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Water-efficient soilless substrates need to be engineered to address diminishing water resources. Therefore, we investigated soilless substrates with varying hydrologies to determine their influence on crop growth and plant water status. Aged loblolly pine (Pinus taeda) bark was graded into four particle size fractions. The coarsest fraction was also blended with either sphagnum peat or coir at rates thatmimic static physical properties of the unfractionated bark or conventional substrate used by specialty crop producers within the eastern United States. Hibiscus rosasinensis ‘FortMyers’ plugs were established in each of the seven substrates andmaintained at optimal substrate water potentials (L50 toL100 hPa). After a salable cropwas produced 93 days after transplanting, substrate was allowed to dry until plants completely wilted. Crop morphology and water use was affected by substrate hydrology. Increased substrate unsaturated hydraulic conductivity (K) allowed for plants to access higher proportions of water and therefore increased crop growth. Maintaining optimal substrate water potential allowed plants to be produced with <18 L water. Measurements of plant water availability showed that the substrate water potential at which the crop ceases to withdraw water varied among substrates. Pore uniformity and connectivity could be increased by both fibrous additions and particle fractionation, which resulted in increased substrate hydraulic conductivity (Ks). Plants grown in substrates with higher hydraulic conductivities were able to usemore water. Soilless substrate hydrology can be modified and used in concert with more efficient irrigation systems to provide more water sustainability in container crop systems. The majority of all specialty crops spend a portion of their lifecycle in a container which uses soilless substrates. About 66% of the total $13 billion in annual greenhouse, floriculture, nursery crops sold annually were produced in containers within the United States (U.S. Department of Agriculture, 2015). Soilless substrates were first introduced for container crop production to increase drainage by maintaining a relatively large proportion of air-filled porosity (AS) as compared with previously used mineral soils (Raviv and Lieth, 2008). These highly porous, initially disease-free, substrates have been designed to ensure ample drainage regardless of irrigation practices or precipitation to prevent hypoxic conditions. This has led to a practice of excess water application to eliminate any Received for publication 30 Aug. 2017. Accepted for publication 8 June 2018. Funding for this work was provided, in part, by the Virginia Agricultural Experiment Station and the Hatch Program of the National Institute of Food and Agriculture (SCRI 2014-51181-22372), U.S. Department of Agriculture. A special thanks to Joshua Heitman and Holly Scoggins for comprehensive review of the manuscript herein. We would also like to thank Pacific Organics and Fibre Dust LLC for supplying materials used in this research. Trade or brand names used in this publication does not constitute a guarantee or warranty of the product by Virginia Tech and does not imply its approval to the exclusion of other products or vendors that also may be suitable. Assistant Professor. Associate Professor. Research Horticulturist. Professor. Corresponding author. E-mail: jsowen@vt.edu. 268 J. AMER. SOC. HORT. SCI. 143(4):268–281. 2018. risk of under-watering (Mathers et al., 2005). Moreover, this excess water application leads to inefficient use of water resources and subsequent leaching or runoff of applied agrichemicals (Million et al., 2007). Freshwater is a finite resource, in which 70% of the freshwater consumed in the world is used for agricultural purposes and nearly 40% of the freshwater withdrawn in the United States is used to irrigate crops (Kenney et al., 2009). Fulcher and Fernandez (2013) reported that nurseries that produce containerized woody ornamentals apply water upward of 177.3 m ha d during the periods of high water demand. Containerized specialty crop producers continue to be more conscientious of water use, whether it is due to economic decisions, governmental restrictions, or increased environmental stewardship (Fulcher et al., 2016) but only innovators or early adopters have only begun to take large steps to implement water saving practices. Varied efforts to reduce the water load in ornamental container production include different irrigation schemes (Warsaw et al., 2009), crop water use modeling (Million et al., 2010), crop spacing and arrangement variation (Beeson and Yeager, 2003), and sensor driven irrigation (Chappell et al., 2013), albeit without a clear alternative to overhead irrigation in container nursery production (Fulcher et al., 2016). About 50% of container nurseries currently use overhead irrigation, without the capability of using more sustainable irrigation delivery methods (Beeson et al., 2004). Further water savings could be realized through reengineering soilless substrates with