| MadSci Network: Earth Sciences |
Aaron: The question "What is soil?" is not as simple as you might suppose. A "Dictionary of geological terms" published by the American Geological Institute gives this definition That earth material which has been so modified and acted upon by physical, chemical, and biological agents that it will support rooted plants. The term as used by engineers includes, in addition to the above, all regolith. The same dictionary defines regolith as The layer or mantle of loose, incoherent rock material, of whatever origin, that nearly everywhere forms the surface of the land in the absence of true soil and rests on bedrock. By these definitions, sand is regolith, and therefore soil (sensu engineer). Some might prefer to define sand as sediment. From the same dictionary, a definition of sediment: Solid material, both mineral and organic, that is in suspension, is being transported, or has been moved from its site of origin by air, water, or ice, and has come to rest on the earth's surface either above or below sea level. Clearly, sand is sediment. However, sand can also be considered soil, in an engineering sense, and since your question is really an engineering question, I think it quite appropriate for you to consider sand to be soil. Your other question: what kind of soil filters water best? I am not sure what you mean. If you mean, what kind of soil passes water through at the greatest rate, then clearly you want a soil composed of large, well sorted, spheroidal particles. This kind of material has the highest porosity and permeability and therefore transmits water best. If gravel is not considered soil and sand is, then sand would be the best soil in this sense. However, you are not really filtering water very much when you pass it through sand. When one talks about filtering water, one usually wants to remove suspended particles from the water, and for this purpose, the smaller holes are better. In fact, minerals that can electrically or chemically bond to the suspended particles are better yet. By this criterion, some kind of clay probably filters water best. I hope this helps, David Kopaska-Merkel Geological Survey of Alabama PO Box 869999 Tuscaloosa AL 35486-6999 (205) 349-2852 FAX (205) 349-2861 www.gsa.state.al.us
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