Posted: 04/03/2017 at 12:47
Agapanthus have proven to be absolute thugs in my borders - huge spreading roots which prevented anything nearby from growing. That's why I dug them all up and now only grow them in pots - not because they like it!
As for the depletion thing, the microbes which break down woody matter require nitrogen to do so and will take it from the soil. However, unless you mix (say) sawdust well into the soil, they can only take it from the top few mm so nothing to worry about when mulching as that is always left on the surface (else it wouldn't actually be a mulch!) Judging by the amount of nitrates in our food and water, a bit of depletion might not be a bad thing..
Lots of gardening 'lore' is probably just misinterpretation. If someone find a method of consistently growing a particular plant well, they will do it again and tell others. Take a (made-up) example that adding crushed egg shells to the bottom of a hole before planting hostas makes them grow better. They do an experiment with 100 'eggshelled' hostas planted side by side with 100 'un-eggshelled'. The differences are clear and repeatable. One might assume "it works because it increases the calcium levels" when, in fact, it may simply be because it improves drainage. After a few rounds of 'chinese whispers', it becomes a 'fact' that hostas need extra calcium.