Mycorrhizal inoculants

Arbuscular mycorrhiza is the oldest and most widespread symbiosis of land plants: fungi of the Glomeromycota enter root cortical cells and build finely branched structures inside them — arbuscules — across which exchange happens. The plant supplies carbon, as sugars and, as was established more recently, as lipids the fungus cannot make for itself; the fungus supplies phosphorus, some nitrogen and micronutrients. The association is not accidental. It is initiated through the common symbiosis signalling pathway, the same one used by the legume–rhizobium partnership: fungal lipochitooligosaccharides are perceived by plant receptors and set off nuclear calcium spiking and the kinase CCaMK.

Why the gain is specifically in phosphorus

Soil phosphate is immobile. It sorbs strongly to iron and aluminium oxides and to carbonates, and diffuses orders of magnitude more slowly than nitrate. A root taking up phosphate creates a depletion zone a few millimetres thick around itself and is then limited not by the soil’s stock but by the rate of supply into that zone. A hypha a few micrometres across extends centimetres to decimetres beyond it, at a fraction of the carbon cost of root of the same length. The phosphate it takes up enters the plant by a separate, mycorrhizal route, through transporters expressed only in arbuscule-containing cells. This is exactly why the symbiosis pays where phosphorus is present but unavailable, and does not pay where phosphorus is simply abundant: at high available P the plant suppresses the symbiosis itself, reducing strigolactone exudation and limiting colonisation. A well-fertilised field is a poor target for a mycorrhizal product for a biological reason, not an agronomic one.

Production is constrained by obligate biotrophy

Arbuscular fungi are obligate biotrophs: without a living root they do not complete their life cycle. They cannot be fermented. Only two industrial routes exist — growing them on a host plant in a substrate and harvesting roots, spores and substrate together, and root organ culture on transformed roots in vitro, which yields clean spores expensively and in limited volume. That explains the cost, the variability, and the fact that dose is expressed not in CFU but in infective propagules, a quantity determined by a dilution bioassay rather than by counting. Ectomycorrhizal fungi — Pisolithus, Laccaria, Hebeloma — which form a mantle and Hartig net around forest nursery seedlings, are easier in this respect: they grow in pure culture.

The real limit is establishment, not activity

Soil is not empty. A gram of it holds on the order of a billion cells of an established, competitive community, and most arable and natural soils already contain their own arbuscular fungi, adapted to that particular place. An introduced strain has to do more than stay alive; it has to occupy the root sooner and more firmly than the residents. So the response to inoculation is reproducibly large where the resident community is destroyed or absent: fumigated substrates, container nursery production, mine spoil and reclamation sites, severely disturbed or phosphorus-poor soils. On an ordinary fertilised field with an intact community the average effect is close to zero, and published audits of commercial products repeatedly find fewer viable propagules than the label claims. That is the state of the field rather than a complaint about particular manufacturers.