When David Latimer planted a seed in a glass bottle on Easter Sunday of
1960 out of pure curiosity, he had no idea that it would flourish into a
mass of greenery that would thrive untouched for several decades. Now, over
half a century later, the sealed bottle garden is still growing as
vigorously as ever, filling the bottle entirely with lush plant life,
despite the fact that the last time Latimer watered it was in 1972.
http://www.mymodernmet.com/profiles/blogs/david-latimer-sealed-bottle-garden

My point is that if you use mulch or compost and plant anything from the outdoors, you get a LOT of living critters that you may not have noticed or planned on. Much of it is too small to see unless that's what you are looking for.
And a broad community of these critters (and bacteria and fungi) may be needed for many plants to thrive.