What type of leaves are they? I ask that because with some leaves you also get thousands of little seeds like ash tree leaves. So if they are old dry leaves and have been shredded and aren't full of seeds you don't have to add anything. If you add fresh grass clippings they could heat up like compost and you don't want that. Old dried grass clippings are fine with leaves. I use a ton of leaves and over the period of a year an inch or two of shredded leaves will decompose and be eaten by worms and other things, so that by next year it will be time to add more.
