The nursery industry is an expanding sector in agriculture that requires intensive use of water and fertilizer to produce salable crops. Many nursery growers rely on bark-based soilless substrates as the primary component of their growing media. Bark-based substrates often have increased proportions of large-diameter pores, which result in relatively low water-holding capacity (Pokorny, 1979) and allow for ample aeration (Jackson et al., 2006). This is advantageous because it reduces risks associated with excessive irrigation/precipitation (Mathers et al., 2005) and subsequent root disease (Baker, 1957). However, the downside to the low water-holding capacity is growers must irrigate frequently to replenish the water reservoir that is rapidly lost (Fields et al., 2021). Thus, nurseries tend to use large quantities of water, with estimates of 19,000 gal/acre/d spent on crop irrigation during the peak season (Fulcher and Fernandez, 2013). Leaching from high-irrigation application volume and frequency has been shown to result in runoff of agrichemicals such as mineral nutrients, pesticides, and large quantities of applied irrigation (Dumroese and Haase, 2018). As the industry seeks to become more sustainable, better water management strategies must be developed.
Soilless substrate stratification is an emerging substrate management strategy. It involves stacking two unique substrate layers atop one another to reorganize water retention characteristics in the container profile (Fields et al., 2020). This practice has been identified as a possible technique to increase irrigation water efficiency while improving plant growth simultaneously (Criscione et al., 2022). The placement of coarse particles in the lower half of the container promotes drainage where a perched water table is observed (Owen and Altland, 2008), whereas using substrates with high water-holding capacity and increased hydraulic and chemical retention in the upper half may provide more available resources for plant roots and may limit drying from evapotranspiration and gravitational forces. Together, this process supports a balanced rooting environment in the early stages of root growth and exploration in the upper half of the container, while allowing ample drainage in the lower half of the container. In practice, layering fine bark or bark amended with fibers such as peatmoss or coir on top of coarse bark will promote water holding in the upper (typically drier) portion of the container and will promote drainage in the lower (typically wetter) portion of the container. Thus, we are encouraging water retention and distribution to be more uniform throughout the container profile (i.e., promoting water retention in dryer regions and promoting drainage in wetter regions), while maintaining a similar overall porosity. Substrate stratification has also been shown to reduce weed germination (Khamare et al., 2022); however, this strategy involves layering coarse particles on top of finer particles. Thus, the upper portion of the container would dry quickly, inhibiting germination.
To date, Fields et al. (2021) and Criscione et al. (2022) have identified benefits associated with water and fertilizer management in nursery production using stratified bark substrates. Their work showed that layering fiber-amended bark atop coarse bark can increase resource (i.e., fertilizer and water) efficiency while producing similar crops. Criscione et al. (2022) also showed that stratified substrates can support quality plant production under irrigation levels that were too low to produce healthy plants grown in traditional pine (Pinus sp.) bark substrates. Both Fields et al. (2021) and Criscione et al. (2022) reduced fertilizer inputs by 20% and produced equal or larger plants in stratified substrates when compared with crops grown in traditional bark substrates.
With prior research effectively demonstrating the benefits of substrate stratification, it is important to identify pragmatic options to integrate this management strategy easily into production. It is hypothesized that a single screen could be used to divide a pine bark substrate into fractions to achieve stratification. Therefore, the objective of this study was to evaluate the growth effects of a nursery crop grown in single-screen, stratified pine bark substrates.
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