In California, fresh-market spinach is grown throughout the year and accounts for 59% of the total U.S. fresh market production [U.S. Department of Agriculture (USDA), 2012]. In 2010, there were 26,900 acres of fresh market spinach grown in California (USDA, 2011). Spinach is marketed as three commodities: 1) fresh market clipped and bagged (clipped), 2) fresh market bunched, and 3) frozen. Clipped spinach is the dominant spinach produced in California and is sold as either small, young leaves (baby) or slightly older, medium-sized leaves (teenage). Clipped spinach as well as a large proportion of bunched product is grown on 80-inch-wide beds planted at 1.5–4.0 million seeds/acre. Occasionally, the clipped spinach fields are regrown for harvest for the freezer industry.
Spinach is a high-value crop that requires sufficient N fertilizer and irrigation to ensure optimal growth and to meet high quality criteria (e.g., deep green leaf color). These production practices combined with a shallow root system (Schenk et al., 1991) and short production cycle (typically 30 d) increase the potential of nitrate (NO3) leaching. Water quality monitoring in coastal vegetable production areas has shown widespread incidences of NO3 levels that exceed the Federal Drinking Water standard of 10 mg·L−1 NO3-N (Harter and Lund, 2012). As a result, growers have come under increasing regulatory pressure to improve crop NUE and thereby minimize NO3 losses from production fields.
Producers can improve NUE by applying fertilizer at the optimal time and rate to match crop N uptake. However, the data necessary to make these fertilizer decisions for spinach is lacking. A few studies have reported N uptake of spinach (Canali et al., 2008, 2011), but no studies have evaluated high-density planting of clipped or bunched spinach grown on 80-inch beds. This study was undertaken to provide data on the N uptake characteristics of spinach and evaluate ways to improve N fertilizer management.
To maximize the NUE of spinach production, it is necessary to account for the levels of residual soil NO3 and adjust fertilizer rates accordingly. In coastal California, two or three crops are grown during the yearly production cycle. Higher N fertilization rates are usually needed in first-cropped fields in spring because of low NO3 concentrations following leaching in wet winters and also because of cool soil conditions that limit soil N mineralization from soil organic matter. Later in the growing season, fertilizer N applications can often be less in the second or third crop than the first crop because residual fertilizer N carryover and increased N mineralization of prior crop residues and soil organic matter increase the pool of soil NO3 available to the subsequent crop (Breschini and Hartz, 2002; Jackson et al., 1994).
Finally, little information is available concerning the water requirements of spinach under the common production systems used on the central coast of California. High-density plantings of spinach for clipped product are irrigated with overhead sprinklers. Spinach yield and quality is sensitive to deficits in soil moisture (Thompson and Doerge, 1995), and consequently growers may apply more water than the evapotranspiration (ET) demand of the crop. The combination of high soil NO3 levels combined with drainage from over-irrigating would presumably result in leaching of soil NO3-N.
The objectives of this study were to 1) understand the nutrient uptake characteristics for fresh market spinach grown at high density on 80-inch-wide beds, 2) evaluate how fertilizer application rates and timing influence the growth and N uptake of spinach in first- and second-cropped fields with low and high residual soil NO3 concentrations, 3) evaluate water applications to production fields relative to crop ET, and 4) provide N management strategies to increase the NUE of spinach. To meet these objectives, grower fertilizer programs were evaluated and spinach N uptake was measured for sites over an entire production season with a range of soil and climatic conditions, and cropping histories. In addition, four replicated fertilizer trials were conducted on first- and second-cropped fields.
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