Rammed earth walls are a mixture of locally obtained sub soils, portland cement and other pozzolans in varying quantities, small amounts of clays and silts, and frequently, iron oxides for enhanced color. Notably absent are organic materials.  There is no topsoil in a rammed earth wall.  There are no organic binders such as straw; so there is nothing to rot or support the growth of unhealthy organisms.  The earthen mixture is transformed by the ramming process into a very hard and durable masonry wall.  Finding appropriate material close to the site is half the fun; sometimes a beautiful material appears where least expected.