Survey sites.

Eleven commercial field sites in the Salinas Valley (Monterey County) and in the San Juan Valley (San Benito County) of California were evaluated in 2011 for spinach growth and N uptake (Table 1). Eight sites were mechanically harvested for fresh market clipped products: baby (2- to 3.5-inch leaf length) and teenage (3.5- to 4.5-inch leaf length); and three hand harvested sites for fresh market bunch. All fields were sprinkler irrigated over the growth cycle. Spinach was planted in 18–32 seed lines on 80-inch-wide beds at a density of 2–3.5 million seeds/acre. Biomass samples were collected one to five times during the crop cycle from each site; samples were collected from a 15-ft² area and replicated three to four times. The entire plant was collected (including crown and cotyledons) by under cutting the spinach with knives, which typically left about 0.25 inch of hypocotyl on the plants. If wet soil adhered to plant tissue, the spinach was washed and blot dried before weighing. Crop residue left in fields following mechanical or hand harvest was measured using the same area and methods described above.

Table 1.Site and production information for commercial spinach fields surveyed in 2011 in the Salinas Valley and San Juan Valley of California. First-cropped fields (first) indicate that the crop was the first of the production season after winter fallow. Second- and third-cropped fields (second and third) followed a previous vegetable crop. All nitrogen (N) applications were applied by the grower.

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Soil samples for mineral N were collected from 0- to 12-inch depth on the same day as the biomass samples. Field moist soil was added to a 2 M potassium chloride (KCl) solution at a ratio of 1 g soil to 5 mL of KCl. The clear supernatant was extracted and the solution was analyzed by the University of California Davis Analytical Laboratory [UCDAL (Davis, CA)] for NO₃ (Knepel, 2003) and ammonium [NH₄ (Hofer, 2003)]. Dried plant samples were ground using a Wiley mill to pass a 40-mesh screen (420 μm) and the samples were sent to UCDAL for total N analysis by combustion (AOAC International, 2006) and selected samples were also analyzed for P and K (Meyer and Keliher, 1992; Sah and Miller, 1992). Soil texture was determined using the hydrometer method (Bouyoucos, 1962).

Fertilizer trials.

Four fertilizer trials were conducted in commercial fields in Spring through Fall 2011: three in the Salinas Valley and one in the San Juan Valley. The effect of at-planting N rates on yield was evaluated for three of the trials (trials 1, 2, and 4), while trial 3 received only a midseason topdress fertilizer application. Each trial consisted of a grower standard (Std) practice and four fertilizer treatments: ammonium sulfate applied at a rate of 0, 20, 40, 80 lb/acre N, (AS0, AS20, AS40, and AS80, respectively). Ammonium sulfate was chosen as the N source because the positively charged NH₄ ion would initially be less susceptible to leaching losses with the germination water. The sulfur content of this fertilizer was not considered a confounding factor because soil and irrigation water sulfate content are sufficient for optimal crop growth in the intensive vegetable production fields of central California. Details for each replicated fertility trial site are given in Tables 1, 2, and 3. For trials 1, 2, and 4, fertilizer (15N–3.5P–3.3K at 80 lb/acre N) was sprayed over top of the Std treatment with a commercial rig at planting; the 15% N in this material was composed of 3.0% NO₃, 5.5% NH₄, and 6.5% urea. Each plot was one 80-inch-bed wide by 20-ft long and replicated four times (three times for trial 3) in a randomized complete block design. All fields were sprinkler irrigated throughout the growth cycle.

Table 2.Soil characteristics of replicated nitrogen (N) fertilizer trials on spinach in the Salinas Valley and San Juan Valley of California.

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Table 3.Nitrogen (N) fertilizer applied to replicate trials in commercial spinach fields in the Salinas Valley and San Juan Valley of California. For trials 1, 2, and 4, four rates of ammonium sulfate were applied at-planting at 0, 20, 40, 80 lb/acre N (AS0, AS20, AS40, and AS80, respectively); midseason applications were grower applied at the rates indicated 2–4 weeks after planting. In trial 3, no at-planting N was applied and all treatments were applied 2 weeks after planting. All trials included a grower standard (Std) for comparison.

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Applied water was measured using 6-inch-diameter propeller flow meters (McPropeller, McCrometer, Hemet, CA) positioned on the main irrigation lines. Canopy cover was estimated weekly from overhead photos taken at a 12-ft height using a digital infrared camera (ADC; Tetracam, Chatsworth, CA). Crop ET (ETc) losses for spinach were estimated using daily reference ET data collected from the nearest California Irrigation Management Information System weather station multiplied by a crop coefficient (Kc). The Kc was estimated using the canopy cover data and procedures described by Gallardo et al., (1996) for lettuce (Lactuca sativa) where soil evaporation and crop transpiration were calculated separately.

