WCN August 2020

Right Source
There are several forms of nitrogen
fertilizers, each having different characteristics
and behaving differently in the
environment, some more mobile and
leaching more readily into the groundwater.
“This is something to talk to your
fertilizer supplier about, especially with
urea and ammonium, as they both can
be lost into the groundwater,” Klassen
said. “Urea is mobile, but converts quickly
to ammonium, which is less mobile,
but ammonium quickly converts to
nitrate by soil microorganisms in warm,
moist soils.”
Urea and ammonium may also be
lost through volatilization if surface is
applied and not incorporated quickly.
Nitrogen fertilization sources should be
selected based on best root uptake to
minimize excess nitrate movement past
the root zone.
“Decisions on what nitrogen source to
use is of course not just based on leaching,
but also on cost and the ability to
apply them in the drip system or broadcast,”
Klassen said.
Right Place
“In general, the principle for the
best place to apply nitrogen fertilizer is
where the roots are, of course, and along
the herbicide-cleaned strip,” Klassen
said. “For tree-to-tree sprinkler irrigated
orchards, many growers broadcast one
or more applications, but remember, root
systems develop where water is applied,
so the best application method is fertigation,
water run through sprinklers or
micro-sprinklers by injecting fertilizer
during last half of the set whether you
do a 24-hour set or a 12-hour set. If at
all possible, apply during the last half of
the set, leaving 15 to 20 minutes at the
end to adequately flush out the system of
residue that could otherwise be sitting in
the system when it is shut off.”
Studies performed over time have
shown a dramatic leaching of nitrogen in
flood irrigation systems when the fertilizer
is applied and then six to eight inches
of water are put on.
“Often, the nitrogen is pushed beyond
the root systems in the orchard in these
types of systems,” Klassen added.
He said a University of California
study showed the majority of root growth
is in the top 20 to 22 inches of soil.
“The vast majority of working roots
are about between 7 to 18 inches down
in the soil, so the principle of putting
fertilization in the back half of the irrigation
set places that fertilizer in the active
roots where it is most efficiently going to
be taken up,” Klassen said.
“One other thing I have to dwell on, as
we have so many of our orchards in high
groundwater nitrate areas, is growers
need to be aware of the nitrogen levels
in their irrigation water as this can be
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a useful source for crop production and
the amount can be a pretty significant
number. However, it is important to
remember the caveat, that number is
based on all 30 acre-inches in walnuts,
for example, being absorbed by the tree
through the year, and we know that is
not likely.”
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