Nutrient Management Research Database

As concern over NO3-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·kgL1 residual soil NO3-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·haL1 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·haL1 (73 kg·haL1 vs. 150 kg·haL1 for the grower N regime) with no reduction in fresh biomass produced and only a slight reduction in crop N uptake (151 kg·haL1 vs. 156 kg·haL1 for the grower N regime). Similarly, an average seasonal N rate reduction of 88 kg·haL1 (96 kg·haL1 vs. 184 kg·haL1 ) was achieved in the replicated commercial trials with no biomass reduction. Seasonal N rates between 111 and 192 kg·haL1 maximized fresh biomass in the research farm trials, which were conducted in fields with lower residual soil NO3-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·haL1 ·dL1 . N uptake then increased linearly until harvest (’9 weeks after planting), averaging ’4 kg·haL1 ·dL1 over that period. Whole plant critical N concentration (Nc, the minimum whole plant N concentration required to maximize growth) was estimated by the equation Nc (g·kgL1 ) = 42 L 2.8 dry mass (DM, Mg·haL1 ); on that basis, critical N uptake (crop N uptake required to maintain whole plant N above Nc) in the commercial fields averaged 116 kg·haL1 compared with the mean uptake of 145 kg·haL1 with the grower N regime. Soil NO3-N greater than 20 mg·kgL1 was a reliable indicator that N application could be reduced or delayed. Neither leaf N nor midrib NO3-N was correlated with concurrently measured soil NO3-N and therefore of limited value in directing in-season N fertilization.

• More than 50% of U.S. lettuce production occurs in California’s Salinas Valley.
• Multiple yearly crop cycles and heavy N fertilization, often significantly greater than plant uptake, has resulted in groundwater nitrate pollution that often exceeds drinking water standards.
• Seasonal lettuce N requirements vary widely due to differences in precipitation, irrigation efficiency, residual soil nitrate, and soil N mineralization rates.
• This study investigated the effects of changes to standard fertilization practices on lettuce nutrient uptake and yield.
• Preplant soil nitrate testing was used to identify the potential for reduced N fertilization rates.
• The usefulness of lettuce tissue analysis for in season nitrate availability was also assessed.

• Reduced N treatments did not result in reduced lettuce yields for fields that had preplant nitrate levels greater than 20mg/kg, supporting the use of preplant soil nitrate testing to identify potential reductions in N fertilization. Marketable yields in the reduced plots averaged 41 Mg/ha while yields in the non-reduced plots averaged 40.8 Mg/ha.
• Reduced N treatments averaging between 73-102 kg N/ha were sufficient to obtain yields equivalent to the higher N fertilization rates. The lower required N fertilization rate was largely the result of high preplant soil nitrate levels.
• Seasonal N uptake from lettuce cropping systems averaged 145 kg/ha with uptake in the latter half of the season averaging 4 kg N/ha/day
• Neither leaf nor midrib nitrate concentration was related to soil nitrate availability, suggesting that these tests are not useful in determining in-season N fertilization rates.
• These findings suggest that the so-called “action threshold” of 20mg/kg soil serves as a useful yardstick for planning seasonal N fertilization.
• In addition to reduced fertilizer costs, minimizing excess N application by application of this metric helps to reduce groundwater nitrate pollution.