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Risks of fall nitrogen application

Risks of fall nitrogen application
Nitrogen losses can be high


November 16, 2007
By Bruce Barker

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12aThe surprising part of Kevin Tiessen's research on fall banded nitrogen fertilizer
application was not that low lying areas of the field were subject to fertilizer
loss due to denitrification. Rather the big surprise was how very small changes
in landscape could affect the yield results.

"Even the subtle areas in a field that just pond water for a few days
in the spring are subject to nitrogen losses when nitrogen is applied early
in the fall," says Professor Don Flaten, who supervised Tiessen's Masters
research at the University of Manitoba. "You could move 20 metres in a
nearly level field and find a 20 percent difference in wheat yield."

Over the years, banded N has consistently improved the efficiency of fall-applied
fertilizers in western Canada compared to broadcast N. However, relatively little
research has been conducted in western Canada to see whether fall banded N fertilizers
require delaying of application date to improve grain yields, as was the case
for broadcast and incorporated fertilizers. So Tiessen conducted two years of
experiments in southern Manitoba to see if N fertilizers applied in subsurface
bands are in fact less sensitive to application date in the fall.

Tiessen looked at the implications of fall versus spring banding of N fertilizer
and how application date and landscape position affected N efficiency and crop
yield. He hoped to answer the question of whether fall banding of N could be
used as an effective way to manage the spring workload, and to allow farmers
to capitalize on lower fertilizer and application costs. He also wanted to find
out if losses of fall banded N are especially large in depressional areas of
the field where soils sit waterlogged for several days or weeks in the early
spring as the snow melts.

Three of the research sites were located on heavy clay soils in the relatively
level landscape of the Red River valley, where the typical elevation differences
are less than two feet per mile. The fourth site was near Brandon, which had
slightly more undulating topography.

At each site, urea (46-0-0) was banded at three different times in the fall,
between September 15 (early fall) and October 20 (late fall) depending on weather
conditions, and once in the spring (mid-row banded) at planting. The sites were
also separated into high (better drained) and low (more poorly drained) areas
to assess the affects of landscape position.

Short-term ponding resulted in N losses and lower
yield

Tiessen's research is best understood by separating out the results by landscape
position. On slightly higher ground, where the water did not pond in the spring,
the yield response was statistically similar for all dates of application, spring
or fall. This indicates that while the early-fall banded N treatments may have
had time to convert from ammonium-N to nitrate-N, which is subject to leaching
and denitrification under waterlogged conditions, the nitrate-N did not move
out of the soil and was available to the crop in the spring.

In the slightly lower landscape positions, though, the spring banded and late-fall
banded N had the highest yields, ahead of the early and mid-fall banded applications.
On these heavy clay soils, leaching is unlikely; therefore, this indicates that
in the early and mid-fall treatments the nitrate-N was probably lost to the
atmosphere through denitrification. Tiessen says that crop responses from fall
banded N were improved by as much as 40 percent simply by delaying application
until late in the fall when soil temperatures had declined to five or six degrees
C.

Presumably, the increased efficiency of late-fall banded N in the low landscape
positions was due to less conversion of the urea fertilizer to nitrate prior
to winter, which reduced spring leaching and denitrification losses.

Based on the research, Flaten says that on well-drained land, early-fall application
of N fertilizer appears to be a viable option. On poorly drained land, though,
where the potential of flooded conditions is high during the fall and spring,
producers should wait as long as possible in the fall, or until spring, to apply
nitrogen fertilizer.

The trick, though, comes in defining what poorly drained land looks like. Flaten
was surprised that just a few days of water lying on what appeared to be a relatively
flat soil surface in the spring could have such a large effect on crop yield.
Even minor changes over a field can leave significant portions of the field
subject to short periods of minor flooding, which can cause loss of nitrate-N.
In these cases, the best recommendation would be to err on the side of caution;
band fertilizer N in the late fall or spring. -30-