Soil separate size boundaries in USDA and ISSS / IUSS systems: Clay upper limit: Both USDA and IUSS/ISSS define clay particles as having equivalent spherical diameters of less than 0.002 mm (\(\lt 2\,\mu\text{m}\)). Thus, the upper limit of clay is 0.002 mm (A). Gravel lower limit: Both systems consider fine earth to have particle sizes up to 2.0 mm; any particle with diameter greater than 2.0 mm (D) is classified as coarse fragments / gravel. Thus, the lower limit of gravel is 2.0 mm (D). Therefore, (A) and (D) only represent these two critical boundaries.