Composted leaves will fertilize it somewhat, but if you just let leaves fall on your lawn, it tends to kill the lawn. I recommend raking into pile, and letting

Composted leaves will fertilize it somewhat, but if you just let leaves fall on your lawn, it tends to kill the lawn. I recommend raking into pile, and letting it sit for a year or so (fallen brown leaves compost rather slowly), then you can spread resulting compost. But, unless you have lots of trees, the amount of nitrogen in fallen leaves will be minimal (trees don’t like parting with valuable nutrients either).