Plants provide bacteria and mycorrhizal fungi with what they don't produce themselves - carbon and calories.Two months ago - the apple tree was not fruiting - but the soil was good due to apples such as this rotting away - since I can ever remember.
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Check out this example:
Vermicomposting Trench - All Natural Fertilizer Factory for Your Garden!
https://www.youtube.com/watch?v=Jl2XybtZ1dA
It isn't as much the composting worms as it is the calories/energy contained in the foodscraps.