Mycorrhizae vs worm castings
Mycorrhizae are living fungi that form partnerships with compatible roots. Castings are a finished amendment containing stable organic matter, modest nutrients, and a broad microbial community. One needs a handshake with a root; the other arrives carrying snacks.
How mycorrhizae work
Compatible fungi colonize living roots and extend fine hyphae into soil. In exchange for plant sugars, the fungi can help the plant explore a larger volume for water and nutrients, especially phosphorus. The inoculant must contact viable roots or the immediate planting zone; scattering it on distant soil is a very expensive way to season dirt.
What castings do
Castings add stable organic matter, modest nutrient content, microbial diversity, and fine structure. They can improve the habitat around roots, but they do not guarantee a particular mycorrhizal species or root colonization.
When to use inoculant
- At sowing or transplanting when product can touch roots
- With a plant species known to form that partnership
- When the soil or media is unlikely to contain a compatible population
- According to the product’s storage date and label rate
Many established soils already contain mycorrhizal fungi. Inoculation is not automatically useful in every garden, and high soluble phosphorus can reduce colonization. Brassicas, including broccoli and cabbage, generally do not form the common arbuscular mycorrhizal partnership.
When to use castings
Use them in potting mixes, planting holes blended with surrounding soil, and top-dresses. They are useful whether or not the plant forms mycorrhizae.
Using both
- Place the inoculant exactly where new roots will touch it.
- Blend a modest amount of castings into the surrounding root-zone mix.
- Use a complete medium with proper drainage and pore space.
- Avoid excessive soluble phosphorus unless a test shows it is needed.
- Water to settle the root zone without saturating it.
Storage matters
Mycorrhizal inoculants contain living propagules with a finite shelf life. Keep them cool and dry as the label directs, and check the use-by date. Castings also benefit from cool, shaded storage with enough moisture to avoid becoming dust.
They are not substitutes. Use mycorrhizae for a targeted root partnership and castings to improve the broader root-zone habitat.
🌱 Keep growing
Three related guides worth reading next:
- What Is Mycorrhizae? — The underground fungal network that trades nutrients with your plants.
- Roots & Nutrient Uptake — How roots actually absorb nutrients — and what helps or hinders them.
- Soil Microbiology — Meet the billions of microbes doing the real work under your feet.
Keep Growing 🌱
Where to go next:
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