Quantifying Root Zone Sensor and Substrate Volumetric Water Content Variability in Pot-In-Pot Tree Production

An eight-month study was conducted in 2011 to quantify (a) capacitance sensor variability and (b) the spatial variability of substrate volumetric water content (VWC) within the root zone of two tree species grown in a pot-in-pot nursery production. Dogwood (Cornus florida) and red maple (Acer rubrum) trees were grown in a 75% pine bark: 25% recycled paper mixed substrate in 57 and 114 L containers, respectively. The water-release curve for this substrate was determined with increasing air pressure using a custom-built tension table, which simultaneously correlated the dielectric permittivity of 10HS sensors (Decagon Devices, Inc., Pullman, WA) to decreasing substrate VWC. 10HS sensors were then inserted at two different depths (15 and 30 cm) in 10 dogwood and 10 red maple trees in two blocks in the nursery.  All cultural practices and irrigation events were followed according to standard practices. Substrate VWC and environmental data were recorded every 15 minutes using EM50R nodes, utilizing a wireless network system and DataTrac software (Decagon Devices, Inc.). We quantified 10HS sensor variability by analyzing steady-state VWC data between 4 and 5 AM, removing daily temporal variability in VWC induced by daily irrigation events from March through October. In dogwood, substrate VWC was maintained at an average of 0.54 ± 0.05 m³∙m^-3 at 15 cm and 0.55 ± 0.04 m³∙m^-3 at 30 cm depth. Results obtained for red maple were very similar (an average VWC of 0.54 ± 0.05 m³∙m^-3 at 15 cm and 0.55 ± 0.06 m³∙m^-3 at 30 cm depth). Throughout the study period, there were no significant differences between the VWC readings of the 10 HS sensors at each depth for either tree species. Standard error (SE) values were used to determine the variability of the sensors across the replicates. SE between sensor VWC values ranged from 0.004 to 0.027 m³∙m^-3 at 15 cm and from 0.006 to 0.041 m³∙m^-3 at 30 cm in dogwood; similar sensor to sensor variability was seen in red maple (with SE values from 0.006 to 0.025 m³∙m^-3 at 15 cm and from 0.006 to 0.047 m³∙m^-3 at 30 cm depth) indicating the uniformity of the 10HS sensors in measuring substrate VWC.  These 10HS capacitance sensors proved reliable for VWC measurement in this substrate with an acceptable variation of < 10% over the season, providing precise VWC information to the grower for scheduling daily irrigations.