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 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which ≈70% of the freshwater consumed in the world is used for agricultural purposes and nearly 40% of the freshwater withdrawn in the United States is used to irrigate crops (Kenney et al., 2009).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 m3·ha−1·d−1 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 (ϴ), water potential (Ψ), 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 ϴ and Ψ 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 measuring K 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 ϴ or substrate Ψ should be better understood to make predictions of water availability in containers. Researchers have observed minimal reductions in substrate ϴ can result in great reductions in unsaturated K (Wallach et al., 1992). These changes become more pronounced as ϴ 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.
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