Improving water and nutrient management in container plant production will help the nursery industry adapt to decreasing water resources and comply with the growing number of laws and regulations regarding nursery water use, fertilizer applications, and nutrient levels in runoff (Beeson et al., 2004; Chappell et al., 2013a). Overirrigation commonly occurs for many reasons, including the belief that maintaining substrates near container capacity is necessary for maximum growth (Beeson, 2006), inefficiencies in irrigation application and poor uniformity of irrigation systems (Fare et al., 1992), and the grower preference to over—rather than under—apply water (Million et al., 2007; Yeager et al., 2010). Also, many growers apply large amounts of fertilizer out of concern that lower fertilizer applications could negatively impact growth (Owen et al., 2008; Tyler et al., 1996). The combination of excessive irrigation and high fertilizer rates often leads to significant leaching of fertilizers, which has a negative environmental impact as the leachate enters local ecosystems (Lea-Cox and Ross, 2001), and can lead to the need for additional fertilizer applications late in the production cycle.
Best management practices (BMPs) have been adopted by many growers in an effort to irrigate and fertilize more efficiently. Cyclic irrigation, which applies daily irrigation via multiple smaller applications, can be used to apply reduced irrigation volumes, and can reduce water and nutrient leaching (Fare et al., 1994; Ruter, 1998). Other BMPs for irrigation management include grouping plants by water requirements and inspecting irrigation systems for uniformity (Chappell et al., 2013a). More recently, soil moisture sensor-automated irrigation has been used to control irrigation with a variety of nursery and greenhouse crops, including Hibiscus acetosella (Bayer et al., 2013), Lantana camara (Bayer et al., 2014), Hydrangea macrophylla (van Iersel et al., 2009), Gaura lindheimeri (Burnett and van Iersel, 2008), and G. jasminoides (Chappell et al., 2013b). Until recently most sensor-controlled irrigation has been for research purposes; however, wireless sensor networks capable of controlling irrigation are being developed for implementation in commercial production (Kohanbash et al., 2013; Lea-Cox et al., 2013).
Fertilization and nutrient leaching BMPs have also been adopted, including using controlled-release fertilizers that last throughout the production period and monitoring substrate nutrient levels (Yeager et al., 2010). Less leaching can help reduce nutrient runoff, but there is concern that this may result in the buildup of salts in the substrate, which can damage roots (Bilderback, 2002). Irrigating to maintain a moderate or high leaching fraction (volume of water leached/volume of water applied) is commonly used to avoid fertilizer salt buildup in substrates. Monitoring EC can ensure that salt levels do not become excessive. The pour-through method of EC measurement is commonly used by growers (Bilderback, 2002; Chappell et al., 2013a). This method produces reliable results, but is labor intensive and can be inconvenient if samples are sent to a laboratory for analysis. In situ methods are instantaneous and provide continuous measurements allowing for a clearer picture of the impact of fertilization and irrigation, but most sensors measure the bulk EC of the soil or substrate, which is a combination of substrate/soil particles, air spaces, and substrate/soil solution. Substrate bulk EC depends on substrate water content (Scoggins and van Iersel, 2006) and is not a reliable measurement of nutrient levels in the substrate. New sensors, which measure the bulk dielectric, temperature, and bulk EC (GS3; Decagon Devices, Pullman, WA), can be used to estimate pore water EC using the Hilhorst model (Hilhorst, 2000; van Iersel et al., 2013). These sensors are affordable and provide real-time information about the growing conditions that can be used on a day-to-day basis to make irrigation and fertilization decisions.
The effects of reduced fertilizer rates and irrigation application have been examined (Fare et al., 1994; Million et al., 2007; Owen et al., 2008; Tyler et al., 1996); however, the effects of reduced irrigation volume based on plant water use and fertilizer rate have not been adequately studied. Using soil moisture sensors to irrigate based on substrate water content allows for efficient irrigation based on plant water use. The effect of sensor-controlled irrigation along with reduced fertilizer application rates on plant growth and leaching will provide further information about the potential for reducing fertilizer inputs with efficient irrigation. Our objective was to determine if fertilizer rate and irrigation volume could be applied more efficiently to reduce leachate volume and leachate nutrient content without negatively impacting growth of G. jasminoides ‘MAGDA I’. Our hypothesis was that more efficient irrigation can be combined with reduced fertilizer inputs to reduce leaching and nutrient levels in leachate without impacting plant growth; with reduced leaching, more fertilizer remains in the container and available to the plant.
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