Improving water use efficiency of containerized crops using biochar and irrigation scheduling through developing precision irrigation scheduling with a physiological-based and a substrate physical properties-based system
Basiri Jahromi, F. R. Walker, A. Fulcher, J. Altland, and N. S. Eash.  2018.  Ph.D. Dissertation.

Controlling irrigation using timers, or manually operated systems are the most common irrigation scheduling methods in 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-rich by-product of pyrolysis or gasification, can increase the amount of available water and improve irrigation efficiency and plant growth. The objective of first experiment was to determine the effect of switchgrass biochar on water and nutrient release and retention in container substrates of Buxus sempervirens L. ×B. microphylla (‘Green Velvet’ boxwood) and Hydrangea paniculata (Pinky Winky® hardy hydrangea). Containers were filled with pine bark and amended with 0%, 10%, or 25% by volume of biochar. Plants were irrigated when the volumetric water content (VWC) reached the water buffering capacity set point of 0.25 cm3·cm-3. Results showed that applications of 10% and 25% biochar amendment to pine bark substrate increased water-holding capacity and reduced water consumption of a high water use crop, Pinky Winky hydrangea. However, reduction of plant biomass in the 25% biochar treatment suggests that sufficient water might not be available to plants in this substrate. The lower irrigation frequency associated with 25% biochar amendment might have resulted in an insufficient portion of the water in each cycle being plant available and eventually exceeding the water buffering capacity. Further research was required to more fully understand the effect of biochar on plant water availability. Therefore, the second experiment was initiated to address this problem by using physiological parameters to monitor plant water status under different irrigation schedules that were designed to maximize plant available water. A precision irrigation scheduling with a physiologically based and a substrate physical properties-based system was developed. Models were evaluated by measuring photosynthesis, growth, and water use of Hydrangea paniculata ‘Silver Dollar’ produced with different irrigation schedules and biochar substrate amendment in two different years. Containers were filled with one-year-old pine bark and amended with either 10% or 25% by volume of hardwood biochar. Plants were automatically irrigated by one of the three irrigation schedules. The irrigation schedules were conventional irrigation, delivering 1.8 cm of water in one event each day, and two on-demand, need-based irrigation schedules. The first was based on the moisture characteristic curve for each of the three substrates developed via the evaporative method. Plants were irrigated when the volumetric water content (VWC) corresponding to a substrate water potential of -10 kPa, generally considered the highest tension for plant available water, was reached. The second was a plant physiology-based irrigation scheduling regime built on the relationship between photosynthesis and substrate moisture content. This schedule actuated irrigation at the VWC that was expected to maintain photosynthesis at 90% of the maximum predicted photosynthetic rate. Scheduling irrigation using a plant physiology or substrate physical properties basis, in combination with biochar substrate amendment reduced the water requirement for ‘Silver Dollar’ hydrangea without any negative effect on plant dry weight by maintaining sufficient plant water status and gas exchange even just prior to irrigation, the driest point in the irrigation cycle. Further research was required to validate the models in an outdoor production environment and develop an applied research program. Eighteen independently controlled irrigation zones were designed to test three irrigation schedules on ‘Silver Dollar’ hydrangea grown outdoors in pine bark amended with 0% or 25% hardwood biochar. Total water use was unaffected or lower in the on-demand irrigation systems. However, plant dry weight and water use efficiency was greater in the substrate-based and plant-based irrigation scheduling systems compared to the conventional irrigation. This research demonstrated that automated irrigation systems coupled with a plant physiology or substrate-based irrigation scheduling and a water retentive substrate amendment have the potential to reduce nursery water use without negatively affecting plant growth