Up to 11,000 lbs of fungi per acre: the workforce you can't see

One acre of healthy soil can hold between 1,100 and 11,000 pounds of living fungi, and according to Drew McCutchan, district conservationist with the Natural Resources Conservation Service in Indiana, that biomass is quietly doing jobs most farmers pay good money to replicate.

Writing for the Indiana Conservation Partnership, McCutchan makes the case that fungi are far more than the morel mushrooms you fry up in spring. Without them, he argues, crop production would be greatly diminished. The maths alone suggests he's right.

What do fungi actually do in soil?

Plenty, as it turns out. Saprophytic fungi break down tough residue that other soil organisms simply can't digest. Once fungi have softened those resilient structures, earthworms, beetles, bacteria and protozoa move in and convert what's left into plant-available nutrients. Fungi run the demolition phase; everyone else handles cleanup.

Then there's structure. Fungi release an exudate called glomalin, which binds soil particles and microaggregates into macroaggregates. Those larger clods increase water infiltration and let more oxygen move into the soil naturally. Compacted ground benefits too: microscopic hyphae — the branching "roots" of fungi — are small enough to find weak points in compaction layers. Over time they expand those cracks until plant roots can grow through and break the layer themselves. Biology doing the work of a ripper, at no charge per acre.

Why mycorrhizae matter to your bottom line

The headline relationship for farmers is the symbiosis between plant roots and mycorrhizal fungi. Roughly 80% to 90% of all plants form these partnerships through hyphae networks.

The advantage is reach. Mycorrhizal hyphae are about one-sixtieth the size of a normal plant root, so they can enter micropores that roots physically cannot. Inside those pores sit water and nutrients the plant needs to stay productive — nitrogen and phosphorus among them.

The transaction runs on sugar. Plants produce root exudates during photosynthesis and trade them, like currency, for the nutrients and water that fungi mine from tiny soil aggregates. The fungi get fed; the plant gets phosphorus. Everybody wins, except perhaps the fertiliser supplier.

Disease suppression comes free with the deal. Fungi can attack plant pathogens with fungal enzymes, and a healthy hyphae network means fewer disease-causing fungi hanging around plant roots.

How do you keep fungi fed over winter?

McCutchan's prescription is blunt: fungi need the same three things humans do — food, shelter and water. If any one of them is limiting in your fields, the benefits of soil biology for future crops and forages will be limited too.

The critical failure mode is starvation between crops. If soil biology only receives residue and exudates while a cash crop is growing, populations decline over winter. Mycorrhizal fungi feed on root exudates; saprophytic fungi feed on residue. The fix is keeping a living root in the ground as long as possible.

Diversification multiplies the menu. McCutchan recommends:

• Planting cereal grains such as oats and cereal rye ahead of soybeans
• Planting legumes such as vetch and winter peas ahead of corn
• Diversifying rotations or cover crop mixes to vary root exudates and residue types

Both combinations double as strong host plants for soil fungi, so the rotation does two jobs at once.

What about shelter and water?

Here soil organisms largely manage their own affairs. Fungi build the macroaggregates that trap water and air between soil particles, and that pore space hosts most subterranean life. The threat comes from us. Tillage, or a bare surface without enough residue to armour it, destroys the macroaggregation and invites water runoff and evaporation.

Keep the soil covered and undisturbed, with diverse living roots and residue, and fungi can get straight to work helping you grow a crop the moment seed goes in the ground. Watch your residue cover and cover crop timing this autumn — the 11,000-pound workforce underground only shows up if you keep it fed.