Mycorrhizal Fungi in Agriculture: How Endomycorrhizae Work and When to Use Them

A healthy crop root is not working alone. Fine fungal threads run from inside the root cells out into soil the root itself will never touch, hauling back phosphorus, zinc, and water in exchange for a share of the plant's sugars. That partnership is the mycorrhizal symbiosis. Plants and mycorrhizal fungi have been trading this way for over 400 million years, since before plants had true roots [1].

Around 70 to 90 percent of land plant species form the partnership [2], including most of the crops that matter commercially: corn, wheat, rice, soybeans, potatoes, tomatoes, onions, grapes, citrus, coffee, and nearly every vegetable and fruit crop you can name. Yet decades of tillage, fallow, and heavy phosphorus fertilization have left many farm soils with weak native mycorrhizal populations. That gap is why concentrated mycorrhizal inoculants exist, and why they have become a standard input in organic, regenerative, and increasingly conventional programs.

This guide covers what mycorrhizal fungi are, the difference between endomycorrhizae and ectomycorrhizae and why only one of them belongs in a crop program, the mechanisms behind the phosphorus and drought benefits, which crops respond and which never will, and how to evaluate and apply a commercial inoculant. It is written for growers, agronomists, and the distributors who supply them. For the broader picture of how soil biology works as a system, start with our pillar guide to how beneficial soil microbes work.

Key takeaways

• Mycorrhizal fungi colonize plant roots and exchange soil nutrients and water for plant carbon. Endomycorrhizae (AMF) partner with most crops; ectomycorrhizae partner mainly with trees.
• The hyphal network functions as a root extension, reaching phosphorus and moisture beyond the depletion zone that forms around every root.
• Benefits with the strongest evidence: improved phosphorus and micronutrient uptake, better drought and salinity tolerance, and improved soil aggregation through glomalin.
• Brassicas (canola, cabbage, broccoli) and beet-family crops (sugar beet, spinach) do not form mycorrhizae. No inoculant changes that.
• Very high soluble phosphorus suppresses colonization. Mycorrhizae perform best in moderate-P programs, which is where growers want to cut fertilizer spend.
• Compare inoculants on endomycorrhizal propagules per gram. Ecto spores are cheap to produce in huge counts and inflate total-count claims without adding function for most crops.
• ABI manufactures concentrated endomycorrhizal inoculant in Wisconsin, as a single product or blended with bacteria and Trichoderma in a custom formulation.

Table of contents

1. What are mycorrhizal fungi?
2. Endo vs ecto: the difference, and which one agriculture uses
3. How arbuscular mycorrhizal fungi work
4. What AMF does for crops
5. Which crops respond, and which cannot
6. How to choose and apply a mycorrhizal inoculant
7. Pairing mycorrhizae with bacteria and Trichoderma

1. What are mycorrhizal fungi?

The word mycorrhiza means fungus-root, and that is a fair description. A mycorrhizal fungus lives partly inside or on a plant root and partly in the surrounding soil, connecting the two. The plant feeds the fungus carbon fixed through photosynthesis. The fungus feeds the plant phosphorus, micronutrients, and water gathered by its hyphae, threads a fraction of the width of a root hair that can explore soil pores no root could enter [1].

This is a symbiosis, not an infection. The plant regulates how much carbon it spends, and under most field conditions the trade pays. A colonized root system behaves like a much larger root system.

Mycorrhizal fungi are obligate symbionts. They cannot complete their life cycle without a living host root, which is why long fallow periods and non-host rotations run down native populations. And "mycorrhizae" covers several distinct types of fungi. The two that dominate the conversation are endomycorrhizae and ectomycorrhizae.

2. Endo vs ecto: the difference, and which one agriculture uses

The endo and ecto prefixes describe where the fungus grows relative to the root's cells.

Endomycorrhizae, formally arbuscular mycorrhizal fungi (AMF), penetrate the walls of root cortical cells and form arbuscules, branched tree-shaped structures where the nutrient trade happens. AMF belong to the phylum Glomeromycota. The two most-studied species in the group, Funneliformis mosseae (long known as Glomus mosseae) and Rhizophagus irregularis (formerly Glomus irregularis, and before that Glomus intraradices), are the workhorses of the commercial inoculant industry and the pair in ABI's Endo Mycorrhizae. They partner with the large majority of herbaceous plants, which means nearly all annual crops, vegetables, fruits, and vines [1][2].

