Leaves are of course organic, and therefor they hold nutrients. Even after they die and fall to the ground, they're still nutrient-rich. However, you don't want to just leave them blanketing your lawn because they'll prevent sun, rain, and oxygen from getting to your lawn's roots.
Instead, break them down into smaller bits and you'll have an incredible fertilizer and mulch. Once the leaves are shredded, they can be added to a compost pile, spread over the soil in your flower bed or garden, or even sprinkled into your lawn.
Chipper shredders are a great way to accomplish this. They're available as electric chipper shredders or gas chipper shredders, though the electric shredder type is all you need unless you're looking to chip branches and limbs as well.
If you're mostly interested in using shredded leaves to fertilize your lawn, you can use mulching kits for lawn tractors, and serve multiple purposes at once. A mulching kit will allow you to mow your lawn while mulching any leaves and evenly spreading them into the grass.
As the shredded leaves break down, they add nutrients to the soil, acting as an organic fertilizer for anything from grass to flowers or vegetable gardens. No need to spend money on inorganic fertilizers at the store when you can recycle what you're already getting for free year-after-year.
