CR1 soil is not a well-defined medium, yet not all soil is suitable for culturing a broad range of algae. Soiil used for CR1 originates in the floodplain of the upper Rio de Flag, Flagstaff, Coconino Co., AZ. It is relatively low in organic content, pH circumneutral and is finely particulate in nature, consisting primarily of volcanic ash and weathered cinders and apparently contains a sufficient array of required trace elements and macronutrients.