Aeration of horticultural media is oft en reported to be a problem. The air filled porosity is generally considered as the quality determining factor for media and is generally determining the shape of the moisture characteristic curve. The objective of this study was to determine affects of different size and rate of pumice mixed with soil on the pore size distribution and bulk density of a mixture medium. Pumice of different size (100 μm diameter) were 98.2 % and 70.3 %, and the decreases in bulk density were 24.8 % and 21.0 % for soil I and II, respectively. While the mesopores (100-30 μm diameter), which are important for water conductivity, decreased significantly with pumice application in soil I, no significant changes were obtained for soil II. The amount of water held at 0.01 MPa – 0.10 MPa decreased significantly with 30 % or more pumice application in both soils. The amount of water held at 0.10 MPa or greater suctions decreased with pumice application. Significant changes in physical properties of soil-pumice mixture media were obtained for different sizes of pumice application
