May want to try something like rammed earth.
Basically make a form that will create the outside of the wall and an inside wall. Mix soil with some portland cement and then compact it between the forms. Move the form to a new section and repeat. The moisture in the air as well as any rain should help to set the soil to near concrete hardness and you can fill the inside area with loose soil if you like. Coat the exposed area's with a water repellent or a layer of portland and sand and they will last a long time. It will eventually brake down with shots directly to it but so will wood and concrete but the cost is pretty much "dirt" cheap.
http://www.yourhome.gov.au/materials/rammed-earth
Basically make a form that will create the outside of the wall and an inside wall. Mix soil with some portland cement and then compact it between the forms. Move the form to a new section and repeat. The moisture in the air as well as any rain should help to set the soil to near concrete hardness and you can fill the inside area with loose soil if you like. Coat the exposed area's with a water repellent or a layer of portland and sand and they will last a long time. It will eventually brake down with shots directly to it but so will wood and concrete but the cost is pretty much "dirt" cheap.
http://www.yourhome.gov.au/materials/rammed-earth