Types of mycorrhiza and why they matter in farming
Most crops share their roots with a fungus. What the two main types of mycorrhiza are, how they help with nutrients, soil carbon and root health, and what on your farm helps or harms them.
In brief
- Mycorrhiza is a partnership between soil fungi and plant roots. The plant feeds the fungus; the fungus's threads reach far beyond the roots and bring back phosphorus, other nutrients and water.
- About 72% of plant species form the arbuscular type (AM, often called VAM), the type in most field crops. Ectomycorrhiza (about 2%) is found mainly on trees such as pine and oak.
- Fungal threads and the sticky protein glomalin bind soil into crumbs that protect soil organic carbon.
- Mycorrhiza can make crops more resistant to some root diseases, but results vary; it supports disease management rather than replacing it.
- Till less, apply phosphorus by soil test and keep living roots in the soil. Less tillage and cover crops raised mycorrhiza in crop roots by about 30% across 54 field studies.
What is mycorrhiza?
Mycorrhiza (from the Greek for "fungus" and "root") is a partnership between certain soil fungi and plant roots. It is one of the oldest partnerships in the plant world, and most plant species have one.2
Both sides gain. The plant feeds the fungus sugars and fats made by photosynthesis; the fungus cannot live without them.3 In return, fine fungal threads called hyphae grow out from the root into the soil. They reach beyond the zone around the root that the plant has already emptied of nutrients, take up phosphorus, zinc, other nutrients and water, and pass them back to the plant.4
For a farmer this matters most for phosphorus. Phosphorus moves very slowly in soil, so roots soon use up what lies close to them. The fungal threads extend the root's reach far beyond that.4
Types of mycorrhiza
Mycorrhiza is grouped by where the fungus sits in the root. The two main types are endomycorrhiza, in which the fungus enters the root cells, and ectomycorrhiza, in which it wraps around the outside of the root.4
Arbuscular mycorrhiza (AM or VAM): the type in most crops
Arbuscular mycorrhizal fungi belong to the group Glomeromycota. They grow into the outer cells of the root without breaking the cell's inner membrane and form tiny, tree-like structures called arbuscules, where plant and fungus exchange nutrients. Many also form vesicles, small storage bodies full of fats, which is why the older name is vesicular-arbuscular mycorrhiza, or VAM.4
About 72% of plant species form this type,2 including most field crops: wheat, maize, sorghum, millets, pulses, soybean, groundnut, cotton, sugarcane, onion, tomato, chilli and most fruit trees. The fungi need air, so standing water in flooded paddy reduces them.4
Ectomycorrhiza: the type in many trees
Ectomycorrhizal fungi, mostly mushroom-forming Basidiomycota and some Ascomycota, do not enter the root cells. They form a thick sheath, or mantle, around the root tip and grow between the outer root cells in a network called the Hartig net, where the exchange takes place.4 Only about 2% of plant species form this type, mainly trees such as pine, oak and eucalyptus.2 It matters for plantations and agroforestry rather than for field crops.
A few other types exist. Orchids have their own mycorrhiza, and heathers and blueberries have ericoid mycorrhiza.2
Some crops form little or no mycorrhiza at all: the mustard and cabbage family (mustard, rapeseed, cabbage, cauliflower, radish) and the beet and spinach family.5 In field trials, maize grown after rapeseed or cabbage yielded less than maize grown after a crop that forms mycorrhiza.14
| Arbuscular mycorrhiza (AM, VAM) | Ectomycorrhiza (ECM) | |
|---|---|---|
| Fungi | Glomeromycota | Mostly Basidiomycota, some Ascomycota |
| Where the fungus grows | Inside the root's outer cells | Around the root tip and between the outer cells |
| Main structures | Arbuscules, vesicles, threads in the soil | Mantle, Hartig net, threads in the soil |
| Plants | About 72% of plant species, including most field crops, vegetables and fruit trees | About 2% of plant species, mainly trees such as pine, oak and eucalyptus |
| Soil carbon | Threads, glomalin and fungal remains help bind soil and hold carbon | Threads and fungal remains, important in forest soils |
| Root protection | Takes up space and food in the root zone, primes the plant's defences | Physical mantle over the root tip; some fungi produce antibiotic compounds |
How mycorrhiza builds soil organic carbon
Mycorrhizal fungi move part of the plant's carbon underground and help keep it there. This is one of the reasons soil organic carbon builds up where roots and fungi stay active.
