Comments and Letters to the Editor

Several contrasting uses of the terms macro(large) and micro(small) pore can be identified in soil science literature. In some cases macropores are (i) considered to be pores > 15 (Marshall, 1959) or 100 /im in diam (Bouma et al., 1977) or (ii) implied to be even larger when described in terms of wormholes (Ehlers, 1975), root channels (Aubertin, 1971). and shrinkage cracks (Lewis, 1977). The size range of micropores is either stated or implied to be less than the corresponding macropore range in each case. Noncapillary porosity has also been used to distinguish large pores from small pores. Nelson and Baver (1940) estimated noncapillary porosity for several materials as the pore volume that became water filled above approximately —3 kPa pressure. Marshall (1959) expressed an updated viewpoint that equated noncapillary porosity with macroporosity, the pore volume that became water filled at pressures above —10 kPa. Edwards et al. (1979) used pore diameters of 5 and 10 mm in a modeling study of noncapillary pore effects on infiltration. The fact that macroand microporosity are not uniformly used can lead to ambiguity, particularly with the renewed interest in soil water channeling phenomena (Thomas and Phillips, 1979). The introduction of the term mesoporosity to soil systems is suggested to be timely along with some operational definitions of soil micro-, meso-, and macroporosity. The definitions suggested (Table 1) correspond with three dominant regimes of soil water behavior and are associated with some "convenient" equivalent pore diameter ranges as calculated from the surface tension-capillary rise equation (Marshall, 1959). The prefix meso-, meaning middle, is used in combination chiefly with scientific terms to differentiate some identified hierarchy of structure, function, or behavior. Thus we have, to name a few: