Nitrogen Requirements and N Status Determination of Lettuce

As concern over NO 3 -N pollution of groundwater increases, California lettuce growers are under pressure to improve nitrogen (N) fertilizer efficiency. Crop growth, N uptake, and the value of soil and plant N diagnostic measures were evaluated in 24 iceberg and romaine lettuce ( Lactuca sativa L. var. capitata L., and longifolia Lam., respectively) field trials from 2007 to 2010. The reliability of presidedressing soil nitrate testing (PSNT) to identify fields in which N application could be reduced or eliminated was evaluated in 16 non-replicated strip trials and five replicated trials on commercial farms. All commercial field sites had greater than 20 mg·kg −1 residual soil NO 3 -N at the time of the first in-season N application. In the strip trials, plots in which the cooperating growers’ initial sidedress N application was eliminated or reduced were compared with the growers’ standard N fertilization program. In the replicated trials, the growers’ N regime was compared with treatments in which one or more N fertigation through drip irrigation was eliminated. Additionally, seasonal N rates from 11 to 336 kg·ha −1 were compared in three replicated drip-irrigated research farm trials. Seasonal N application in the strip trials was reduced by an average of 77 kg·ha −1 (73 kg·ha −1 vs. 150 kg·ha −1 for the grower N regime) with no reduction in fresh biomass produced and only a slight reduction in crop N uptake (151 kg·ha −1 vs. 156 kg·ha −1 for the grower N regime). Similarly, an average seasonal N rate reduction of 88 kg·ha −1 (96 kg·ha −1 vs. 184 kg·ha −1 ) was achieved in the replicated commercial trials with no biomass reduction. Seasonal N rates between 111 and 192 kg·ha −1 maximized fresh biomass in the research farm trials, which were conducted in fields with lower residual soil NO 3 -N than the commercial trials. Across fields, lettuce N uptake was slow in the first 4 weeks after planting, averaging less than 0.5 kg·ha −1 ·d −1 . N uptake then increased linearly until harvest (≈9 weeks after planting), averaging ≈4 kg·ha −1 ·d −1 over that period. Whole plant critical N concentration (N c , the minimum whole plant N concentration required to maximize growth) was estimated by the equation N c (g·kg −1 ) = 42 − 2.8 dry mass (DM, Mg·ha −1 ); on that basis, critical N uptake (crop N uptake required to maintain whole plant N above N c ) in the commercial fields averaged 116 kg·ha −1 compared with the mean uptake of 145 kg·ha −1 with the grower N regime. Soil NO 3 -N greater than 20 mg·kg −1 was a reliable indicator that N application could be reduced or delayed. Neither leaf N nor midrib NO 3 -N was correlated with concurrently measured soil NO 3 -N and therefore of limited value in directing in-season N fertilization.