Routine garden work does most of the damage. The practices that kill mycorrhizae start with deep or frequent tillage and a heavy phosphorus feed. Bare fallow ground, water-logged soil, extreme fire heat and some systemic fungicides fill out the rest of the list. The Alaska Fairbanks Extension list of harmful practices names every one of them. None of it looks harsh while you work, and you get no dead patch and no wilted leaf to warn you.
The winter rye came up thin and pale in the raised bed I turned over with a spade every spring. I planted the same seed in my own no-dig bed a week later, and it filled in thick and dark green. The dug bed took a complete fertilizer each April as well. Frequent or deep tillage and heavy phosphorus feeding both suppress mycorrhizal colonization, and that bed had taken both for three seasons.
Reports on tillage and mycorrhizae describe an effect and stop there. Digging deep or digging often cuts the presence and activity of these fungi in the soil. That is the whole of the finding. The likely reason is physical. The fungi grow as fine threads that spread from root to root through ground nobody has touched, and a spade tears that web apart faster than it can knit back together. Take that as a fair guess, not a proven cause.
Feeding works against them from the other side. Nitrogen, phosphorus and complete blends all cut how present and active these fungi are, so the bag you reach for in April can undo the digging you skipped. In the Alaska Fairbanks figures, roots do not need fungal help above roughly 100 parts per million of soil phosphorus. A soil test is the only honest way to know where your beds sit before you feed them again.
The story with fungicides and mycorrhizae is narrower than the internet makes it sound. Some systemic products suppress them. These fungi are sensitive to conventional agrochemicals as a group, so spot-treat a problem plant instead of soaking the whole bed. Bare ground does its own quiet harm. Fallow soil holds no living roots and offers the fungi nothing to partner with, and a garden planted to one crop year after year sits on the same list.
Mycorrhizal fungi generally do not survive composting or compost-tea processes. You cannot brew back what a tiller or a spray knocked down, so guard what your soil already holds.
Fixing this costs nothing and mostly means doing less to the bed. Work through the changes in the order below. The first two undo the most common damage, though no source here promises a timeline for how fast the fungi come back.
- Stop deep digging: Leave the spade in the shed and plant into the soil as it sits.
- Ease off the fertilizer: Hold back complete blends and phosphorus until a soil test shows you are short.
- Keep living roots: Follow one crop with the next or sow a cover crop so roots stay in the ground most of the year.
- Fix the wet spots: Drain or raise any bed where water sits for days after rain.
Living roots matter more than any product sold in a bag. These fungi need a host to stay active, so a bed that carries a plant for most of the year keeps its own supply going and asks nothing of you. Start with the digging if you change only one thing. Most gardeners repeat it every spring, and it is the easiest of the four to drop.
Read the full article: Mycorrhizae: What Works and What Is Hype
