Aeration in growing media: Recent developments

Aeration in soilless mixes is often reported to be a problem. After the partial saturation and the complete drainage of potted media, a very small perched water table occurs at the bottom of the pot, resulting in media equilibrated at very high water potentials. The air-filled porosity of soilless mixes may be very low at these high water potentials, despite a very high total porosity. Air-filled porosity is therefore used as a standard index to guide substrate manufacturing, and is generally measured on disturbed media packed into cylinders and equilibrated at an arbitrary potential. Simulation studies have shown the importance of characterizing air-filled porosity at more than one water potential since air-filled porosity is dependent on container geometry for a given substrate. Latest developments also suggest that the air exchange properties within the substrate be considered to obtain more reliable indexes of aeration. Indeed, in three recent studies in which the substrate physical properties were varied, air exchange properties (gas relative diffusivity) were more closely correlated to plant growth than was the air-filled porosity. Recent studies have also focused on the in situ characterization of artificial mixes, providing new ways to measure gas relative diffusivity, air-filled porosity and water desorption curves directly in the pot, with or without actively growing plants.