Physical and Chemical Properties of Peat-based Substrate as Affected by Volcanic Cinder Amendment

Utilizing locally available materials to create potting mix is vital for sustainable production of ornamental crops. Volcanic cinder is the most commonly used substrate in Hawaii because it is inexpensive, inert and highly porous, and thus provides good drainage for plants.  Experiment was conducted to characterize the physical and chemical properties of the substrates formulated by blending either 0%, 25%, 40%, 50%, 60%, 75% or 100% (by volume) volcanic cinder and peat-moss as compared to commercial potting mix (75% peat-moss and 25% perlite).  All substrates containing cinder had higher bulk density than that of 100% peat-moss (0.14g/cm³) or commercial potting mix (0.12g/cm³). The bulk density increased with higher percentages of cinder ranging from 0.22g/cm³ with 25% cinder to 0.43g/cm³ with 100% cinder. Total porosity was high in both 100% peat-moss (74%) and commercial potting mix (65%) compared to the substrates formulated with cinder, and  significantly decreased with higher percentages of cinder.  The water holding capacities of both 100% peat-moss (53%) and commercial potting mix (52%) were high compared to the substrates created with cinder, and higher percentages of cinder significantly decreased water holding capacity.  Air-filled porosity was high in 100% cinder (33%) followed by 100% peat-moss (21%), while it was similar for the substrate with 25% cinder (14%) and commercial potting mix (13%). Electrical conductivity (EC) was higher in commercial potting mix compared 100% peat-moss and decreased when higher percentages of cinder were mixed. Similarly, the pH was also higher in potting mix (5.23) compared to 100% peat-moss (4.09), but increased with higher percentages of cinder. These results suggest that inclusion of cinder in the substrate may provide optimum water holding capacity, better drainage and air-filled pore space as did perlite in commercial potting mix. The proper ratio of cinder to peat-moss should be determined based on the irrigation and drainage requirements of the specific crop.