Freshwater is a finite resource that is rapidly becoming more scrutinized in agricultural consumption. Specialty crop producers, especially ornamental crop producers, must continually improve production sustainability, with regards to water resource management, in order to continue to stay economically viable. Soilless substrates were initially developed to have increased porosity and relatively low water holding capacity to ensure container crops would not remain overhydrated after irrigations or rain events. As a result, substrates were selected that are now considered to be in efficient in regards to water resource management. Therefore, to provide growers with additional means to improve production sustainability, soilless substrate hydrology needs be innovated to provide increased water availability while continuing to provide ample air filled porosity to ensure productive and efficient water interactions. Historically, soilless substrates have been characterized using "static" physical properties (i.e. maximum water holding capacity and minimum air-filled porosity). The research herein involves integrating dynamic soilless substrate hydraulic properties to understand how substrate hydrology can be manipulated to design sustainable substrates. This task involved adapting new technologies to analyze hydrological properties of peat and pine bark substrates by employing evaporative moisture characteristic measurements, which were originally designed for mineral soils, for soilless substrate analyses. Utilizing these evaporative measurements provide more accurate measures of substrate water potentials between -10 and -800 hPa than traditional pressure plate measurements. Soilless substrates were engineered, utilizing only three common substrate components [stabilized pine bark (Pinus taedea L.), Sphagnum peatmoss, and coconut coir fiber], via particle fractionation and fibrous additions. The engineering process yielded substrates with increased unsaturated hydraulic conductivity, pore connectivity, and more uniform pore size distributions. These substrates were tested in a greenhouse with irrigation systems designed to hold substrates at (-100 to -300 hPa) or approaching (-50 to -100 hPa) water potentials associated with drought stress. Substrate-water dynamics were monitored, as were plant morphology and drought stress indicators. It was determined that increased substrate unsaturated hydraulic conductivity within the production water potentials, allowed for increased crop growth, reduction in drought stress indicators, while producing marketable plants. Furthermore, individual plants were produced using as low as 5.3 L per plant. Increased production range substrate hydraulic conductivity was able to maintain necessary levels of air-filled porosity due to reduced irrigation volumes, while providing water for plants when needed. The substrates were able to conduct water from throughout the container volume to the plant roots for uptake when roots reduced substrate water potential. Furthermore, increased substrate hydraulic conductivity allowed plants within the substrate to continue absorbing water at much lower water potentials than those in unaltered (control) pine bark. Finally, HYDRUS models were utilized to simulate water flux through containerized substrates. These models allowed for better understanding of how individual hydraulic properties influence substrate water flux, and provided insight towards proportions of inaccessible pores, which do not maintain sufficient levels of available water. With the models, researchers will be able to simulate new substrates, and utilize model predictions to provide insight toward new substrates prior to implementing production tests. It has been determined, that increasing substrate hydraulic conductivity, which can be done with just commonly used components, water requirements for production can be reduced, to produce crops with minimal wasted water resources. Concluding, that re-engineering substrate hydrology can ameliorate production sustainability and decrease environmental impact. / Ph. D. / The world is rapidly approaching a time when water will become a limited resource, not only for agriculture, but all daily uses. As a result specialty crop production must continue to increase sustainability in order to continue to thrive. One area where growers and researchers believe environmental stewardship can be increased is through designing more resource efficient soilless substrates. Soilless substrates (potting media) are utilized world-wide by container crop producers as a rooting medium for specialty crops. These substrates were developed to be very forgiving for growers. By that, growers could apply excess water through irrigation or precipitations and these substrates were designed to readily drain excess water. This provides an opportunity to create more water efficient substrates to help reduce water consumption by container nurseries. The processes involving water-air-substrate interactions within the container are not well understood. As a result, my research involves measuring, manipulating, maintaining, and modeling substrate hydrology in an effort to design substrates that will conserve water in container production. I incorporated new technology used in Soil Science to measure hydraulic properties of soilless substrates through the evaporative method. I then understood how growers and allied suppliers can easily modify these substrate hydraulic properties. Next, I researched how these manipulated hydraulic properties would influence plant growth and vitality, by maintaining drought level irrigation levels over multiple crops. Finally, I modeled substrate hydraulic properties to better understand water movement through a container. Through the research herein, I was able to determine that substrate hydrology can be easily modified to provide container crops with more easily accessed water, while still keeping sufficient air-space for plant growth. Increasing unsaturated hydraulic conductivity in soilless substrates, allows ornamental crops to be held at lower water regimes moisture levels traditionally considered to be drought levels. Utilizing the HYDRUS model, I was able to determine how to develop future substrate models that will accurately simulate real-world outcomes, providing researchers with another tool to quickly predict impacts of newly developed (or still in development) soilless substrates on water status in container production.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/74925 |
| Date | 03 February 2017 |
| Creators | Fields, Jeb Stuart |
| Contributors | Horticulture, Owen, James Stetter, Scoggins, Holly L., Altland, James E., van Iersel, Marc, Heitman, Joshua L. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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