Glomalin, a sticky protein. Arbuscular fungi produce glomalin, a sticky, water-repelling protein that coats their threads and stays in the soil after the threads die. It breaks down slowly and acts as a glue in soil crumbs.6 How much of the soil's carbon is glomalin is debated: published estimates vary widely, and the usual laboratory test for "glomalin-related soil protein" also picks up proteins and other material from plants, bacteria and humus, so it overstates the true amount.7
Soil crumbs that protect carbon. The threads work like a net, holding fine silt and clay particles together in crumbs that do not fall apart in water. Organic matter trapped inside these crumbs is harder for microbes to reach, so it is broken down more slowly.8
Fungal remains. Fungal threads grow and die quickly. Their remains contain tough materials such as chitin, and the remains of microbes, especially fungi, make up a large share of soil organic matter.9
How mycorrhiza helps protect roots from disease
Plants with a well-developed mycorrhiza often suffer less from soil-borne diseases such as Fusarium, Phytophthora, Pythium and Rhizoctonia, and from some nematodes. The effect varies with the crop, the fungus and the soil.10
Taking the space first. The fungus occupies root tissue and, in ectomycorrhiza, wraps the root tip in a sheath, leaving fewer places where disease organisms can get in.10
Competing for food. The fungus uses sugars and other substances leaking from the root that would otherwise feed disease organisms around it.10
A better-fed plant. A plant with enough phosphorus copes better with root damage.10
Priming the plant's defences. Mycorrhiza puts the plant's defence system on alert, largely through the jasmonic acid pathway, so the plant responds faster and more strongly when it is attacked.11 Some ectomycorrhizal fungi also produce antibiotic compounds.10
Mycorrhiza supports disease management; it does not replace healthy seed, crop rotation and other control measures. For another beneficial fungus at work against a crop disease, see biochar and Trichoderma against brinjal blight.
What weakens mycorrhiza in your field
- Deep, frequent tillage breaks up the fungal network in the soil.12
- High doses of soluble phosphorus. When a plant has plenty of easily available phosphorus, it sends less carbon to the fungus and the partnership weakens. Across field studies, phosphorus fertiliser reduced mycorrhiza by 32% on average.13
- Long bare fallows. The fungi need living roots; without them, their numbers fall.4
- Crops that do not form mycorrhiza, such as mustard, cabbage and beet, grown one after another.5, 14
- Standing water. The fungi need air, so flooding reduces them.4
How to protect and use mycorrhiza on your farm
- Till less. Zero-till or strip-till leaves the fungal network in place for the next crop. Across 54 field studies, less intensive tillage and winter cover crops each raised mycorrhiza in the next crop's roots by about 30%.12
- Apply phosphorus according to a soil test. A balanced dose feeds the crop without switching off the partnership.13
- Keep living roots in the soil. Cover crops, especially legumes, between main crops keep the fungi alive.12
- Plan your rotation. Where you can, follow mustard, cabbage-family or beet crops with a crop that forms mycorrhiza, such as a pulse or maize.14
- Use inoculants with care. Mycorrhizal products applied to seed, as a root dip or in the furrow at sowing can help, but results vary. A review of rainfed trials found an average yield increase of 23%,16 while in 54 Swiss maize fields the effect ranged from 12% less growth to 40% more.15 A product works only if its spores are alive when they reach the root: see why many VAM products give uneven results.
The surest way to benefit is to look after the mycorrhiza already in your soil: fewer disturbances, sensible phosphorus and living roots for as long as possible.
Editor's note: this article is based on a technical note by Dr. Abhishek Mathur. NanoKheti's editors rewrote it in plain language, checked each statement against the published research listed under Sources, and added the summary, table, questions and links.
