Trials were conducted in 15 commercial fields in the central coast region of California in 1999 and 2000 to evaluate the use of presidedress soil nitrate testing (PSNT) to determine sidedress N requirements for production of iceberg and romaine lettuce (Lactuca sativa L.). In each field a large plot (0.2-1.2 ha) was established in which sidedress N application was based on presidedress soil NO3-N concentration. Prior to each sidedress N application scheduled by the cooperating growers, a composite soil sample (top 30 cm) was collected and analyzed for NO3-N. No fertilizer was applied in the PSNT plot at that sidedressing if NO3-N was >20 mg·kg-1; if NO3-N was lower than that threshold, only enough N was applied to increase soil available N to ≈20 mg·kg-1. The productivity and N status of PSNT plots were compared to adjacent plots receiving the growers' standard N fertilization. Cooperating growers applied a seasonal average of 257 kg·ha-1 N, including one to three sidedressings containing 194 kg·ha-1 N. Sidedressing based on PSNT decreased total seasonal and sidedress N application by an average of 43% and 57%, respectively. The majority of the N savings achieved with PSNT occurred at the first sidedressing. There was no significant difference between PSNT and grower N management across fields in lettuce yield or postharvest quality, and only small differences in crop N uptake. At harvest, PSNT plots had on average 8 mg·kg-1 lower residual NO3-N in the top 90 cm of soil than the grower fertilization rate plots, indicating a substantial reduction in subsequent NO3-N leaching hazard. We conclude that PSNT is a reliable management tool that can substantially reduce unnecessary N fertilization in lettuce production.

Trials in nine commercial celery (Apium graveolens L.) fields were conducted between 1997-99 to evaluate grower drip irrigation management practices and their effects on yield and quality. Surface drip irrigation tapes with flow rates higher and lower than the grower-installed tapes were spliced into the field system; as the cooperating growers irrigated and applied N fertigation according to their routine practices these drip tapes delivered either more or less water and N than the field drip system. Total grower water application during the drip-irrigated portion of the season ranged from 85% to 414% of seasonal reference evapotranspiration (ETo). Water volume per irrigation varied among fields from 1.8 to 3.8 cm, with irrigation frequency varying from an average of every other day to once a week. Grower management of drip irrigation was not consistently successful in maintaining soil water tension (SWT) in a desirable range. SWT was often below -30 kPa, and in some cases below -70 kPa. These transient stresses were more often a result of inappropriate irrigation frequency than applied water volume. In four of the fields plots receiving less water than that delivered by the field system produced equivalent marketable yield and quality, indicating a significant potential for water savings. An economically important incidence of petiole pithiness (collapse of parenchyma tissue) was observed in four fields. Infrequent irrigation under high ETo summer conditions, rather than irrigation volume applied, appeared to be the major factor in pith development. N fertigation amount and crop N status appeared to be unrelated to pithiness severity. We conclude that celery drip irrigation management could be substantially improved by maintaining a closer proportionality between irrigation and crop evapotranspiration (ETc), increasing irrigation frequency, and reducing volume per irrigation.

High rates of N fertilization of cool-season vegetables has contributed to NO3-N pollution of groundwater in the Salinas Valley of central California. Ten field demonstrations were conducted in 1999 to document the utility of presidedress soil NO3-N testing in maximizing N fertilizer efficiency in iceberg lettuce (Lactuca sativa L.). In each demonstration, a plot 36 beds wide × the entire field length was established in a commercial lettuce field. The cooperating growers applied 1 to 3 N sidedressings in these fields. Before each sidedressing the soil NO3-N concentration in the top 30 cm of the plot was determined by an on-farm quick test technique. If NO3-N was >20 mg·kg-1, no N was applied at that sidedressing; for NO3-N <20 mg·kg-1, ≈4 kg N/ha was applied for each milligram per kilogram below the 20 mg·kg-1 threshold. Plot yields, harvested by commercial crews, were compared to the yield of adjacent areas of the field that received the growers' full sidedress N regime. Across fields, seasonal sidedress N application in the PSNT plots averaged N only at 86 kg·ha-1, almost 60% less than the average N (212 kg·ha-1) applied by the growers. Yields in the PSNT plots averaged 1824 boxes/ha, compared with 1829 boxes/ha in the companion field plots. Whole leaf N concentration at heading was above published sufficiency standards in all PSNT plots. Evaluation of heads after 10 days of storage at 5 °C showed that sidedress N application rate did not affect visual quality, decay, or midrib discoloration. We conclude that PSNT can reliably be used to minimize wasteful sidedress N applications in lettuce.