Living SoilGarden centers sell little bags and bottles of mycorrhizal fungi to mix into planting holes and potting soil. The science behind the fungi is real. So should you be buying them?
This is one of those questions where the honest answer saves you money. Let us start with what these fungi are, then look at what the research says about buying them.
What mycorrhizal fungi are
Mycorrhizal fungi form partnerships with plant roots. They spread fine threads out through the soil that act like an extension of the root system, enlarging the roots' absorbing area by 100 to 1,000 times by one estimate. They help the plant take up nutrients, especially phosphorus.
Most plants form these partnerships. There are exceptions, including spinach, sugar beets, lupins and brassicas.
One important limit: mycorrhizae help with uptake, but they do not cure nutrient deficiencies, especially when the soil simply lacks those minerals. They are partners, not a fertilizer.
Do you need to add them?
Here is what the research says, and it is fairly blunt:
- Most soils already contain mycorrhizal fungi, so they do not need to be added.
- Studies show inoculation is unnecessary and ineffective in most landscapes.
- One extension service does not recommend mycorrhizal products for general use at all.
- Research on commonly available inoculants found that over 50 percent of the products sold to consumers were not even viable.
To be fair to the other side: research written mainly for farms notes that helpful fungi and bacteria can be introduced by inoculating soil or seed. And inoculants have worked on extremely disturbed or toxic ground, such as mine spoils or sites where all the topsoil has been stripped away and plants are growing in subsoil. If that describes your garden, the picture is different.
What actually helps your fungi
If the fungi are already there, the job is to stop harming them. Compacted, flooded or contaminated soil is harmful to them. And tilling is one of the biggest threats: unnecessary rototilling destroys mycorrhizae along with soil structure, and research on tillage finds it reduces beneficial fungi.
So the cheapest mycorrhizal product in the world is a garden fork you leave in the shed. Dig less, keep off the beds, keep the soil covered, and the fungi already living there will do the rest.
The habits that protect fungi are the same habits that build living soil in general. They are all in how to make living soil.