Microbial Inoculants

Microbial inoculants put the soil food web back to work

Microbial inoculants are concentrated populations of beneficial bacteria and fungi that colonise roots and the soil around them, forming partnerships that extend a plant's reach far beyond what its own roots could manage. They earn their keep most in the places where soil biology has been stripped out: containers, new beds, freshly worked ground, and anywhere that has seen years of synthetic feeding.

What a microbial inoculant is, and what it is not

A microbial inoculant is a dose of living organisms, applied to soil or directly to roots, intended to establish populations that are absent or depleted. It is not a fertiliser. It supplies almost no nutrition itself; what it does is make existing nutrition more available, chiefly by extending the surface area that can absorb it and by cycling organic matter into forms a root can take up.

That distinction matters when you are deciding whether to buy one. On old, undisturbed, well-fed ground with plenty of organic matter, the biology is usually already there and an inoculant adds little. On a new raised bed of bought compost, a container, or a bed that has been dug over and fed on synthetics for a decade, it has something to fix.

What is in the collection

• Mycorrhizal Fungi Powder, endo and ecto strains. The fungal network effectively extends the root system, reaching water and phosphorus the plant could not otherwise access. Worth knowing before you buy: it works brilliantly for most garden plants and does nothing at all for brassicas, beets or rhododendrons. The full list of which plants benefit, and when to apply.
• EM-1 Effective Microorganisms, the original Dr Higa mother culture of beneficial bacteria, yeasts and phototrophs. The foundation of bokashi bran, and a capable soil drench in its own right.
• Soil Smiths The Goop, a dense microbial concentrate for top-dressing, drenching and inoculating at planting.
• Grow-Kashi, a fermented probiotic top-dress that goes straight onto established beds and pots.
• Unsulphured molasses, the carbon source microbes need to multiply once they are in the soil. Inoculating without feeding is half a job.

Endomycorrhizal inoculant or ectomycorrhizal: which your plants form

Most garden plants form endomycorrhizal associations, where the fungus grows into the root cells themselves. Vegetables, fruit, roses, herbaceous borders, grasses and most broadleaved trees sit in this group, which is why an endomycorrhizal inoculant covers the large majority of an ordinary garden. Ectomycorrhizal fungi work differently, sheathing the outside of the root, and partner mainly with pine, spruce, birch, oak and beech.

The powder here carries both, which is the sensible choice when one bag has to serve a mixed garden. Then there are the plants that form no mycorrhizal association at all, brassicas, beets, spinach and rhododendrons among them, where the inoculant simply does nothing. Check the bed before you dust it.

Soil biology, in plain terms

The soil food web is the chain that turns dead material into plant nutrition. Bacteria and fungi break down organic matter; larger organisms graze on them and release nutrients in forms roots can absorb; worms move the whole lot through the profile. Soil biology is the mechanism by which an organic garden feeds itself, and it is the reason organic matter and mineral amendments behave differently from a soluble feed.

Inoculants work best where there is something for the biology to eat and somewhere for it to live, so pair them with humic acid, biochar and soil conditioners. The mineral side of the same argument is in the volcanic rock dust guide. If you want to make your own microbial food from kitchen waste, that is what EM and bokashi composting is for.

Judging a mycorrhizal product honestly

This category has a quality problem, and we would rather say so than let you find out for yourself. Independent testing of commercial mycorrhizal products has repeatedly found far fewer viable propagules than the labels imply, and in a number of cases none at all (Koziol et al., 2024 and 2025). That finding applies to the category we sell into, not only to other people's shelves, and it is the reason we would rather you bought on evidence than on packaging.

What follows from it is practical. Ask what species a product contains and what viable propagule count it carries, and be sceptical of any that will not say. Apply at planting, where the fungus can reach live roots, because that is the only route by which it can establish. And treat inoculation as one part of rebuilding soil biology rather than a substitute for organic matter, which is the thing that actually keeps a population alive once it is there.

When and how to apply

Mycorrhizal fungi must physically touch live roots to colonise, so dust the powder directly onto the root ball at planting or transplanting. Broadcasting it over bare soil is largely wasted. EM and microbial concentrates go on diluted as a soil drench; Grow-Kashi and molasses as a top-dress. Rates are on each product page.

Read our reviews.

Common questions

When is the best time to apply mycorrhizal fungi?

At planting or transplanting, dusted directly onto the root ball or into the planting hole. Contact with live roots is the whole mechanism.

Which plants does it not work on?

Brassicas, beets, spinach and rhododendrons, among others. Some plant families simply do not form mycorrhizal associations, so the inoculant does nothing for them. Check before you dust a whole bed.

What is the difference between mycorrhizal fungi and beneficial bacteria?

Mycorrhizal fungi form a physical partnership with the root and extend it, working over centimetres and months. Beneficial bacteria work at a smaller scale and a faster pace, cycling organic matter and occupying root-surface space. They do different jobs, which is why the collection carries both.

Will the microbes survive in my soil?

If the conditions are right: moisture, organic matter, a carbon food source such as molasses, and minimal synthetic chemical use. Inoculating soil that has none of those is like putting fish in an empty tank.

Is a microbial inoculant worth it on established beds?

Less than you might hope. Established, well-mulched ground usually has functioning biology already, and the money is better spent on organic matter. Inoculants pay for themselves on new beds, containers and ground that has been worked hard.

Can I use these alongside my regular fertiliser?

Yes. Avoid applying live microbes within twenty-four hours of any high-salt or strongly alkaline product, but day-to-day organic feeding is fully compatible.

Do I need all of these, or just one?

Mycorrhizal fungi at planting plus a regular microbial drench covers most gardens. The full range is for growers running living soil or regenerative systems.

How often should I re-apply?

Mycorrhizal fungi once, at planting, since a successful colonisation persists. Microbial drenches every few weeks through the growing season, more often on containers.