The coastal valleys of central California produce nearly 60,000 ha of lettuce annually, more than half of the nation’s supply. In this region, lettuce is typically produced in rotation with other leafy vegetables. Production systems are characterized by two to three crops per year with frequent irrigation and heavy N fertilization. Water quality monitoring in the agricultural watersheds in this region has shown that both surface water and groundwater often exceed the federal drinking water standard of 10 mg·L−1 NO3-N. Vegetable growers are under increasing regulatory pressure to improve both their fertilization and irrigation practices to protect environmental water quality. Recently proposed regulations would require growers to report N fertilization rates and to bring N loading from fertilizer and irrigation water into approximate balance with crop N uptake. In this region, lettuce N uptake has been reported to average 130 kg·ha−1 for iceberg and 107 kg·ha−1 for romaine (Breschini and Hartz, 2002). However, a recent field survey found that lettuce received an average seasonal N fertilization rate of 184 kg N/ha (Hartz et al., 2007), suggesting that significant N rate reduction would be required to meet these new regulations.
Studies on lettuce response to N fertilization have reported widely varying results. Seasonal N rates required to maximize crop yield have ranged from 100 to 150 kg·ha−1 (Gardner and Pew, 1972, 1974, 1979; Tei et al., 2003) to greater than 220 kg·ha−1 (Hoque et al., 2010; Welch et al., 1979). Much of this variability may be attributed to field-specific factors affecting crop yield potential and N fertilizer efficiency; these factors include plant population, precipitation, irrigation efficiency, residual soil NO3-N, and soil N mineralization potential. Given the high crop value and strict market standards for lettuce, growers commonly use standard fertilization programs with little field-specific modification; they are reluctant to modify current N fertilizer practices without a sound understanding of the interaction of these factors and reliable diagnostic techniques to guide field-specific N fertilization.
Adding to the uncertainty regarding efficient N management of lettuce, California growers continue to modify production practices to increase yield. Average lettuce yield rose ≈11% between 2000 and 2010 (Monterey County Agricultural Commissioner, 2000, 2010); factors potentially responsible included modified planting configurations that increased plant population and widespread adoption of drip irrigation. We undertook this study to develop detailed information on lettuce N requirements under current production practices used in California’s central coast region and to critically evaluate the value of soil and plant diagnostic techniques to guide in-season N fertilizer management.
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