I really need to get this off my chest. I hear too many people who ought to know better saying it wrongly.
The word is mycorrhizae. Mycorrhizae. NOT microrrhizae.
Mykos = fungus
Rhiza = root
Micro = very small
Mycorrhizae are various types of fungi in the soil that associate themselves very closely with plant roots in a mutualistic* relationship. The MYCOrrhizae facilitate the uptake of water and dissolved nutrients by the roots. In return the plant provides food to the fungus in the form of sugars (and possibly starches) that it manufactures by photosynthesis.
Theoretically, there could be a word “microrrhizae” that simply means “very small roots”. Such a word is not in use, to my knowledge.
Despite their popularity on gardening shows on television and in books and magazines, assuming you have decent soil in your garden (i.e. it supports some plant growth already), you do not need to add store-bought mycorrhizal pellets when you plant new plants (healthy ones). Those pellets only contain a few types, which are almost certainly different from the several hundred types that are already present in your garden – making them unnecessary at best and unsuitable at worst. They are also heat sensitive, and you don’t know how the containers have been stored/transported.
Also, not every plant needs them, and other conditions might make them less useful.
If you have bare sterile soil, then perhaps – but such soil isn’t suitable for growing plants in the first place. You’d be better off finding out exactly WHY that area is bare first (getting a lab soil test done, for example) and fixing that problem.
*Mutualism is the situation where all parties benefit from the close relationship between two (possibly more) organisms (symbiosis); it’s what most people think of when they hear/see the word “symbiosis”. Other forms of symbiosis are parasitism (one party benefits at the clear expense of the other) and commensalism (one party benefits while the other is not significantly impacted either way).