Ectomycorrhizae never enter the root's cells. They sheath the root tip in a mantle of fungal tissue and grow between cells in a lattice called the Hartig net. Their hosts are mostly woody: pine, fir, spruce, oak, beech, birch, and a few others. Many familiar forest mushrooms, including porcini and chanterelles, are the fruiting bodies of ectomycorrhizal fungi.

Unless you are planting pines or oaks, the mycorrhizae in your program should be endomycorrhizae. Ectomycorrhizal species on a vegetable or row-crop label will not colonize those crops at all, and there is an economic reason they show up on labels anyway, covered in the buying section below.

3. How arbuscular mycorrhizal fungi work

Colonization starts when an AMF propagule, a spore or a fragment of colonized root or hypha, germinates near a host root. The fungus senses root exudates, grows toward the root, and enters the cortex. Inside cortical cells it builds arbuscules, and the exchange happens across the membrane surface of each arbuscule: phosphate and micronutrients move to the plant, sugars and lipids move to the fungus [1].

Outside the root, the fungus does the work growers care about. Extraradical hyphae extend centimeters beyond the root surface, past the depletion zone. Phosphate barely moves in soil, so a root quickly strips the few millimeters around itself and then sits next to its own shortage. Hyphae cross that gap, mine undepleted soil, and pipe phosphate back to the plant. The same network takes up zinc and copper, two micronutrients that share phosphorus's immobility problem, and conducts water from micropores too small for roots [1][3].

The network also changes the soil itself. AMF hyphae produce glomalin-related soil proteins, sticky compounds that bind soil particles into stable aggregates [4]. Aggregated soil drains better, resists crusting and erosion, and holds more plant-available water. This is a slower benefit than nutrient uptake, building over seasons rather than weeks, and it is one reason regenerative programs treat mycorrhizae as infrastructure rather than a seasonal input.

4. What AMF does for crops

Phosphorus uptake is the best-documented benefit. In responsive crops and moderate-P soils, a functioning AMF network can supply a large share of the plant's phosphorus demand, because hyphae reach phosphate the root system cannot [1]. This makes AMF a natural partner to the microbes that dissolve locked phosphorus in the first place. Solubilizing organisms convert bound phosphate into available forms, and the hyphal network delivers it. Our guide to phosphate-solubilizing bacteria and fungi covers that first half of the pipeline.

Drought and salinity tolerance follow from water access. A review of more than 200 studies found mycorrhizal plants frequently maintain better water status and keep growing longer into a dry spell than non-mycorrhizal plants [3]. The mechanisms: more absorptive surface, access to smaller pores, and better ion balance under salt stress. For growers on sand, on limited irrigation allocations, or in climates with hard mid-season gaps, this is often the benefit they notice first.

Yield responses vary, and the reasons are known. A meta-analysis of field studies found AMF inoculation raised yield by roughly 23 percent on average [5], and a broad synthesis across hundreds of experiments confirmed plants respond most strongly where phosphorus is limiting and native mycorrhizae are scarce [6]. Those conditions describe a lot of farmland, but not all of it. A biologically active soil with strong native AMF may show little response to added inoculant, while a fumigated, eroded, or long-fallowed field can show a dramatic one. The evidence does not support quoting a guaranteed percentage.

Nutrient efficiency compounds across the program. Because colonized plants capture more of what is already in the soil, AMF supports the same input-reduction logic we lay out in our nitrogen use efficiency guide: hold yield while trimming applied fertilizer, and let biology close the gap.

5. Which crops respond, and which cannot

Most crops are hosts, and a short list of crop families is not. The table below covers the categories growers ask about.

The non-host families evolved away from the symbiosis and their roots do not admit AMF. Applying mycorrhizal inoculant to a canola or sugar beet field is money spent on biology that cannot attach. It also means a rotation heavy in non-host crops starves the native AMF population, since obligate symbionts need living host roots. Growers coming out of a canola year or a long fallow are inoculating at the right moment, because that is when native propagule counts are lowest.

