What it is
Mycorrhiza means fungus-root. Ninety percent of land plants form one.
Fungal threads colonise the root and fan out into the soil. The plant pays in sugar. The fungus pays in water and phosphorus that roots alone cannot reach.
The partnership is 400 million years old. The earliest land plants had no true roots. The fungi were the roots. Forests still cannot exist without them.
Ectomycorrhizal fungi sheath the root tips of oaks, beeches, birches, pines, and eucalyptus. The hyphae fan meters out into the soil. Their fruiting bodies are the chanterelles, boletes, and truffles you find under native oaks.
Arbuscular fungi (AMF) push inside the root cells of vegetables, grasses, and most tropical trees. They build tiny branching arbuscules where exchange happens. No mushrooms. They feed most of the food supply on Earth.
Why it works
The numbers are absurd. Hyphae are about one-fiftieth the width of the finest root hair. They slip into soil pores roots cannot enter. One plant's network can multiply its absorptive reach by a hundred.
A mycorrhizal tomato is not feeding from the dirt around its roots. It is feeding from a volume far wider, stitched together by thread.
Phosphorus is the headline. Phosphorus locks tight to soil particles and barely moves. Roots strip a thin halo around themselves within days. Hyphae cross that depletion zone and bring more back. In poor soils, mycorrhizal plants take up three to five times the phosphorus of plants without the partnership.
The network does more than feed. Suzanne Simard's work showed trees swap carbon, water, and alarm signals through it. Big established trees subsidise shaded seedlings. Attacked trees warn their neighbours before the insects arrive. The wood wide web is real, and it is what makes a food forest more than a list of plants.
Protect what you have
Native mycorrhizae are already in most soils. Your job is to stop killing them.
Tillage is the worst. One pass of a plough or rotavator severs hyphae that took years to build. A single digging event can drop colonisation by 50 to 90 percent. This is the real case for no-dig. Masanobu Fukuoka knew it without ever using the word mycorrhiza.
Fungicides kill fungi without asking which ones. Systemic products move into roots and take the partners with the pathogens. Copper-based sprays suppress colonisation at high rates. Use them only as the last move inside an IPM plan.
Bare soil starves the network. No living root means no carbon, and the hyphae die back within weeks. Keep something growing year-round. Cover crops, perennials, living mulch. Transplants establish faster in soil that just grew something than in soil that sat fallow.
Too much phosphorus breaks the deal. When phosphorus is cheap, the plant stops paying the fungus. Decades of over-fertilising leave soils with broken networks. Run a soil test before you add more.
When to inoculate
Most healthy soil does not need it. A few situations do.
Construction sites. Mine spoils. Fields under chemical agriculture for forty years. These can be empty of viable spores. New plantings stall until the network rebuilds, which can take years. Inoculation cuts that wait.
Apply at planting. Dust it onto bare roots. Mix it into the planting hole. Water it in around seedlings. Contact with the root is the whole point.
Commercial inoculants vary wildly. Read the label. You want named species and a guaranteed spore count per gram. For vegetables, look for Rhizophagus irregularis. For temperate forest trees, look for Pisolithus, Rhizopogon, or Scleroderma.
The cheapest method is dirt. Two handfuls of soil from an old forest floor, an undisturbed garden, or a native grassland carries a locally adapted community for free. Sprinkle it in the planting hole. Pioneer species on degraded sites respond strongly to this trick.