Erosion, volcanic eruption, earthquakes, floods, tectonic grinding, landslides, and other natural forces act continuously on the earth’s crustal rock, creating various types of debris: gravel deposits, mudflats in the tidal estuaries of creeks, cobble terraces, and beaches of black lava sand. When chemical agents such as phosphorus and nitrogen infuse this debris, and biological entities including microbes and earthworms work material into it organic enough to support plants, it becomes soil. A soil that is chemically or organically exhausted, that has been pulverized or become deeply parched, that has been invaded by decomposing rock, or that has been fouled by sewage or industrial pollution to the point where it can not support plant life is called dirt. Barry Lopez in Home Ground /Language for an American Landscape, a very nice book of very short essays pretending to be definitions. Thanks JR !