Evaluating on-demand irrigation systems for container-grown woody plants grown in biochar-amended pine bark

Authors: JAHROMI, NASTARAN - University Of Tennessee; FULCHER, AMY - University Of Tennessee; WALKER, FORBES - University Of Tennessee; Altland, James; WRIGHT, WESLEY - University Of Tennessee; EASH, NEAL - University Of Tennessee

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2018
Publication Date: 12/1/2018
Citation: Jahromi, N., Fulcher, A., Walker, F., Altland, J.E., Wright, W., Eash, N. 2018. Evaluating on-demand irrigation systems for container-grown woody plants grown in biochar-amended pine bark. HortScience. 53(12):1891-1896. https://doi.org/10.21273/HORTSCI13541-18.
DOI: https://doi.org/10.21273/HORTSCI13541-18

Interpretive Summary: Overhead irrigation controlled by timers or manually operated systems is the most common method of irrigation for container nursery production systems. Using a fixed irrigation rate in a timer-based or manual system can result in over- or underwatering. Improving irrigation efficiency can be achieved by scheduling irrigation with the appropriate use of sensors rather than by relying on periodically adjusting irrigation volume based on plant or substrate visual appearance. The goal of this study was to evaluate the impact of on-demand irrigation scheduling and hardwood biochar amendment on plant water use and biomass gain of container-grown Hydrangea paniculata ‘Silver Dollar’ grown outdoors. The three irrigation schedules were conventional irrigation and two on-demand schedules, which were based on substrate physical properties or plant physiology. Conventional irrigation delivered 1.8 cm water in one event each day. The scheduling of substrate-based irrigation was based on the soilless substrate moisture characteristic curve, applying water whenever the substrate water content corresponding to a substrate water potential of –10 kPa was reached. The plant-based irrigation schedule was based on a specific substrate moisture content derived from a previously defined relationship between substrate moisture content and photosynthetic rate, maintaining the volumetric water content (VWC) to support photosynthesis at 90% of the maximum predicted photosynthetic rate. The plant-based irrigation used less water than the two other irrigation systems while still meeting crop demand, which makes it the optimal irrigation scheduling system in this experiment. Both on-demand irrigation scheduling regimes had greater plant biomass metrics and water use efficiency, and less leachate volume compared with conventional irrigation. Nursery industry professionals should consider adopting plant-based on-demand irrigation systems to increase water savings or expand production using existing and/or limited water supplies.

Technical Abstract: Controlling irrigation using timers or manually operated systems is the most common irrigation scheduling method in outdoor container production systems. Improving irrigation efficiency can be achieved by scheduling irrigation based on plant water needs and the appropriate use of sensors rather than relying on periodically adjusting irrigation volume based on perceived water needs. Substrate amendments such as biochar, a carbon (C)-rich by-product of pyrolysis or gasification, can increase the amount of available water and improve irrigation efficiency and plant growth. Previous work examined two on-demand irrigation schedules in controlled indoor (greenhouse) environments. The goal of this study was to evaluate the impact of these on-demand irrigation schedules and hardwood biochar on water use and biomass gain of container-grown Hydrangea paniculata ‘Silver Dollar’ in a typical outdoor nursery production environment. Eighteen independently controlled irrigation zones were designed to test three irrigation schedules on ‘Silver Dollar’ hydrangea grown in pine bark amended with 0% or 25% hardwood biochar. The three irrigation schedules were conventional irrigation and two on-demand schedules, which were based on substrate physical properties or plant physiology. Conventional irrigation delivered 1.8 cm water in one event each day. The scheduling of substrate-based irrigation was based on the soilless substrate moisture characteristic curve, applying water whenever the substrate water content corresponding to a substrate water potential of –10 kPa was reached. The plant-based irrigation schedule was based on a specific substrate moisture content derived from a previously defined relationship between substrate moisture content and photosynthetic rate, maintaining the volumetric water content (VWC) to support photosynthesis at 90% of the maximum predicted photosynthetic rate. Total water use for the substrate-based irrigation was the same as for the conventional system; the plant-based system used significantly less water. However, plant dry weight was 22% and 15% greater, water use efficiency (WUE) was 40% and 40% greater, and total leachate volume was 25% and 30% less for the substrate-based and plant-based irrigation scheduling systems, respectively, than for conventional irrigation. The 25% biochar amendment rate reduced leachate volume per irrigation event, and leaching fraction, but did not affect total water use or plant dry weight. This research demonstrated that on-demand irrigation scheduling that is plant based or substrate based could be an effective approach to increase WUE for container-grown nursery crops without affecting plant growth negatively.