Engineering:
The chief variable in any soil-cement mix is in the native materials available. Sand and rock is good; clays give shrinkage problems & increase the percentage of portland cement - plant roots and humus are bad for strength and a problem when finishing. Every case has its own unique characteristics. When one is pouring thick walls of soil cement, the strength of the mix is critical. Concrete is tremendously heavy stuff - it weighs 144 pounds per cubic foot, which ironically works out to 1 psi (pounds per square inch) per vertical foot of wall load imposed. But, considering that the concrete we normally use in footings is 2,500 psi, there's a lot of latitude! One may wish to consult an engineer about such things. The loads imposed by walls typically 9 or more feet tall must be considered, both in the design of footings and in the mix for the walls. For cost-saving, the ratio of portland cement in the mix may be reduced as the wall height increases and the imposed loads are smaller. Footings: This house will be HEAVY - you'll need decent bearing - solid ground or rock. A conservative rule of thumb is that undisturbed soil below frost-line should bear 1000 # / SqFt. The minimum horizontal rebar would be 2 #4 (1/2") running horizontally; you may need more. Of course the advantage of all this weight is in its utilization as thermal mass, the essential element in any passive solar home. Testing: Where there are concerns about the strength of the mix, pouring test cylinders is simple and inexpensive; testing companies can be found almost everywhere. Reinforcing with re-bar and/or wire mesh would be required by building codes for most soil-cement wall construction. It's not especially expensive, and as it greatly enhances the strength and stability of the footings and walls, one would be foolish to ignore it! Again, consulting an engineer is prudent, and may well save money by avoiding over-reinforcement just to cover our own ignorance! | 
