Nutrient Management Research Database

http://ucanr.edu/sites/nm/files/76580.pdf

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 fieldspecific 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.

• Pre-sidedress soil nitrate testing (PSNT) allows for identification of fields that will or will not show a yield increase upon side-dress N fertilization
• A threshold of 20 ppm nitrate-N in the top foot of soil has been identified.This threshold by tested by comparing standard grower fertilization practice with fertilization rates tailored to maintain, but not exceed, this 20 ppm cutoff:
  

  1)      No N was applied if soil nitrate-N levels exceeded 20 ppm.

  2)      Whenever levels dropped below this threshold, the following fertilization rates were used:

  • 0-5 ppm -> 80 lbs N/acre applied
  • 5-10 ppm -> 60 lbs N/acre applied
  • 10-15 ppm -> 40 lbs N/acre applied
  • 15-20 ppm -> 20 lbs N/acre applied

• By adopting the PSNT informed fertilization regime, the rates of N applied were lowered from an average of 254 lbs N/acre to 142 lbs N/acre without any yield penalties.
• Only 6% of N applied in excess of the PSNT based rates was taken up by plants, with the remaining N staying in the soil N pool. This residual N represents an avoidable cost for growers while also posing an environmental risk due to nitrate leaching into groundwater.
• Plant midrib tissue analysis did not provide precise enough information to serve as the basis for fertilizer rate adjustments.