Particle size and the physical properties of growing media for containers

The effects of particle size distribution on the physical properties of growing media based on Pinus radiata bark were measured in laboratory experiments. Varying the proportions of particles larger than 0.5mm had little effect on air‐filled porosity and ability to release water between 1 and 10kPa suctions. Particles in the 0.1 to 0.25mm range decreased air‐filled porosity and increased water release to a greater extent than did either those in the 0.25 to 0.5mm range or those smaller than 0.1mm. Pine bark fractions generally had a greater ability to supply water at suctions below 10kPa than did sand of similar size. Guidelines for formulating media with differing abilities to supply air and water are given.