Question on Mycorrhizal fungi

Does the mycorrhizal fungi always need to be added only when putting a new plant in the soil or can it be dug in (watered in?) around existing plants?
“Every day is ordinary, until it isn't.” - Bernard Cornwell-Death of Kings
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I expect tv Gardeners such as Monty Don are told to mention certain garden products for sponsorship and for advertising purposes.
Wisdom is not putting it in a fruit salad.
Reminds me of the roots of my eucalyptus
Wisdom is not putting it in a fruit salad.