ADOBE SOIL, a term used in the western United States, particularly in California, to designate the structural quality of various clay soils, referring usually to the characteristic breakage of such soil material into small angular blocks when it dries. These soils are very heavy in texture, being composed of extremely fine-grained material, a large part of which is colloidal. Sands and loose sandy loams, for example, never display an adobe soil structure. By many, however, adobe soils are often thought of as constituting the material from which adobe or sun-dried bricks are made ; but, as in the case of ordinary brick, adobe building materials are derived from a great variety of clayey soils. Under proper conditions of moisture soils of adobe structure are readily worked, but when too dry their hardness and coherence make tillage extremely difficult. When placed under irrigation soils of this type usually display great fertility.