Agriculture Reference
In-Depth Information
Fertilizer application options
For a predicted yield of
79
t ha
-
1
in the
Columbia Basin of Washington State, Lang
et al
.
(1999) recommended a nitrogen fertilizer rate
of 392 kg ha
-
1
,
after accounting for residual soil
nitrogen and potential microbial tie-up. They
also suggested applying approximately
17-
35%
(
67-
134 kg N ha
-
1
for a
79
t ha
-
1
yield) in the top
30 cm of soil prior to or at planting, with the re-
mainder applied in-season. They estimated
30-
40% of total nitrogen uptake occurred by the
end of tuber initiation, and the largest daily ni-
trogen demand came during rapid tuber bulk
(~early July), when plants were extracting up-
wards of 4.5 kg N ha
-1
day
-
1
.
It is common for Columbia Basin growers to
fertigate
22-
34 kg N ha
-
1
during one irrigation
cycle, with weekly totals of up to 84 kg N ha
-
1
,
especially during early, rapid tuber bulk. Due to
differences in seasons, varieties, and mineraliza-
tion rates, it is important to track petiole and soil
nitrogen and to adjust in-season application
rates accordingly. To allow for proper plant and
tuber maturity, soil nitrogen should be depleted
and in-season applications should cease approxi-
mately
30-
45 days prior to harvest.
Various methods exist to deliver fertilizer to
potatoes. Growers with overhead irrigation,
like that from center pivots, have the ability to
fertigate as needed throughout the season.
Applying fertilizers like nitrogen through an
overhead irrigation system allows the plants
to receive the nutrient as needed with extreme
precision and uniform distribution. Growers
without overhead irrigation have fewer op-
tions, and their fertilizer is typically applied
and incorporated with tillage equipment prior
to planting, during planting, and/or prior to
row closure (100% ground cover); however, fo-
liar nutrients are occasionally applied after row
closure via ground or air applicators. Foliar
applications are typically used for smaller,
supplemental nutrient applications. By using
nutrient applications via the overhead-irrigation
system, losses due to nitrogen leaching and
volatilization are minimized because the appli-
cation is matched closely to real-time plant
growth during the season (Waddell
et al
.,
2000). Moreover, applying all fertilizer pre-
plant or early in the season is inefficient, as
plants may recover only 60% of that applied
(Westermann
et al
., 1988). To maximize effi-
ciency and minimize leaching and volatilization,
many growers apply nutrients via fertigation in
small quantities twice or more each week dur-
ing the growing season. The ability to “spoon feed”
nutrients to the plants through the irrigation
system enables growers to tweak plant growth
during crucial plant development stages in an
effort to optimize yield, postharvest quality and
storability, and in turn, economic return.
Potato source-sink relationships
One relationship that is crucial to profitable
potato production is the source-sink relation-
ship. The term “source-sink” is somewhat
self-descriptive in that plant organs that pro-
duce synthesized food material are referred to
as the “source” and those that store them are
called the “sink”. Sources are net carbon pro-
ducers, while sinks are net carbon users
(Dwelle, 1990). Mature green potato leaves
act as the main source; tubers become the
major sink when they start to bulk. Keeping
the source-sink ratio in balance is thought to
be important when trying to maximize profits
via crop production. The sink provides the
income for potato growers, and it is therefore
essential that the relationship between the
leaves and tubers is maintained in a manner
that will maximize final tuber weight and
quality. Through the use of in-season nitrogen
fertigation, growers can manipulate plant
growth at key stages to maximize their profits.
For an in-depth discussion on this concept and
potato growth, see Chapter
5
of this volume.
Nitrogen fertigation
Compared with other macronutrients, nitrogen
has the largest effect on plant growth and final
tuber yield (Rykbost
et al
., 1993). Nitrogen fertiga-
tion quantities and timing should match plant
growth. This can be accomplished by maintaining
petiole and soil levels in accordance with recom-
mendations based on previous research (as indicated
previously), and by having a thorough under-
standing of plant growth and development, espe-
cially as it relates to a particular variety and region.
