A NEW METHOD FOR MEASURING TOTAL POROSITY IN HORTICULTURAL SUBSTRATES

The determination of air and water holding capacities of horticultural substrates has been plagued by errors in measurement. The amount of air and water held at container capacity is influenced by the substrate and container height. Container capacity can be established through specific measurement. Air space, the difference between total porosity and container capacity, is usually poorly determined because of errors in total porosity measurement. Most researchers calculate total porosity (S t ) from the formula: S t = 1-(ρ b /ρ p ), where ρ b is the dry bulk density and ρ p is the particle density. While bulk density is usually measured, particle density is not. Many times an average ρ p of 2.65 Mg·m -3 for mineral soils is used. This sometimes creates large errors in calculating total porosity because the values of ρ p for horticultural substrates range from 0.35 to 2.1 Mg·m -3 . Total porosity can be measured with great accuracy at 0 kPa tension on a pressure plate apparatus, but is costly in equipment and time. Using a modified method of extraction and a new apparatus, using standard aluminum soil sampling cylinders, total porosity was measured with an 85% reduction in time end no decrease in accuracy.