The container nursery industry is continuously seeking new irrigation and fertilization strategies to improve application efficiencies and reduce negative environmental impacts. The objective of these strategies is to strike a balance between the rewards of reduced water and fertilizer inputs and the risks of reduced plant growth and quality. One strategy to optimize irrigation entails supplying an amount of water that is proportional to that lost from ET. Water loss from ET can be measured directly by weighing representative containers or indirectly through monitoring substrate water potential. Estimations of ET can also be made with weather data and appropriate crop coefficients (Beeson, 1993, 2005, 2010; Schuch and Burger, 1997). Furthermore, irrigation can be withheld until substrate water loss exceeds a managed allowable deficit (MAD) (Welsh and Zajicek, 1993). A delay in irrigation afforded by MAD scheduling increases the chance for precipitation to replace some or all of the irrigation demand. Beeson (2006) found that 20% MAD produced acceptable sweet viburnum growth.
By minimizing leaching volume, ET-based irrigation has the potential to reduce nutrient leaching losses during production and, therefore, allow smaller quantities of fertilizer to be applied. Reducing the leaching fraction (volume of leachate divided by the volume of irrigation water) from 0.4–0.6 to 0.0–0.2 decreased N and P leaching losses by greater than 60% for Cotoneaster dammeri grown in 3.8-L containers (Tyler et al., 1996). A decrease in the leaching fraction from 56% to 12% reduced total N leaching by ≈50% but resulted in an extremely high (greater than 800 mg·L−1) NO3-N concentration in leachate from 0.34-L (10-cm diameter) containers (Huett, 1997). By decreasing the daily irrigation rate from 1.3 to 0.6 cm during production of Ilex cornuta in 2.3-L containers, cumulative leachate volume was reduced from 3.1 to 0.9 L per container, respectively, and resulted in a 50% reduction in estimated NO3-N losses (Fare et al., 1999). Constant feed fertilizer rates can generally be halved using ebb-and-flow subirrigation compared with overhead irrigation (Barrett, 1991) indicating that lower fertilizer rates may be possible with water-conserving irrigation practices. Huett (1997) indicated that occasional heavy precipitation can substantially reduce fertilizer nutrient buildup that can occur under minimal-leach irrigation treatments. This suggests that the potential for zero-leach or minimal-leach irrigation strategies to reduce nutrient leaching may be limited in regions with significant precipitation.
The objective of our research was to determine if an ET-based irrigation schedule could reduce irrigation runoff volume and nutrient loss compared with a fixed-rate irrigation schedule. The two irrigation schedules were evaluated using two controlled-release fertilizer (CRF) application rates to determine if fertilizer efficiency could be increased with ET-based irrigation.
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