increased K, coupled with reduced irrigation, to increase water efficiency of containerized specialty crop production. Therefore, research should, in part, move toward engineering new soilless substrates to increase water resource efficiency. Current best management practices (Bilderback et al., 2013) for container production of ornamental nursery crops in the southeastern United States recommend static physical properties with a large proportion of AS (10% to 30%) at container capacity [CC (45% to 65%)]. Instead of solely using recommended static measures (AS and CC), dynamic hydraulic properties, including the inherent relationship between volumetric water content (W), water potential (Y), and K, can provide greater insight in determining suitability of soilless substrates (Caron et al., 2014). Moisture characteristic curves (MCCs), which represent the relationship between W and Y of a porous media are used by researchers to not only determine static physical properties (Milks et al., 1989), but also provide information regarding degree of water availability (de Boodt and Verdonck, 1972) and infer pore structure (Drzal et al., 1999). Campbell and Campbell (1982) discussed the importance of high K to allow roots to access water from greater distances within mineral soil matrices. da Silva et al. (1993) discussed the merits of measuringK in soilless substrates primarily composed of sphagnum peat to create more efficient irrigation scheduling to help reduce water stress. Raviv et al. (1999) deemed decreasing K as the primary limiting factor for water uptake by roots in soilless substrates; however, they noted that in situ measurements of substrate K are not easily obtained and relationships between K and W or substrate Y should be better understood to make predictions of water availability in containers. Researchers have observed minimal reductions in substrate W can result in great reductions in unsaturated K (Wallach et al., 1992). These changes become more pronounced as W decreases further away from saturation in peat-based substrates (O’Meara et al., 2014). The overarching objective of this research is to determine if basic modifications of soilless substrate hydrophysical properties can increase water retention or enhance root accessibility of substrate water and subsequently increase crop water efficiency while retaining or improving crop growth. This objective was accomplished by 1) determining the influence of substrate modifications on hydraulic properties, 2) assessing the effects of substrate hydraulic properties on water dynamics for crops grown in optimal substrate water potentials (between –50 and –100 hPa), and 3) measuring how water availability for a containerized crop is affected by varying hydraulic properties. Materials and Methods SUBSTRATE PREPARATION. On 17 Mar. 2015, 2.5 m of stabilized (aged between 4 and 6 months) loblolly pine bark screened through a 12.6-mm screen was attained from a commercial nursery substrate company (Pacific Organics, Henderson, NC). The bark was further separated into four different size fractions by sieving through a shaker rotating at 8 oscillations/s (Custom Fabricated Shaker; Steve’s Welding, Williamston, SC) with 6.3-, 4.0-, and 2.3-mm screens (W.S. Tyler, Mentor, OH) at the Substrates Processing and Research Center located at North Carolina State University, Raleigh. The sieve sizes used in this research were chosen based on preliminary experimentation to provide four unique fractions. The process entailed incrementally placing 0.05 m of pine bark (64.6% ± 0.8% moisture content measured via random sampling throughout the screening process) into the shaker tray, smoothing the pine bark to an even depth of 6 cm, shaking through a 6.3-mm screen for 90 s, and then placing particles that did not pass ($6.3 mm) and which passed through (<6.3 mm) the screen into separate 0.19-m drums. Once 0.38 m of each substrate was collected, the process was repeated on the bark that passed through the initial screen using 4.0and 2.3-mm screens to collect 0.19 m of each size fraction. This process resulted in four particle size fractions of pine bark: >6.3 mm (PF6), 4.0– 6.3 mm (PF4), 2.3–4.0 mm (PF2), and #2.3 mm (PF0). In addition, 0.38 m of unprocessed pine bark (UPB) was placed in sealed drums for later use. All substrates were then transported to the Virginia Tech Hampton Roads Agricultural Research and Extension Center in Virginia Beach for analysis and experimentation. Compressed bales of sphagnum peat (Fafard, Agawam,MA) and coir (FibreDust LLC, Glastonbury, CT) were hydrated, reconstituted, and stored individually in sealed plastic tubs for 24 h to allow for moisture equilibrium. Peat and coir were blended with PF6 to produce two additional substrates: 65% (by volume) PF6: 35% (by volume) peat (P35) and 60% (by volume) PF6: 40% (by volume) coir (C40) with the goal of mimicking the stat

journal of the american society for horticultural science

Soilless Substrate Hydrology Can Be Engineered to Influence Plant Water Status for an Ornamental Containerized Crop Grown within Optimal Water Potentials