Timing matters for the same reason. AMF benefits compound from early colonization, so the inoculant belongs at planting or transplant, in the furrow, on the seed, or in the transplant hole, where germinating propagules meet new roots immediately.

6. How to choose and apply a mycorrhizal inoculant

Evaluating a mycorrhizal inoculant comes down to five questions.

Is it endo, ecto, or a mix? For crops, you want endomycorrhizae, and this is the question labels are least straightforward about.

How to read a mycorrhizal label. AMF are obligate symbionts, so they can only be multiplied on living host roots. That caps how many propagules a gram of finished product can hold and makes genuine endo inoculant slow and costly to produce. Ectomycorrhizal spores are the opposite: species like Pisolithus tinctorius fruit in huge puffball-like structures whose spores can be harvested by the trillion at very low cost. So a blended label can advertise an enormous total propagule count where nearly all of it is ecto spores that will never colonize a vegetable, row crop, or vine. The count is real. The function, for most crops, is not. Compare products on endomycorrhizal propagules per gram, and treat a giant total count with a small endo fraction accordingly.

What is the propagule concentration? AMF products are specified in propagules per gram, not CFU, because AMF cannot be fermented like bacteria; they are grown on living roots. Propagules include spores, colonized root fragments, and hyphal fragments, all of which can start a colonization. Compare products on propagules per gram and cost per treated acre, not package weight.

Which species are in it? Species differ in how they perform across soils and conditions [6], so the names on the label matter more than the number of them. Funneliformis mosseae and Rhizophagus irregularis are the two most-studied AMF species in the scientific literature, both broad-host generalists with documented performance across field crops, vegetables, and perennials. That pair is the basis of ABI's Endo Mycorrhizae. A long species list is not a quality signal by itself; two proven generalists at honest concentrations beat ten names at trace amounts.

How will it be applied? ABI's Endo Mycorrhizae is a wettable powder, which is the most flexible format: it runs through transplant water, drench applications, and drip systems where screen sizes allow, and it can be applied in-furrow, dusted onto seed, or blended into transplant media. Granular products exist in the market and are limited to dry placement. Whatever the format, the non-negotiable is root contact. Mycorrhizal propagules broadcast on the surface, away from roots, accomplish little.

What will the chemistry around it do? Two cautions. Very high rates of soluble phosphorus suppress colonization, because a plant swimming in easy phosphate spends less carbon on fungal partners [1]. Moderate starter-P programs and mycorrhizae get along; heavy broadcast P and mycorrhizae do not. Second, certain systemic fungicide seed treatments can slow early colonization. Check compatibility for the specific active before combining, or separate the inoculant from the treated seed by placing it in-furrow.

Storage is the easy part. AMF propagules are dormant survival structures and hold up well in cool, dry storage. Reputable manufacturers state shelf life and storage conditions on the technical sheet, and ABI publishes both for every product.

7. Pairing mycorrhizae with bacteria and Trichoderma

Mycorrhizae rarely work alone in a serious biological program, and they should not.

Phosphate-solubilizing bacteria such as Bacillus megaterium and potassium-mobilizing Bacillus mucilaginosus free bound nutrients; the AMF network delivers them. Rhizosphere colonizers like Bacillus subtilis and Pseudomonas fluorescens promote the root growth that gives the fungus more territory to colonize. Trichoderma harzianum is combined with AMF in commercial practice, and greenhouse and field studies of the pairing report growth results as good as or better than either organism alone [7].

This is the logic behind multi-function blends, and it is why ABI builds AMF into custom microbial formulations alongside bacterial and Trichoderma components. Distributors and private-label brands can spec the endomycorrhizal component, the bacterial package, the carrier, and the propagule and CFU targets through the custom blend builder, or talk to our team about bulk and OEM supply from our Wisconsin facility.

For buyers: ABI manufactures concentrated endomycorrhizal inoculant and supplies it as a single product, in bulk, or blended to spec for private-label and OEM programs. Request pricing or build a custom blend.