Minimizing the load of mineral nutrients that are leached from container-grown crops is a goal of both horticultural scientists and members of the industry for two reasons. First, leached mineral nutrients are no longer available for crop growth, and improvements in fertilizer use efficiency may help growers maintain profitability as fertilizer costs increase. Second, reducing the runoff nutrient load minimizes non-point source agrichemical contributions to local watersheds while simultaneously helping growers to comply with current or future regulatory standards such as the total maximum daily load limits for agrichemical contributions to the Chesapeake Bay watershed (Majsztrik and Lea-Cox, 2013). A set of best management practices (BMPs) (Bilderback et al., 2013) is available to growers as a tool set that can be used to improve water and nutrient use efficiency of container-grown ornamental crops. The industry has responded positively, as indicated by generally favorable BMP adoption rates in regional and national surveys of major nursery crop production regions in the United States (Garber et al., 2002; Mangiafico et al., 2010; Schoene et al., 2006).
Among these BMPs are the use of controlled-release fertilizers (CRFs) and nutrient monitoring techniques. A recent nationwide survey reported that 66.4% of nurseries were currently using CRFs (Dennis et al., 2010), a fertilizer technology that has been shown to be effective in reducing nitrogen (N) and phosphorus runoff as compared with fertigation (Wilson and Albano, 2011).
Controlled-release fertilizer performance is well understood as a result of season-long (≈4 to 12 months) studies (Broschat and Moore, 2007; Cabrera, 1997) that use techniques like the pour-through procedure or effluent (leachate) collection to evaluate trends in nutrient release. This knowledge enables growers to make informed decisions regarding irrigation and fertilizer management in an effort to improve crop quality and reduce nutrient loss through leaching. However, considerably less is known about nutrient leaching trends on a short time scale, i.e., how water and fertilizers move through and leach from a soilless substrate during and immediately after an irrigation event. Developing this knowledge may allow for the refinement of production practices (e.g., irrigation, fertilizer use, substrate selection, etc.) that lead to improved water and nutrient use efficiency.
Research on water and solute transport in soils (Beven and Germann, 2013; Mohammadi et al., 2009; Russo, 1993) provides a good foundation for understanding soilless systems. However, as a result of the substantial differences in physical properties between soils and soilless substrates, an independent body of research on water and solute transport is warranted to develop a more direct and thorough understanding of water and solute transport in soilless systems. Physical properties of the pine-bark and sand blends commonly used in the mid-Atlantic and southeastern U.S. nursery industry for the production of woody ornamentals (similar to the substrate used in this study) consists predominantly of large, low-density particles of organic origin. Consequently, substrates are highly porous and of low bulk density (Drzal et al., 1999). Fields et al. (2014) evaluated the wettability (rehydration efficiency) of pine-bark and found that the initial substrate MC and use of wetting agents had a significant impact on the number of hydration events that it takes to rewet a pine-bark substrate. It stands to reason that the pre-irrigation substrate MC would affect the movement and retention of water during an irrigation event. Therefore, characterizing how water moves through soilless substrates during irrigation is warranted, because it may lead to improved production practices that maximize water application efficiency (i.e., substrate retention of applied water).
An understanding of how water flows through a bark-based substrate also necessitates an investigation of the effect of root growth on water movement. Altland et al. (2011) demonstrated that plant roots will decrease air space and increase container capacity (CC), an effect that has been attributed to roots growing into and occupying pore spaces as well as the decomposition of organic substrate components. Gish and Jury (1983) observed that in loamy sand columns, an infiltrating chloride tracer solution applied to columns under pre-established, steady-state flow conditions moved through the column with less dispersion (i.e., more evenly) in treatments containing a wheat plant than a fallow column. They postulated that roots grew into large pore spaces and effectively created a homogenous pore size distribution that reduced the preferential flow of water through large pores and created a more uniform network of flow paths. However, their experiments were conducted under steady-state flow conditions, not under irrigation conditions (i.e., when the soil or substrate is of relatively low water content). Nash and Laiche (1981) assessed the hydraulic conductivity (HC) of water moving through bark, peat, and sand-based horticultural substrates in which ryegrass was grown. They reported high and variable HCs that generally ranged from 1.0 to 4.5 cm·min−1 and in extreme cases reported values of 26 cm·min−1. They theorized that roots that were concentrated near the substrate surface and along container walls may have caused channeling along container walls and lead to localized high HC values. Johnson and Lehmann (2006) discussed how, in shrink-swell soils, live roots that compress adjacent soil and decomposed roots allow for the preferential flow of infiltrating water through root-generated paths. Selker (1996) discussed the concept of preferential flow under field conditions and highlighted three types of preferential flow as 1) fingered flow (i.e., fingers or channels create uneven flow paths through coarse textured soils); 2) macropore flow (i.e., water flow is dominated by large over small pores); and 3) funnel flow (i.e., different textural layers redirect the flow of water). These flow types may provide insight into how applied irrigation water moves through soilless substrates. However, a more direct study of preferential flow path formation in common horticultural soilless substrates would be useful when developing BMPs that maximize irrigation and fertilizer efficiency.
This research is focused on characterizing the movement of irrigation water throughout a 17-week production cycle using Ilex crenata Thunb. ‘Bennett’s Compactum’ grown in 2.7-L nursery containers and a bark-based substrate. The objectives of this study were 1) to evaluate the patterns in which water moves through a pine-bark based substrate at different depths in the container profile; and 2) to determine the subsequent effect of root growth on water movement.
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