Growth Rate of Lettuce: Implications for Nitrogen Fertilization

Lettuce (Lactuca sativa L.) matures in 65 to 70 days in summer and typically takes up 100-120 lbs of nitrogen (N) in the above ground biomass. Grower N fertilization programs vary widely but average from 150 to over 200 lbs of N/A. In 2008 three trials on head and romaine lettuce were conducted to compare best management practices (BMP) and standard growing practices in commercial fields in the Salinas Valley, CA, USA. In the BMP treatments irrigations were scheduled from estimated consumptive water use for lettuce which was based on CIMIS evapotranspiration data and the water holding capacity of the soil; nitrogen fertilizer applications were determined by measuring soil residual nitrate-N using the soil nitrate quick test. Standard treatments were based on the grower’s practices. Suction lysimeters were installed to capture leachate at two feet soil depth. Suction was maintained at 10-20 cbar and leachate was collected during irrigation events and total nitrate leaching was calculated based on estimates of water movement through the soil. Soil mineral N, crop biomass and biomass N were evaluated several times during the crop cycle, and yield evaluations were conducted.  Total N uptake by lettuce during the first 30 days is less than 6 lbs of N per acre. A high proportion of total seasonal applied water is used during germination and establishment of the crop. Given low N uptake by lettuce in the first 30 days, prudent N and water management are needed to reduce nitrate-N leaching during the early phase of the growth cycle.  Following thinning (app. 30 days after planting) lettuce grows exponentially and the rates of nitrogen uptake increase substantially. Using N biomass data normalized for growing degrees days (GDD, using 4 and 30 C threshold temperatures) indicated that, for a site in the northern Salinas Valley with a summer temperature regime that averages 21 GDD per day, on average lettuce take up 3.3 lbs of N/A/day. Lettuce grown in the southern Salinas Valley with higher GDD would on average have N uptake > 4.0 lbs of N/A/day. The post thinning period is the most critical period for fertilizing lettuce. In the BMP treatment, by accounting for residual soil nitrate-N and careful water management, we reduced N fertilizer application by 66 lbs/A as well as nitrate leaching by 22.1 lbs/A in two irrigation events. BMP yields equaled the standard in two of three trials.