Evaluation of physical properties of several growing media for use in aerial seeding containers

Several personally blended and commercially prepared growing media were evaluated as potential substrates for coniferous tree seedlings in specialized seeding containers (aerial darts). Water retention curves were developed using a porous pressure plate apparatus. Bark–vermiculite–sand and Ball Growing Mix II retained less water at pressures from 0 to 0.1 MPa than all other media observed. At pressures less than 20 × 10 −3 MPa the greatest water retention was in Pro-mix A, while above 33 × 10 −3 MPa the greatest water retention was in bark–vermiculite. As the proportion of bark in bark–vermiculite was increased, the air space increased significantly but water retention was unaffected. Addition of a hydrophilic gel to media containing a surfactant significantly increased water retention. The easily available water, water buffering capacity, and air space were unaffected by gel but total porosity increased in some of the media. When the surfactant was excluded the addition of gel at the recommended rate did not significantly increase water retention. As the concentration of gel increased there was an increase in water retention, a linear increase in total porosity, a slight increase in air space, a decrease in easily available water, and no change in water buffering capacity. Media amended with gels of different absorbing capacities retained water similarly, releasing approximately 50% of the absorbed water at pressures less than 10 × 10 −3 MPa.