The Garden’s Homegrown Ally

Mycorrhizal fungi could be the secret ingredient to making the perfect soil.

By Christine Ziegler, research and science editor, Rodale Institute

Put mycorrhizal fungi to work in the soilGet the underground army of mycorrhizal fungi to work in the garden by following a few simple management practices:
  • If the soil is already high in phosphorus (a simple, inexpensive soil test can answer this), do not fertilize with a phosphorus-rich amendment, because high phosphorus levels inhibit development of associations between plants and mycorrhizal fungi. Manures and manure-based composts can be high in phosphorus, so test these amendments before adding them.
  • Minimize digging (especially rototilling), as it can break mycorrhizal hyphae, preventing them from colonizing new plant roots and transporting nutrients.
  • Grow a diverse mix of plants in your soil for as much of the year as possible, because mycorrhizae need active plant roots in order to develop. 

Some techniques to keep the mix diverse:

  • Rotate crops each year (as long as there aren't too many successive brassicas). Crop rotations are vital to mycorrhizal fungus populations because, in addition to providing a continuous succession of root hosts, different crops also tend to favor different species of mycorrhizal fungi.
  • Plant an overwintering cover crop. In addition to adding organic matter and retaining soil nutrients, the cover crop offers host roots for the mycorrhizal fungi to colonize and helps them proliferate in preparation for next spring's planting. A good mix of crops above ground is the best way to support a mix of beneficial fungi below ground.
  • Lighten up a bit on weed control, because, surprising as this may be, weed roots can also be excellent mycorrhizal hosts.
These simple, no-cost steps help keep the soil's native population of mycorrhizal fungi healthy and diverse, harnessing yet another gift of the natural environment to create a vibrant and abundant garden.