The use of nitrogen fertilizer
is coming under increased scrutiny in many parts of the United States
due to concern over nitrate contamination of groundwater. Groundwater
nitrate contamination is of particular concern in the coastal valleys
of central California, where many wells now exceed the U.S. Environmental
Protection Agency drinking water standard of 10 mg·kg-1 NO3-N.
Vegetable production dominates agriculture in these valleys, with
lettuce by far the most common crop. Seasonal N application > 200
kg·ha-1 is common for lettuce production, substantially more
than crop N uptake, and perhaps three time the amount of N removed
in harvested product. Similar imbalances between the amount of N applied
and that removed in harvested product exists for the other common
vegetable crops grown in rotation with lettuce in this region.
Such high rates of fertilization are due in part to conflicting research
results, and in part to conflicting research results, and in part
to economic relationships. In one study it was found that head lettuce
yield peaked with N at 100 – 150 kg·ha-1, while in other
reports yield increase up to at least 250 kg·ha-1. Given the
high potential value of lettuce (frequently exceeding $10,000 per
ha), exacting market standards for size and appearance, and the low
cost of fertilizer N (currently = $0.80 – 1.20 per kilogram)
there has been little economic incentive to minimize N application
in the absence of regulatory pressure. Such pressure is now being
brought to bear.
Presidedress soil nitrate testing (PSNT) is a potential strategy for
minimizing unnecessary N application. PSNT has been widely shown to
identify corn fields in which crop response to sidedressed N was unlikely.
These studies reported that a soil NO3-N concentration (top 30 cm)
greater than = 20 mg·kg-1 when corn was 15 cm tall (the stage
at which sidedressing is usually done) indicated that no sidedress
N was required to achieve maximum yield. Work in California found
this 20 mg·kg-1 NO3-N threshold successfully identified lettuce
and celery fields in which sidedress N application could be delayed
or eliminated. They did not attempt to use PSNT to predict sidedress
N requirements in fields below 20 mg·kg-1. The current study
was undertaken to evaluate the utility of PSNT as a general N management
technique in lettuce production, regardless of residual soil NO3-N
concentration. Our main objective was to document the effects of N
management using PSNT on crop yield and quality, N application, and
postseason NO3-N leaching potential across a wide range of field conditions
and management practices representative of the commercial industry.
Trials were conducted in 15 commercial fields in the central coast
region of California in 1999 and 2000 to evaluate the use of presidedress
soil nitrate testing (PSNT) to determine sidedress N requirements
for production of iceberg and romaine lettuce (Lactuca sativa L.).
In each field a large plot (0.2 – 1.2 ha) was established in
which sidedress N application scheduled by the cooperating growers,
a composite soil sample (top 30 cm) was collected and analyzed for
NO3-N. No fertilizer was applied in the PSNT plot at that sidedressing
if NO3-N was >20 mg·kg-1; if NO3-N was lower than that threshold,
only enough N was applied to increase soil available N to = 20 mg·kg-1.
The productivity and N status of PSNT plots were compared to adjacent
plots receiving the grower’s standard N fertilization. Cooperating
growers applied a seasonal average of 257 kg·ha-1 N, including
one to three sidedressings containing 194 kg·ha-1 N. Sidedressing
based on PSNT decreased total seasonal and sideress N application
by an average of 43% and 57% respectively. The majority of the N savings
achieved with PSNT and grower N management across fields in lettuce
yield or postharvest quality, and only small differences in crop N
uptake. At harvest, PSNT plots had on average 8 mg·kg-1lower
residual NO3-N in the top 90 cm of soil than the grower fertilization
rate plots, indicating a substantial reduction in subsequent NO3-N
leaching hazard. We conclude that PSNT is a reliable management tool
that can substantially reduce unnecessary N fertilization in lettuce
production.
