Defining critical capillary rise properties for growing media in nurseries

Water availability for landscape nursery irrigation is foreseen as a major impediment for this industry within the next decade. Among various solutions proposed to increase irrigation efficiency, thereby reducing the water volumes required, are closed and semi-closed subirrigation systems designed to grow plants potted in organic growing media. These systems, however, require organic substrates that have good capillary properties. However, standards for such capillary properties are not available. This study compared substrates composed of peat, bark, and sand having contrasting capillary properties, in a nursery experiment to establish guideline values for the proper and efficient operation on capillary mat devices. It also proposes a theoretical model of capillary rise using the hydraulic characteristics of growing media to predict the suitability of various substrates. Substrates with 60% (per volume) sphagnum peat were found to provide the best capillary rise and best growth, based on empirical measurements, relative to substrates with 30% sphagnum or 30% sedge peat. The proposed theoretical model concurred with these observations.