Soil NO₃ and NH₄ were measured at weekly intervals from the 0- to 12-inch depth. Field moist soil was extracted with a 2 M KCl solution and analyzed as previously described for NO₃ and NH₄. Biomass and tissue N evaluations were made two to three times during the growth cycle from a 5.4-ft² area in each plot and plant samples were collected and processed as described above.

Root and NO₃ distribution.

A root pit (32-inch wide by 24-inch deep) was excavated at four sites (fertilizer trials 1, 3, and 4 and survey site 4) to evaluate rooting depth and density, and NO₃ distribution in the soil profile. A smooth face was prepared on the excavated area and then a hand-pressurized sprayer (model no. 418–1L; Solo, Newport News, VA) was used to wash soil from the roots. A grid with 48 4 × 4-inch sections was placed over the exposed soil and roots in each square were counted. Soil samples were collected from each grid and analyzed for NO₃.

Data analysis and statistics.

Yield data were subjected to ANOVA in SAS (version 9.2; SAS Institute, Cary, NC) using the GLM procedure. Mean separation was determined using Fisher’s least significant difference test (P < 0.05). Linear regression analysis was performed using SigmaPlot (version 12.0; Systat Software, San Jose, CA).

Nutrient uptake.

Mean N, P, and K uptake over a range of soil and climatic conditions during the 2011 production season (March–September) is shown in Table 4. The average days after germination water (DAGW) to harvest were 30 but varied by the time of year and the spinach product grown (Table 4). Spinach N uptake was 0.45 lb/acre per day (7.0 lb/acre) in the first 2 weeks of the crop cycle (including the germination period) and increased to 4.3 lb/acre per day [R² = 0.67 (Fig. 1A)] in the 2 weeks before harvest. However, N uptake was even greater the week preceding harvest; during this part of the growth cycle mean N uptake was 7.3 lb/acre per day and was as high as 10.0 lb/acre per day for several fields. Graphing N uptake as a function of growing degree-days did not improve the coefficient of determination (data not shown).

Table 4.Harvest characteristics and total nitrogen (N), phosphorus (P), and potassium (K) uptake, and N in residue and soil for commercial spinach fields sampled in 2011 in the Salinas Valley and San Juan Valley of California.

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More than 90% of total N uptake occurred from ≈2 weeks after the germination water until harvest. At harvest, N uptake for baby, teenage, and bunch spinach averaged 74, 94, and 120 lb/acre N, respectively (Table 4). The N uptake for bunch spinach given here is likely an underestimation. Bunch fields were usually sampled on the first day of harvest, but bunch harvest may occur over a period of up to 1 week depending on field size and market demand. Additional days of growth could significantly increase N uptake.

The pattern of K uptake is similar to that for N (Fig. 1B), with a low K demand in the first 2 weeks of the cropping cycle (0.52 lb/acre per day) followed by a high K uptake rate (7.8 lb/acre per day) during the 2 weeks before harvest. At harvest, K uptake for baby, teenage, and bunch spinach were 76, 124, and 196 lb/acre, respectively (Table 4) indicating a higher demand for K than for N. The demand for P was also low in the first 2 weeks with 0.05 lb/acre per day removed by the crop. This was followed by a P uptake of 0.6 lb/acre per day (Fig. 1C). At harvest, P uptake for baby, teenage, and bunch spinach was 6, 9, and 14 lb/acre, respectively (Table 4).

N fertilizer trials.

Fertilizer trials 1 and 2 were first-cropped fields that were fallowed the preceding winter. Due to wet winter and spring conditions in 2010–11, initial soil mineral N concentrations were low [<10 mg·kg⁻¹ NO₃-N, <1.2 mg·kg⁻¹ NH₄-N (Fig. 2A and B)]. In trial 1, following at-planting fertilizer applications, soil mineral N levels increased above 20 mg·kg⁻¹ for treatments receiving ≥40 lb/acre ammonium sulfate, and remained above 20 mg·kg⁻¹ for the rest of the crop cycle for the two highest fertilizer treatments (Fig. 2A). This is at or above the 20–25 mg·kg⁻¹ NO₃-N threshold for N fertilizer response established for cool season vegetables in this region (Breschini and Hartz, 2002; Hartz et al., 2000). After the first biomass evaluation in trial 1, there was not a statistical difference in biomass for treatments that received at-planting applications of ≥40 lb/acre N (Fig. 3A). At harvest, there was not a statistical difference in tissue N content or N uptake between the three highest fertilizer treatments [AS40, AS80, and Std (data not shown)].

