Optimizing substrate available water and coir amendment rate in pine bark substrates

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

Submitted to: Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2020
Publication Date: 1/29/2020
Citation: Jahromi, N.B., Fulcher, A., Walker, F., Altland, J.E. 2020. Optimizing substrate available water and coir amendment rate in pine bark substrates. Water. 12(2). Article 362. https://doi.org/10.3390/w12020362.
DOI: https://doi.org/10.3390/w12020362

Interpretive Summary: Agricultural irrigation is a major water consumer, responsible for about 70% of worldwide consumptive use and about 30 percent of total withdrawals in the U.S. Nursery production is an intensive form of agriculture that uses relatively large amounts of water, nutrients, and pesticides. Developing management practices that make more efficient use of water is important for economically and environmentally viable production systems. Coconut fiber, known as coir, is a readily available, plant-derived by-product of the coconut (Cocos nucifera) industry and has been shown to provide aeration and water holding capacity for plant roots. The first objective of this study was to evaluate the impact of coir amendment rate on plant available water and plant gas exchange, with the goal of optimizing substrate available water and determining the optimum coir amendment rate in a greenhouse environment. The second objective was to establish the optimum method of determining plant available water using either plant gas exchange parameters or substrate physical properties. This research demonstrated that addition of coir to pine bark substrate increases plant water availability, which increased plant gas exchange rates. Of the rates tested in this greenhouse experiment, the highest rate, 65% coir, was the ideal rate of coir amendment to pine bark. Compared to other gas exchange parameters, photosynthetic rate was a better indicator of plant water status and can be used for irrigation scheduling. Cost feasibility studies should be conducted to determine if consulting and instrumentation expenses necessary to implement a gas exchange-based irrigation schedule would outweigh the financial savings from reduced water and pumping costs.

Technical Abstract: Water resources can be used more efficiently by including sustainable substrate components like coir that increase water-holding capacity. The first objective of this study was to evaluate the impact of coir amendment rate on plant available water and plant gas exchange, with the goal of optimizing substrate available water and determining the optimum coir amendment rate in a greenhouse environment. The second objective was to establish the optimum method of determining plant available water using either plant gas exchange parameters or substrate physical properties. Greenhouse experiments were conducted with Hydrangea paniculata ‘Jane’ (Little Lime® hardy hydrangea) potted with one of five different coir rates (0%, 10%, 25%, 40% and 65%) mixed with pine bark on a volume basis. Plant gas exchange parameters and substrate water content were measured daily over a range of increasingly drier substrate moisture contents. Actual photosynthetic rates increased with increasing coir amendment rate and were highest with 65% coir amendment. Amending pine bark with coir increased the water storage capacity, plant available water, and plant gas exchange parameters. Results suggest that 65% coir amendment rate was the optimum amendment rate among those tested in a greenhouse environment and plant photosynthetic rate was the better method of determining plant available water.