A soil test answers this before your wallet does. The University of Alaska Fairbanks puts little fungal need above about 100 parts per million soil phosphorus. Roots with that much on hand have no reason to trade sugar for a nutrient they can already reach. In high phosphorus soil mycorrhizae also struggle to establish at all. So skip the product when your own test reads high, and skip it again when the crop is a non-host. Tilled and fallow ground is a third case, and there the practice itself is the problem.
Treat those numbers as examples, not as a line drawn for every garden. That same Alaska guidance calls the fungi important below about 50 ppm. In between sits a band where the answer depends on your crop and how you work the ground. A 2020 review in 3 Biotech used a far lower mark and put the trouble above 10 ppm in the soils it covered. Your own laboratory prints its interpretation next to the result, and that reading beats any figure you pull off a bag.
The trade behind all of this is simple. The plant hands over sugar and the fungus hands back phosphorus it reaches through thread-fine hyphae far past the root tips. Take away the shortage and the plant has little reason to pay. That is why high phosphorus soil mycorrhizae never get far into the root. A second dose of spores will not restart a deal the roots have already dropped.
Two beds tell you to skip mycorrhizal inoculant before a test does. The first is a brassica rotation. Cabbage and broccoli are non-host plants for the most part, so nothing in the bag can attach to those roots. The second is a vegetable plot you feed hard. Added nitrogen, phosphorus or a complete fertilizer cuts both the presence and the activity of the fungi you are trying to add.
A bed rototilled every spring is a different case, and so is ground left bare between crops. Tillage and fallow conditions both suppress these fungi. Some systemic fungicides do the same, so a spray after seeding works against the thing you just paid for. None of that stops the day you open a bag. The conditions that wiped out the native fungi are still there, and they go straight after whatever you apply next. Fix the practice first, then decide whether you still need to buy anything.
- Soil phosphorus reads high under your own laboratory's interpretation of the test.
- The bed sits in a brassica rotation, so those roots host nothing.
- You feed the plot with nitrogen or a complete fertilizer all season.
- The bed gets rototilled every spring, which breaks the network before it spreads.
- Ground sits bare and fallow between crops with no living roots in it.
- A systemic fungicide goes on the same bed you planned to inoculate.
Product quality is its own hurdle. A 2025 New Phytologist paper pooled 302 trials across 250 commercial products. 84% of those products produced under 5% root colonization. Those are the odds even in a bed where the fungi would help. Put the money into a soil test instead. You learn whether the question is worth asking at all, and that beats paying for a bag that never gets far into a root.
Read your test first. If phosphorus comes back high under your own laboratory's reading, put the bag down and keep the money. If the bed grows non-host crops, that answer holds no matter what the phosphorus number says. Till less, feed less and keep something rooted in the ground between crops. Those changes cost nothing and they work on the fungi already there.
Read the full article: Mycorrhizae: What Works and What Is Hype
