium.
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Letting the inoculant rest several more weeks for increased spore formation.
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Applying the finished product by mixing into planting holes or seed rows at 1-2 tablespoons per hole/foot of row.
Homemade inoculant works best when using live cultures immediately. You can also propagate specific strains with lab methods. This simple soil-based method harnesses beneficial native fungi.
Adding mycorrhizal fungi gives plants an upgraded root system! Follow organic practices that protect and promote mycorrhizal fungi to enhance your garden’s health without heavy chemical inputs. Implement some of these methods this season – your plants will thank you!
On-farm production of inoculum
The goal of our research was to develop an on-farm inoculum production system that generated a potent, effective, species rich inoculum that was inexpensive to produce. By avoiding the associated costs of commercially produced inoculum, on-farm production makes the economic and environmental benefits of AM fungi available to a larger number of farmers.
Although significant yield increases vary by year, crop, and cultivar, our system was designed with routine use in mind. With yearly use, farmers will be poised to profit when mycorrhizae ease the conditions that depress yields, but in years when inoculum does not impact yields, the inexpensive system will not be an economic burden. We are targeting vegetable farmers who produce their own seedlings on site for later outplanting to the field, so adapting their current systems to produce colonized seedlings is simple. Although large scale, direct application of inoculum to fields is possible, the return on investment for crops such as field corn and soybean is too low to make inoculum application economically attractive.
The on-farm system starts by planting “host plant” seedlings into black plastic bags filled with a mix of compost, vermiculite and local field soil. AM fungi present in the field soil colonize the root of the host plants and over the growing season, the mycorrhizae proliferate as the host plants grow. When the host is frost killed, the mycorrhizae will naturally overwinter in the compost and vermiculite mixture, and the inoculum will be ready for use in the spring. Most aspects of the system, from the type of compost used to the dilution rate, have been examined through experimentation to pinpoint the best methods to maximize production. The result is a system that has successfully propagated all AM fungi tested and can produce hundreds of propagules per cubic centimeter. In one trial, 465 propagules cm-3 were produced, a 7000 fold increase relative to the concentration in the field soil that was initially added to the bag.
Here we walk through the process step by step and analyze the details that will make your on-farm system a success. Citations are provided to direct you to articles for further reading (see the end of this article for all reference information).
Choosing a host plant
The most important factor in choosing a host plant is selecting a plant that supports mycorrhiza growth. Crops such as spinach, sugar beet, lupine and members of the mustard family do not form a symbiosis with AM fungi. A dependable host for the majority of AM fungi species, bahiagrass (Paspalum notatum Flugge) has been used extensively. Additionally, to prevent the spread of pathogens, the host plant should be from a different family than the inoculated crop. Due to the fact that the inoculum system targets vegetable producers, bahiagrass, a member of the grass family, is an ideal general host. Finally, as a tropical plant, bahiagrass will be frost killed and will not become a weed pest in the field.
As of yet, bahiagrass seedlings are not commercially available. In order to use bahiagrass as a host plant, farmers must establish their own seedlings. We simply germinate bahiagrass seeds in either vermiculite or seed starter and transplant the seedlings into conical plastic pots filled with a sand and soil mixture. Taller than the typical greenhouse flat, these conical pots produce seedlings with a long root system. When outplanted into the bags of diluted compost, the long root ball will come into contact with propagules deep in the bag more quickly than would seedlings produced in shallower pots. The sand and soil mixture we use in the conical pots is a 1:3 soil:sand (volume basis) mix that uses sterilized field soil and coarse swimming pool sand. If the bahiagrass seedlings are grown in a typical greenhouse potting medium, we have found that they become iron deficient.
The process of germinating bahiagrass and establishing seedlings is usually started in the greenhouse four months before the last frost date so the plants can be transplanted as soon as possible after the frost. (For a printable timeline and information on where find these materials see our Quick and Easy Guide to On-Farm Mycorrhizae Inoculum Production.)
How to Grow Your Own Mycorrhizal Fungi in Chicken Manure and Wood Shavings
FAQ
How long does it take for mycorrhizal fungi to grow?
Though it varies by plant species, growing protocol, etc., it generally takes about 8 weeks for benefits to become visible to the grower in comparison trials. Differences may be visible sooner in more stressful growing conditions, as this is when the mycorrhizae can bring the most benefits to the plants.
How do you encourage mycorrhizal fungi?
- Reduce tilling. …
- Leave a layer of organic matter on top of the soil. …
- Use native plants. …
- Don’t leave the soil bare.
Can you sprinkle mycorrhizae on top of soil?
Mycorrhizal products are often used by gardeners when sowing seeds, when transplanting, or to inoculate a bed before planting, working them into the top 4-6 inches. Inoculated soils will actually improve year after year, so it’s a sustainable product.
What are the disadvantages of mycorrhizal fungi?
stramonium, positive associations between AM fungi and plant fitness may not be proportional and, that at high colonization densities, mycorrhizae may have detrimental effects, perhaps by competing with plants for nutrients, or by interfering with other essential interactions.