Physical properties of pine bark substrate amended with industrial mineral aggregate

Pine bark is the most common container substrate in the Southeastern United States nursery industry. Pine bark based substrates provide excellent aeration and a moderate amount of available water (AW), however, they have little water buffering capacity. Thus, frequent irrigation events are required to maintain adequate water. This often leads to low water use efficiency. Current studies have shown reduced water application needs and increased plant stomatal conductance and carbon assimilation when plants were grown in a mineral aggregate amended pine bark substrate compared to pine bark alone. An increase in water buffering capacity was reported which may explain the plant response. Our objective was to determine if increased substrate water buffering capacity could be explained as a function of substrate physical properties. To accomplish this objective, pine bark was amended with a calcined Georgiana palygorksite-bentonite aggregate (0.85-0.25 mm) at 0%, 4%, 8%, 12%, 16%, 20% and 24% (by vol.). Physical properties consisting of total porosity, container capacity, air space (AS), bulk density, AW, and unavailable water (UW) were determined. Soil moisture characteristic curves were determined for amendment rates of 0%, 8%, 12%, 16% and 20% (by vol.). Container capacity and AW increased linearly with increasing amendment rate, whereas UW and AS decreased linearly with increasing rate of mineral aggregate. Substrate moisture characteristic curves showed that more water was retained at greater substrate moisture tensions with increasing mineral aggregate rate, thereby increasing readily available water. Volumetric water content was initially greater at the 0% rate, however it quickly decreased at approximately 2 cm substrate moisture tension below those substrates amended with the mineral. The physical properties of the substrate in association with the inherent zeolitic and absorbed water of the mineral increased water content, resulting in increased buffering capacity which could reduce plant water stress.