Pore sizes have traditionally been divided into macropores and micropores with the division between the two being arbitrary. Since most mixes used in container production are ≥ 80% pores by volume, a more detailed pore-fraction analysis seems warranted. Taking into account hydraulic properties and irrigation parameters, pore-size distribution curves were separated into four ranges. Macropores were selected as pore sizes > 416 µ. Pore sizes within the macropore range cannot hold water under tension induced by gravity when allowed to drain after saturation. Mesopores were selected as being in the pore size range of ≤ 416 to ≥ 10 µ. Micropores were categorized into the pore-size range of 0.2 to 10 µ. This would be equivalent to volumes of water held between 30 kPa and 1.5 MPa. The water in these pores may be viewed as a type of water stress "buffer" not commonly used under normal irrigations but extracted by plant roots when suctions exceed 30 kPa. Ultramicropores hold water at suctions > 1.5 MPa and would be found in pores with effective pore diameters < 0.2 µ. This water would be considered unavailable to plants. Data derived from this analysis were in good agreement with traditional measures of pore space and particle size distributions for peat-based, bark-based, and soilbased substrates.