posted 11 years ago
Think of it as "brown vs white". In this case brown (the leaves) contains a lot of tannins, which break down to humic acids, and the white (chips) has a lot less tannins and a lot more cellulose. Cellulose feeds termites and bacteria and brown rot fungi, and tannins are broken down principally by the white rot fungi.
If you chop and drop, you are going to have a balance of both, so the soil food web is going to be pretty diverse. If you have only leaves, you are going to push the soil food web toward the white rot fungi, which is what you would find in old growth forests that have lots of leaf litter on the forest floor. This is desirable for perennial plants, but if you want to grow annuals or biennials, like brassicas, they may not like it as much as soil that has more cellulose digesting soil critters present.