you can mix some cement powder and some wood chips in with the dirt to strenghten it but it would sorta suck when you wanna change something if the jumps are pure cement and wood. I cant see that working out well either.
ya i agree with SUZUKIRM125. when u go to change something that is gonna be a hell of a job. Enless u awalys like riding the same track. LOL. atleast when ur buddies come over to race u you will be like a god at your home track.
The dirt at my house is really sandy. This is what I did. I have a small amount of clay piled up. I used the sandy soil to build up the jumps just shy of the height I wanted. I then used clay to cover them so they hold their shape. I'd check around and see what you can get a load of clay for.
I've also heard of people with sandy soil using carpet (yes, carpet) to cover the jumps. It looks weird but the jumps will hold their shape.