Well first of all, if you had clover growing, clover will root down to 8' deep if you're not in a short grass prairie area with a layer of hard carbonates at 18". And rye will root down 5 to 6 feet, which in your case, if you are in a wet area on fill, should easily be attainable, unless its too wet, at which point the soil conditions will be too acidic for rye. Rye usually has a 4 leader root system compared to 3 for most other grain species, and around 1/3 more total root length than most of the others, which is why rye does better in dry conditions than most grains. If things work out this spring, that rye should be bringing a lot of carbon to the surface to offset the nitrogen of the clover.