West Coast Nut September 2019

Nitrogen in Walnuts—
What, When, Where
and How Much?
By JULIE R. JOHNSON | Contributing Writer
TO KNOW HOW TO CORRECTLY
use nitrogen in walnut orchards,
you first have to know some of the
basic principles of nitrogen, according
to Katherine Jarvis-Shean, University of
California Cooperative Extension area
orchard systems adviser for Sacramento,
Solano, and Yolo counties. Jarvis-Shean
shared this information during the Nickels
Soil Lab Annual Field Day in her presentation—Nitrogen
in Walnuts: What, When,
Where and How Much?
Nitrogen Principles
Jarvis-Shean said growers have a few
different forms of nitrogen as options of
use in their agriculture systems.
“Nitrogen is one part of soil organic
matter, one part of urea, as in ammonium,
and then nitrate,” she explained. “So those
are ways we encounter nitrogen in our
soils when we are trying to get it up into
our plants.”
Unless a grower is in an organic system,
soil organic matter is not a huge source
of nitrogen for most ag systems. In those
circumstances, the nitrogen input sources
from soil organic matter must come
through nitrogen-based cover crops or
manure compost.
“It is like a savings account from which
you get a small dividend every month,
it is a slow leak out of nitrogen from
soil organic matter, but it is never a big
wallop,” Jarvis-Shean said.
Urea is a neutral that moves through
the soil with water, she added. Urea
doesn't stick in the soil profile, but it does
change to ammonium in a matter of days
after it has been applied because of micro
activity in the soil.
“Think of urea as acting like ammonium,
as it changes to ammonium pretty
quickly,” Jarvis-Shean said. “This leaves
us with ammonium and nitrate, our two
big hitters in our nitrogen system.”
She explained that ammonium is
positively charged, and while this doesn't
seem to be a big deal, it is.
“Because the surface of the soil is negatively
charged, so ammonium will stick
in the soil, which we like, as that means
it doesn't leach out of the soil system
and the roots can take up nitrogen in the
form of ammonium,” she stated.
Nitrate, on the other hand, is negatively
charged, which means it wants to
stay away from the negatively charged
soil, so it doesn't want to stick.
“This keeps it moving through the soil
system when water is applied,” Jarvis-
Shean said. “Nitrate leaches out very
Katherine Jarvis-Shean, UCCE area orchard systems adviser for Sacramento, Solano, and Yolo counties, gives
her presentation—Nitrogen in Walnuts: What, When, Where and How Much?, to a group during this year's
Nickels Soil Lab Annual Field Day. All photos courtesy of Julie R. Johnson.
easily through the root zone when
putting water on top of it, and moving it
through the soil and root zone.”
Nitrate is easily taken up by the roots
so we like that part of it, she added, but it
has this liability of being leachable.
“So you might say, let's just get around
all these leaching problems by just
applying ammonium—unfortunately it is
not that easy, because ammonium will be
munched on by microbes and turn into
nitrate—we call it nitrification,” Jarvis-
Shean advised. “And that, in California's
soil systems, happens pretty quickly. So
with the temperatures we have in the
growing season, about half the ammonium
applied will turn into nitrate in
about two weeks if it hasn't been taken
up by the tree.
That's a pretty quick turn around, so
we can't put a lot of nitrogen on in the
form of ammonium in the beginning of
the growing season and hope it will get
us through the whole growing season
because a lot of that can turn into nitrate
and leach out of the root zone.”
The Four R's of Nitrogen
Management
With the basic principals of nitrogen
in hand, Jarvis-Shean talked about the
four R's of nitrogen management in
walnuts—What, When, Where and How
Much or otherwise known as “the right
time, the right place, the right rate, and
the right source.”
She said:
• Right rate is about matching the
overall seasonal demand with your
overall season's nitrogen application.
• Right time is keeping in mind that
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West Coast Nut September 2019