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In trial 2 at-planting additions of ammonium sulfate did not increase soil mineral N concentrations as greatly as in trial 1 (Fig. 2A and B). Soil mineral N concentrations were at or above the 20 mg·kg⁻¹ NO₃-N threshold following the at-planting N addition in the high rate fertilizer treatments (AS80 and Std) only. Although soil mineral N concentrations were below the 20 mg·kg⁻¹ threshold during most of the growing season for the lower rate fertilizer treatments (AS20 and AS40), there was no difference in biomass accumulation in the first 18 d of growth for all fertilizer treatments (Fig. 3B). As in trial 1, there was not a statistical difference in biomass for treatments that received at-planting applications of ≥40 lb/acre N (Fig. 3B). Also at harvest, there was not a statistical difference in tissue N content or N uptake between the three highest fertilizer treatments [AS40, AS80, and Std (data not shown)]. The lower yield at harvest in trial 2 compared with trial 1 is the result of harvesting the crop as a baby spinach product.

Trials 3 and 4 were second- and third-cropped fields, respectively, and there was no yield response to any of the fertilizer treatments (Fig. 3C and D) or difference in tissue N or N uptake among the treatments (results not shown). The at-planting residual soil NO₃ concentrations in Trial 3 were 27 mg·kg⁻¹ and the grower decided to not make at-planting fertilizer N applications. Fertilizer N treatments were applied 2 weeks after planting (Table 3). Soil NO₃ levels remained above 20 mg·kg⁻¹ until harvest for the three highest fertilizer treatments (Fig. 2C) and no yield response was observed in any of the fertilizer treatments (Fig. 3C). High residual soil NO₃ levels were probably the result of residual fertilizer and N mineralization from prior crop residues.

In trial 4, at-planting residual soil NO₃ concentrations were >20 mg·kg⁻¹ N (Fig. 2D), but the grower decided to apply at-planting fertilizer because he was concerned that much of this NO₃ would leach when the germination water was applied to the coarse-textured sandy loam soil. Before the midseason fertilizer application 15 DAGW, all treatments were at or well above the 20 mg·kg⁻¹ soil NO₃ threshold. The midseason fertilizer application of 86 lb/acre N increased mineral N to high levels (Fig. 2D), which remained high until harvest. As in trial 3, there was no statistical difference in biomass (Fig. 3D), tissue N, or N uptake among all treatments (data not shown).

Irrigation and ETc are shown in Table 5. Reference ET averaged 0.15 inches/d across all trial sites and ranged from 0.11 to 0.22 inches/d. The Kc for spinach during germination averaged 0.53 and increased to 0.85 by harvest. Growers applied water equal to 399% of ETc during germination, typically in three irrigations over a 6-d period. Growers applied an average of 122% of ETc after germination to harvest. On average, 5 inches of water was applied from seeding to harvest which was 192% of ETc.

Table 5.Applied irrigation water and crop evapotranspiration (ETc) for fertilizer trials at field sites in commercial spinach fields sampled in 2011 in the Salinas Valley and San Juan Valley of California.

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Grower N fertilizer rates and timing.

Grower applied at-planting fertilizer applications averaged 66 lb/acre N, with first-cropped fields receiving slightly more (Table 1). The average date for the first midseason fertilizer application was 16 DAGW. Although the growers did a good job synchronizing this fertilizer application with the rapid increase in spinach N uptake that began around 14 d (Fig. 1A), most growers did not considered soil NO₃ concentrations when determining the fertilizer rate.

Only survey site 5 had more N in the biomass than was applied as fertilizer (Fig. 4). The grower at this site eliminated the at-planting fertilizer application based on a soil test that indicated sufficient residual soil NO₃ for optimal crop growth. For the other 14 sites, growers applied an average of 111% more N than was taken up in aboveground biomass at harvest. As a result of these high fertilizer applications, residual mineral N concentrations were often moderate to high at harvest, especially for second-cropped spinach (Table 4). After harvest, soil NO₃ concentrations are assumed to increase further because 41% of the total biomass N remains in the field after harvest (Table 4). This residue mineralizes rapidly when incorporated into the soil because the tissue is succulent (i.e., low lignin content) and has a high N content (about 5% N).

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Root and NO₃ distribution.

