Normally a two to four inch layer of shredded leaves can be composted naturally into the soil over the winter. Spread the leaves over the garden area and spade into the soil as deeply as possible, six inches or more is best. A small addition of fertilizer may help decompose thick oak leaves quicker. As the leaves break down they supply the needed organic matter for our heavy clay soils. It is almost impossible to get too much organic matter in our soils.
Organic matter is like magic.
Soils with a healthy amount are easy to till and naturally break apart. High organic matter soils also retain moisture while providing good aeration. This combination of properties is often what is lacking in average garden soil. Fall tillage is also a good time to take a soil test. Additions of phosphorus and potassium can be worked into the soil as well as materials to adjust the soil pH. If these nutrients are not worked into the soil they provide little benefit.