The coastal valleys of central California produce more than 60,000 ha of lettuce annually, accounting for more than half of the nation's supply. Significant changes have occurred in the industry over the past decade; newer cultivars, increased planting density, and the widespread use of drip irrigation are among the factors that have increased average yields and led to modified fertilization practices. Considerable research on N and P fertilization of lettuce in this region has been conducted in recent years (Breschini and Hartz, 2002; Hartz et al., 2000; Johnstone et al., 2005). This research called into question the validity of the plant tissue nutrient sufficiency ranges currently used by the industry (Lorenz and Tyler, 1983: Ludwick, 2002).
Foliar nutrient sufficiency guidelines have often been developed from limited numbers of fertilization trials in which typically only one or two nutrients have been manipulated. That approach may not adequately reflect the influences of the wide range in soil characteristics and environmental conditions that characterize a regional industry. Furthermore, the interaction among the various macro- and micronutrients is not easily captured by traditional fertilizer trials.
An alternative approach to the development of foliar nutrient sufficiency guidelines is the Diagnosis and Recommendation Integrated System (DRIS; Beaufils, 1973). In the DRIS approach, differences in nutrient concentrations and nutrient ratios between high- and low-yielding populations are used to estimate the degree to which various nutrients may limit yield either by deficiency or excess (Walworth and Sumner, 1987). DRIS evaluation criteria have been developed for a range of agronomic and horticultural crops (Angeles et al., 1990; Beverly et al., 1984; Elwali et al., 1985; Parent and Granger, 1989; Walworth et al., 1986), including lettuce (Sanchez et al., 1991). DRIS was originally conceived as a diagnostic tool with which tissue nutrient concentrations in a field of interest could be compared with a set of established standards, or “norms”, through the calculation of nutrient indices; these indices would rank the relative degree of deficiency or excess for each nutrient. However, the complexity of this system has limited its practical application within the commercial horticultural industry.
The DRIS framework for comparing the nutrient concentration differences between high- and low-yielding populations can also be used to establish optimum leaf nutrient ranges (Needham et al., 1990). Hartz et al. (1998) used DRIS analysis to calculate leaf nutrient norms for processing tomato (Lycopersicon esculentum Mill.) and used those norms and the variance in nutrient concentrations among high-yield fields to establish leaf nutrient optimum ranges. The objective of the current study was to apply DRIS analysis to data collected in a survey of commercial lettuce fields to establish whole leaf macro- and micronutrient optimum ranges of broad applicability to the California lettuce industry.
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