Nutrient Management Research Database

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.

• 15 trials were implemented in commercial lettuce fields in the central coast region of CA in 1999 and 2000
• Fields were direct seeded from March-July for harvest in June-September, and a standard planting of 2 rows per 1m raised bed was used
• A single plot was designated in the center of each field where sidedress N application was based on presidedress soil N03-N test level
• Composite soil samples of the top 30cm were collected in both treatment plots and plots with the grower's standard N application
• Is soil N0₃-N was greater than 20 mg kg^-1, no sidedress application was made, and if the soil N0₃-N was lower, N was applied to bring the soil up to approximately 20 mg kg^-1
• At harvest the effect of treatment on postharvest quality, biomass, percentage of plants that were marketable and marketable yield were assessed
• Soil sampling to determine change in soil profile NO₃-N was also evaluated

• Cooperating growers applied an average of 257 kg ha^-1 N including 1 to 3 sidedressings.
• Total N applied  was 43% lower in presidedress soil nitrate testing (PSNT) treatments,  with a reduction in sidedress N of 57%
• In 11 of 15 trials in the PSNT treatments, the first sidedressing was eliminated and 2 fields received no sidedress N at all.
• There was no significant difference in marketable yield between PSNT and treatments
• Tissue N levels in both growers and PSNT plots were generally above sufficiency standards and there were no N treatment effects on postharvest quality 
• From the first sidedressing to harvest, PSNT plots showed a marked decrease in NO_3-N in the upper 30cm of soil (>12 mg kg^-1)
• Grower plots showed an increase in soil N0_3-N in the 30-90cm soil depth and PSNT plots showed a decline

Iceberg and romaine lettuce varieties were used