Over 95% of plant families form a partnership with mycorrhizal fungi, and the share climbs in ground nobody has dug or sprayed. The biology is not in doubt. So adding mycorrhizae to soil comes down to whether the product in the bag is any good, and most of the ones researchers have tested are not. In new, disturbed or low phosphorus ground, a bag is worth a shot. In an old, fertile bed you have left undug, it is money you will not get back.
The trade is straightforward. Fungal threads reach far past the root tips and pull in phosphorus, water and micronutrients the roots would never touch, and the plant pays in sugar for the service. That bill runs as high as 20% of the carbon a plant fixes in a season. Plants keep the deal going when the soil is poor enough to make it pay, and they pull back when it is not.
The products are the weak link. A 2025 meta-analysis in New Phytologist pooled 302 trials covering 250 commercial products. In 84% of those products, root colonization stayed under 5%, which is far too little to help the plant. Just 12% reached a useful level, against 63% for lab-grown stock. A mycorrhizal inoculant is a living product, and the bag gives you no way to check what is still alive inside it.
Disturbance is what opens the door for a product in the first place. Tilling shreds the fungal network, and fumigants and heavy phosphorus fertilizer knock it back further. Ground that gets dug over, scraped by machines or built from bagged mix is where native fungi are most likely to be missing, so there may be a real gap to fill. Soil under an old hedge is far less likely to have one.
Soil phosphorus decides most of the rest, and it is the one test worth running before adding mycorrhizae to soil. Above about 100 parts per million, roots gather all the phosphorus they need on their own and the fungi add little you can see. Below about 50 parts per million, the fungi do real work, and that is where a product has room to pay for itself.
Gains do show up when the conditions line up. A snap pea trial in Alaska grew inoculated plants about 10% taller than the untreated rows. That is a fair result from one Alaska field trial. On a ten-year-old vegetable bed loaded with compost, do not expect anything close to it.
Test your soil before you spend a cent. A basic lab test costs a few dollars and gives you the one number that settles this. If phosphorus is low and the ground is new, a bag is a fair gamble. That covers raised beds filled with bagged mix, borders cut into the subsoil a builder left behind, and anything you dug this spring.
- Test: Run a soil phosphorus test and read the number before you shop for anything.
- Check the date: Buy the freshest stock you can find and look for a packing date on the bag.
- Place it right: Put the product down in the planting hole where roots will touch it, not on the surface.
- Add organic matter either way, because it feeds the fungi you already have.
Everywhere else, feed the fungi you already own. Most soil fungi benefits arrive free of charge once you top the ground with organic matter such as arborist wood chips and hold back on the sprays and the tiller. Native fungi colonize roots on their own from there, and they are already matched to your ground in a way no bagged product can be.
Read the full article: Mycorrhizae: What Works and What Is Hype