Spinach, when grown as a clipped product, has a shallow root system. At harvest (≈30 DAGW), 95% of all roots were located in the surface16 inches (Table 6). Three of the four soil pits had a restrictive layer, due to a plow pan or textural change that may have prevented deeper root penetration. However, 96% of the roots recorded in trial 3, which did not have a restrictive layer, were also found in the surface16 inches, which indicates that even without an impeding layer, few roots extend below 16 inches. These results are similar to Schenk et al., (1991) where at harvest (6-week old plants) 80% and 15% of total spinach root length were located in the top 0–6 and 6–12 inches of soil, respectively.

Table 6.Root density distribution in six soil depths at harvest for three fertilizer trials and one survey site in commercial spinach fields sampled in 2011 in the Salinas Valley and San Juan Valley of California.

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At harvest, all sites showed increasing levels of soil NO₃ below 12 inches (Table 7). This NO₃ was unused by the spinach crop and would be at risk to NO₃ leaching with the application of irrigation or rainfall amounts that are beyond the ability of the soil to retain the moisture.

Table 7.Nitrate-nitrogen (N) distribution in soil profile at harvest for three fertilizer trials and one survey site in commercial spinach fields sampled in 2011 in the Salinas Valley and San Juan Valley of California.

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Implications for N fertilizer management of fresh market spinach.

Two to three vegetable crops are grown per year in the coastal vegetable production districts of California. This intensive production can result in high end-of-season residual NO₃ concentrations, which increases the NO₃ leaching hazard in the region. Over the growing season, NO₃ levels in the soil can build up due to a combination of unused fertilizer and mineralization of crop residue and soil organic matter (Fig. 2C and D, Table 4).

These evaluations show that soil NO₃ testing can be used to improve the NUE of spinach. Soil testing would be most effective in spinach production at two critical points: 1) at-planting and before the midseason fertilizer application when N use by spinach greatly increases (Fig. 1A). Soil NO₃ concentrations >20 mg·kg⁻¹ NO₃-N were shown to be sufficient for lettuce grown in central California (Breschini and Hartz, 2002; Hartz et al., 2000), and the results from these evaluations show that this threshold is applicable to spinach.

For first-cropped fields with low residual soil NO₃ levels, an at-planting fertilizer application was necessary to achieve maximum yields. Although N uptake in the first 2 weeks of the cropping cycle was low (7.0 lb/acre N), 40 lb/acre fertilizer N was necessary to build soil NO₃ levels to ≥20 ppm (Fig. 2A and B) and to achieve maximum yields (Fig. 3A and B). These results indicate that spinach requires high soil NO₃ levels early in the crop cycle, possibly to overcome intercrop competition in the high-density plantings. In addition, germination water in these studies was applied in excess of ETc to establish the crop (Table 5), which may leach soil NO₃ beyond the reach of spinach roots. The choice of fertilizers is also important to prevent loss of at-planting fertilizer. NH₄ or NH₄-producing fertilizers may be a better choice than fertilizers containing a significant portion of NO₃-N because the positively charged NH₄ ion is much less mobile in soil than NO₃ and would be less likely to leach with the germination water.

The N management strategy for second-cropped fields was different from first-cropped fields. Residual soil NO₃ remaining after harvest of the prior crop as well as N mineralizing from incorporated crop residues provided a substantial portion of the subsequent spinach crop’s N requirements. Therefore, at-planting and possibly even midseason fertilizer applications for second-cropped fields could be reduced or, in some cases, eliminated without suffering a yield loss (Fig. 3C and D).

If residual soil NO₃ levels are not accounted for and fertilizer is applied in excess of crop requirements, a large pool of NO₃ may be left in the soil after harvest (Fig. 2C and D). Although any residual soil NO₃ left by spinach after harvest may be used by a subsequent vegetable crop, it could also be at risk of loss to leaching during germination of the subsequent crop or during the winter fallow.

N management is incomplete without efficiently managing irrigations to minimize NO₃ leaching below the crop’s root zone. Fresh market spinach has a shallow root system (Table 6) and appears to be unable to effectively use NO₃ deeper than 16 inches. Therefore, to maximize the NUE of spinach, NO₃ movement below the root zone should be minimized by closely matching irrigations with crop water requirements.

Reducing N use in spinach will be a challenge. The crop has a shallow root system, a high N demand, which occurs over a short period, and strict quality standards for a deep green color that tends to encourage N applications beyond the agronomic requirements to maximize yield. Addressing these challenges will require that fertilizer N is applied at the optimal time and rate based on crop uptake and soil NO₃ testing, and that irrigation water is efficiently applied to minimize leaching and runoff.

Units

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