Nitrogen Requirements for Lettuce Production

As California vegetable growers face increasing regulation to protect environmental water quality there is a need to reevaluate established N fertilization practices. Growth and N uptake were studied in lettuce (Lactuca sativa L.) grown in the temperate coastal valleys of central California. Growth and N uptake in 15 commercial iceberg and romaine lettuce fields were monitored; all fields had >20 mg kg^-1 residual soil NO­₃-N at the time of first N sidedressing, a common phenomenon in this production area. Two levels of N fertilization were established in each field by eliminating or reducing the cooperating growers’ first sidedress application in a portion of the field. Whole plant samples were collected every 7 to10 d for determination of above-ground biomass and N content. Seasonal N fertilization by the growers averaged 153 kg ha^-1, while the reduced N plots averaged 76 kg ha^-1. Total fresh and dry biomass at commercial harvest averaged 99 and 4.5 Mg ha^-1, respectively, with no significant difference between the grower and reduced N regimes. Crop N uptake and biomass accumulation was minimal up to the rosette stage of growth (approximately 4 weeks after planting), and then increased linearly until harvest (approximately 9 weeks after planting).  Mean dry biomass accumulation during the rapid growth phase was 110 to 130 kg ha^-1 d^-1 and mean N uptake was 4 to 5 kg ha^-1 d^-1 at the summer temperatures prevalent in this production region.  Biomass N at harvest averaged 154 and 146 kg ha^-1 in the grower and reduced N regimes, respectively; the small difference indicated highly inefficient crop utilization of the first sidedress N application. Lettuce showed modest seasonal N uptake; when significant residual soil N is present, limited N fertilization is necessary.