Monitor
in-season soil and plant N status
In commercial vegetable production only a small portion of seasonal
N fertilization is usually applied preplant. This provides the opportunity
to reevaluate N requirements during the season. In non-irrigated culture,
or when furrow irrigation is used, most N is applied in one or two sidedressings
before peak crop N uptake occurs. Most soil NO3 – N present at
sidedressing will remain available for crop uptake, since crop N uptake
rate is increasing at that point in the season, and subsequent in-season
leaching losses tend to be small. Pre-sidedress soil nitrate testing
(PSNT) can provide a direct measure of current mineral N concentration,
eliminating the uncertainty associated with pre-plant estimates of soil
N availability.
PSNT is a very useful tool for identifying fields in which additional
N fertilization can be delayed or reduced. Originally developed for
use with field corn, this technique has been successfully adapted for
use in cabbage, celery, lettuce, sweet corn and tomato production. Across
crops and production regions, the soil NO3 – N threshold above
which crop response to additional fertilization was unlikely ranged
from approximately 20 to 25 mg · kg-1. In fields with soil NO3
– N below the response threshold, applying only enough fetilizer
to bring the soil up to the threshold was an efficient practice. Sampling
the top foot of soil has been the standard PSNT approach; for deep-rooted
crops deeper soil sampling may be used, but the correlation of NO3 –
N concentration of deeper samples with the surface foot of soil tends
to be sufficiently strong to make deeper sampling unnecessary.
The use of PSNT can be tailored to different cropping systems. For crops
that typically receive only one sidedressing a single soil test is appropriate;
for crops that receive multiple sidedressings, the test can be repeated
prior to each. Where multiple sidedressings are applied, a lower PSNT
response threshold may be appropriate than when a single sidedressing
is done. Care must be exercised to collect samples representative of
the active root zone, particularly avoiding zones of recent banded fertilizer
application; while this may underestimate actual N availability, it
avoids the economic risk of overestimating N availability and compromising
crop production or quality. The potential to reduce N fertilization
by using PSNT can be substantial; trials in commercial lettuce fields
in California showed that seasonal N application could be reduced by
>40%. The cost/benefit ratio of PSNT in these studies was very high
and was estimated that a reduction of 10 mg · ha-1 N would more
than offset the monitoring costs.
PSNT can also be useful in drip-irrigated fields. With drip irrigation,
N is often applied in small fertigations throughout the season, allowing
a grower to tailor the fertigation program to match crop uptake. Early
in the season PSNT can identify fields with significant residual NO3
– N, allowing a grower to delay the initiation of fertigation.
While PSNT has been shown to be a valuable tool in the production of
a variety of crops in a wide range of production environments, it is
not universally applicable. PSNT is maximally effective in situations
where large sidedress N applications are made early in the cropping
season, and in fields in which significant in-season leaching is unlikely.
In vegetable production on light-textured soils in high rainfall environments,
PSNT may have little applicability; under such circumstances other approaches
(small N applications throughout the season, use of slow-release fertilizers,
etc.) are more useful.
Plant tissue analysis is often advocated as a BMP. However, the practical
value of plant analysis in improving N management in vegetable production
has generally been overstated. An important limitation of plant analysis
is that both leaf N and petiole NO3 – N are relatively insensitive
indicators of current soil N availability, at least in fields of moderate
to high N supply. Researches have provided evidence of such insensitivity
in lettuce by documenting a lack of correlation between either midrib
NO3 – N or leaf N and concurrently measured soil NO3 – N.
They have, with lettuce, concluded that plant analysis was an insensitive
diagnostic tool for N management during the first half of the growing
season, the time when most N application occurs. In lettuce, broccoli,
and cauliflower fertilization trials in Arizona showed that midrib NO3
– N incorrectly predicted crop response to sidedress N fertilization
in approximately half of the trials. Similarly, results from another
trial in another area found tissue analysis to be unreliable in predicting
sidedress N requirement in cabbage, carrot, and onion production.
A fundamental problem with tissue analysis is that cultivar-and-field
specific factors confound the relationship between soil N availability
and tissue N level. In test plots it has been found that tissue N varied
considerably more among fields than between N fertilizer rates within
fields, even when those N rates varied by more then 100 kg ·ha-1.
These reports discussed here strongly suggest that conventional plant
tissue analysis, while a potentially useful technique to detect N deficiency,
is generally ineffective in helping vegetable growers reduce unnecessary
N application. In recent years an alternative approach to plant analysis,
the evaluation of leaf color using the SPAD chlorophyll meter (Minolta
Corp., Kyoto, Japan), has been extensively evaluated. The initial cost
of the SPAD meter is substantial (approximately $1400), but there are
no recurring costs for monitoring other than labor. The meter provides
a leaf chlorophyll index (LCI), which is correlated with leaf N status.
Since the relationship of leaf chlorophyll index (LCI) to leaf N can
be confounded by cultivar, soil, and environmental factors, the comparison
of LCI of a field to that of well-fertilized in-field “reference”
plots increases the utility of the measurement. Delaying additional
fertilization as long as the field of LCI remains within 5% to 10% of
the reference plot LCI shows promise as a technique to improve N management.
