Bark particle sizes and the modification of the physical properties of peat substrates

The physical quality of peat mixes is in part related to the capacity of the substrate to store and supply air and water to plant roots. During manufacturing, the mixing of various substrate components modifies the substrate characteristics. The objective of this study was to assess the changes in air storage and supply properties caused by varying the particle size of the substrate components. The substrate was composed of 40% wood bark (WB), 50% peat, and 10% coarse gravel (volume basis). Wood bark particle Size was varied in a first (0-2, 2-4, 4-8 and 8-25 mm) and a second (1-2, 2-4, 4-8 and 8-16 mm) experiment. When increasing bark particle sizes to 8-25 mm or 8-16 mm, air supply characteristics, as assessed with gas diffusivity measurements, decreased to 0.78 or 0.45 its value for the 2-4 or 1-2 mm average bark particle size. This occurred despite no significant changes in air storage, as assessed from air-filled porosity measurements.