Mycorrhizal Fungi: The Underground Internet

What Mycorrhizae Are

The word mycorrhiza means "fungus-root," and the partnership it describes is one of the oldest and most important in the living world. Roughly ninety percent of all land plants form mycorrhizal associations — fungal filaments that colonise plant roots and extend outward into the soil, creating a secondary root system vastly more extensive than the roots themselves. The plant provides the fungus with sugars produced through photosynthesis. The fungus provides the plant with water and mineral nutrients — particularly phosphorus — that its roots alone could never reach.

This partnership predates roots themselves. Fossil evidence from four hundred million years ago shows mycorrhizal fungi associated with the earliest land plants, which had no true roots at all. The fungi were the roots. Plants colonised land not alone but in partnership with fungi, and that partnership has been maintained through every major group of land plants since. When you look at a forest, you are seeing an ecosystem that literally could not exist without the invisible fungal networks beneath it.

Two main types dominate. Ectomycorrhizal fungi form a sheath around the root tips of trees — primarily oaks, beeches, birches, pines, and eucalyptus — without penetrating the root cells. The network of fine hyphae radiating from this sheath can extend meters from the tree, accessing nutrient pools far beyond root reach. Many familiar forest mushrooms — chanterelles, boletes, truffles — are the fruiting bodies of ectomycorrhizal fungi associated with native oaks and other forest trees.

Arbuscular mycorrhizal fungi (AMF) penetrate the root cells of most herbaceous plants, grasses, vegetables, and many tropical trees, forming intricate branching structures called arbuscules inside the cells where nutrient exchange occurs. AMF are less visible — they produce no mushrooms — but they are arguably more important, partnering with the vast majority of food crops and wild plants worldwide.

What Mycorrhizal Networks Do

The numbers are staggering. Mycorrhizal hyphae are roughly one-fiftieth the diameter of the finest root hairs, allowing them to penetrate soil pores that roots cannot enter. A single plant's mycorrhizal network can extend the effective absorptive area of its root system by a factor of one hundred or more. In practical terms, this means a mycorrhizal tomato plant is not feeding from the soil volume around its visible roots — it is feeding from a vastly larger volume connected by invisible fungal threads.

Phosphorus transport is the best-documented benefit. Phosphorus binds tightly to soil particles and moves very slowly through soil solution — roots deplete the phosphorus in their immediate vicinity within days, creating a "depletion zone" around each root. Mycorrhizal hyphae extend beyond this zone, accessing fresh phosphorus sources and transporting it back to the plant. In phosphorus-limited soils, mycorrhizal plants can absorb three to five times more phosphorus than non-mycorrhizal plants of the same species.

But the network does far more than transport nutrients. Research by Suzanne Simard and others has shown that mycorrhizal networks connect individual trees in a forest, allowing the transfer of carbon, water, and chemical defence signals between them. Large established trees — "mother trees" — funnel carbon through the network to shaded seedlings on the forest floor, subsidising their survival until a gap opens in the canopy. When a tree is attacked by insects, it can send chemical alarm signals through the mycorrhizal network, triggering neighbouring trees to upregulate their defences before the attack reaches them. This is the "wood wide web" — a communication and resource-sharing network that makes a food forest far more than the sum of its individual plants.

How to Protect Mycorrhizal Networks

Mycorrhizal fungi are resilient but sensitive to certain kinds of disturbance. The most damaging practices are also the most common in conventional agriculture and gardening.

Tillage is the primary destroyer. Every pass of a plough or rotavator physically severs hyphal networks that took months or years to develop. A single digging event can reduce mycorrhizal colonisation of roots by fifty to ninety percent. This is one of the strongest arguments for no-dig management — not just that it preserves soil structure, but that it preserves the fungal networks that make soil structure functional. Masanobu Fukuoka understood this intuitively, arguing that the less humans disturb the soil, the more effectively it functions.

Fungicides — including many common garden products — kill fungi indiscriminately. Systemic fungicides applied to plants can be translocated to roots and kill mycorrhizal partners along with target pathogens. Even some organic fungicides, particularly copper-based products, suppress mycorrhizal colonisation at high application rates. Use fungicides only when absolutely necessary and as a last resort within an integrated pest management framework.

Bare soil and fallow periods starve mycorrhizal fungi. Without a living root to supply carbon, the fungal network depletes its energy reserves and dies back. Keeping living roots in the soil year-round — through cover crops, perennial plantings, or living mulch — maintains the mycorrhizal network in a ready state for the next crop. This is why transplants establish faster in soil that was recently growing something than in soil that sat bare for months.

Excessive phosphorus fertilisation short-circuits the partnership. When phosphorus is abundant in soil solution, plants reduce their investment in mycorrhizal fungi — why pay the carbon cost of a fungal partner when the nutrient is already available? High-phosphorus soils from years of over-fertilisation often have severely reduced mycorrhizal communities. A soil test can tell you whether you actually need phosphorus before you apply it.

When and How to Inoculate

In many soils, native mycorrhizal fungi are already present and need only to be protected rather than introduced. But in several situations, deliberate inoculation can dramatically improve plant establishment and growth.

Severely degraded soils — construction sites, mine spoils, land that has been under continuous tillage and chemical agriculture for decades — may lack viable mycorrhizal spore banks. New plantings in these soils establish slowly and struggle until mycorrhizal networks rebuild naturally, which can take years. Inoculation jumpstarts the process. Apply mycorrhizal inoculant directly to the root zone at planting time — either dusted onto bare roots of transplants, mixed into the planting hole backfill, or watered in as a slurry around seedlings.

Commercial mycorrhizal inoculants vary widely in quality. Look for products that list specific species of fungi and guarantee a minimum spore count per gram. For vegetable gardens and herbaceous plantings, products containing Rhizophagus irregularis (formerly Glomus intraradices) — the most widely effective arbuscular species — are a reliable choice. For tree plantings, ectomycorrhizal inoculants containing Pisolithus, Rhizopogon, or Scleroderma species suit most temperate forest trees.

The simplest inoculation method requires no commercial product at all. Soil from a healthy, undisturbed site — a forest floor, a long-established garden, a native grassland — contains a diverse community of mycorrhizal spores and hyphal fragments. A few handfuls of this soil mixed into planting holes or scattered on the surface of new beds introduces a locally adapted mycorrhizal community for free. Pioneer species in restoration projects particularly benefit from this approach, as early mycorrhizal colonisation helps them tolerate the harsh conditions of degraded sites.

See Also

• The Soil Food Web — the broader biological community in which mycorrhizae operate
• No-Dig Gardening — the management approach that best preserves fungal networks
• Food Forest Design — designing systems that leverage mycorrhizal connectivity
• Native Oaks — keystone trees whose health depends on ectomycorrhizal partnerships
• Cover Cropping — keeping living roots in the soil to sustain mycorrhizal networks year-round