Nutrient Management Research Database

A series of 11 replicated trials in commercial iceberg lettuce (Lactuca sativa L.) fields conducted in 1996-97 showed that sidedressing could be delayed with no loss of crop productivity or quality as long as residual soil NO3-N in the top foot of soil exceeded 20 ppm at the time of initial sidedress N application. Ten non-replicated field demonstrations were conducted in 1999 on commercial farms in the Salinas Valley to determine the reliability of the pre-sidedress soil nitrate testing (PSNT) approach in determining sidedress N requirements of lettuce on a field specific basis, regardless of initial residual soil NO3-N concentration. The majority of each field received the growers’ standard N management program. A 36 row-wide plot the full length of the field was established at each site, in which sidedress N was applied based on residual
nitrate-nitrogen (NO3-N) in the top foot of soil prior to each sidedress application the grower made. In these PSNT plots no sidedress N was applied as long as soil NO3-N was > 20 ppm; whenever soil NO3-N was < 20 ppm the N application rate at that sidedressing was calibrated to raise soil NO3-N up to the 20 ppm threshold. Both the PSNT plots and adjacent portions of the field receiving the grower standard N treatment were harvested by commercial crews. The cooperating growers applied an average of 254 lb N/acre, 194 lb/acre of which was applied as sidedress or water-run. Following the PSNT approach reduced seasonal N application by 44%, to an average of 142 lb/acre; sidedress N application in PSNT plots averaged only 81 lb/acre. Evaluations made after 10-14 days of cold storage showed that N treatment had no effect on postharvest quality. Plant N monitoring showed that all plots remained above established tissue
critical levels throughout the season. Less than 10% of the N applied by the growers above that applied in the PSNT plots was even taken up by the crop; soil sampling showed that the majority of this extra fertilizer N remained, in NO3-N form, in the soil profile after harvest. We conclude that the use of PSNT is a reliable technique that can dramatically improve N use efficiency and reduce NO3-N pollution potential.

• N fertilization trials were conducted in 10 commercial lettuce fields in the Salinas valley
• 2 N management practices were conducted:
  • the majority of each field received the cooperating growers' normal N
  • Pres sidedress nitrate plots were created where no N was applied as long as soil N0₃-N in the top foot of soil was > 20 ppm.  If N0₃-N was <20ppm, N was sidedressed to bring soil N0₃-N up to 20ppm.
• N status of both soil and crop was monitored throughout the season and potential contribution of soil organic N to mineral N supply was assessed in the laboratory
• All fields were harvested by commercial crews
• The effect of N on postharvest quality was evaluated on 48 heads of lettuce per treatment

• The Pre-sidedress nitrate testing (PSNT) approach reduced seasonal N application by 44%, much of the reduction coming at the first sidedress
• Despite difference in N application, crop N status was similar between N treatments at all sites
• Ave. commercial yield was similar between PSNT and grower treatments
• At harvest, grower plots had about 60lb/acre more NO₃-N in the top 3 feet of soil than PSNT plots, representing the possibility of increased leaching

All fields were commercial iceberg lettuce (Lactuca sativa L.)