grab 6 used pallets from the back of stores.(ask first)nail a box together for a cheap 4by8 compost bin .add used dirt with the roots shook out.add leaves ,any manure you can pick up , left over vegies, trash fish from fishing.a couple gallons of woodstove ash,a good dusting of bonemeal ,i like a layer of seaweed. wet down every couple of weeks stirring a few times add some worms and in 4-5 months after filled.all the salts and left overs from previous grow will be gone and the compost will be very good.add perlite to thin if you like.it will be very rich.the 3rd time it goes to my wifes flowers. this will save you more money than you realize. and fun making that perfect blend for your grow.