February 2019 - 11

The on-farm research looked at
how late-season, nitrogen applications
affected carrot yield, shoot growth
and root nitrate accumulation. It also
evaluated how foliar disease severity
affected root nitrate levels. The two-year
study took place in 2017 and 2018.
In both years, urea topdress
applications in September of 30
pounds to 90 pounds of actual
nitrogen per acre produced somewhat
larger shoots, but no significant effects
on root yield or quality.
" Nitrogen puts more
nitrate in the soil and
exacerbates the risks
of high nitrate levels
in the roots. "
− Zack Hayden,
Michigan State University
" We did not see an increase in
yield with increased, late-season
nitrogen rates, " Hayden said. " Nitrogen
applications in September have the
potential to increase shoot size, but
otherwise we saw little benefit - as
long as all other management practices
are sound. " Shoot size can be related to
mechanical harvest efficiency.
The plots in MSU's trials already
had a total of about 80 pounds of
actual nitrogen applied per acre before
the September application. This
came from applications of about 20
pounds of nitrogen at planting in the
spring, followed by about another 30
pounds topdressed in July and another
30 topdressed in August. These
applications are vital since adequate,
full-season, nitrogen fertility following
MSU's recommended rates remains
essential for optimizing carrot yield
and quality, Hayden said.
The September nitrogen applications
did increase nitrate levels in carrot roots
in both 2017 and 2018, with higher rates
of nitrogen leading to higher levels of
root nitrates. For nitrogen rates in excess
of 30 pounds per acre, root nitrate levels
had not started to decrease in 2017 by
the Oct.12 harvest date. Nitrate levels
for all nitrogen rates in 2018 were lower
overall and declining significantly by the
Oct. 27 harvest date. This is likely due in
part to later harvest and more vigorous
carrot growth that year.
" Generally, you're going to see that
nitrate peak three to four weeks after
application, but then it will go down, "
Hayden said. Moving a late-season
nitrogen application up a little earlier
or harvesting a little later will both
reduce root nitrate risk at harvest.
Foliar disease inhibits top growth in
carrots and increases the risk of higher
nitrate accumulation in the roots
later in the season. MSU's study also
evaluated the effects of low, medium
and high levels of foliar disease on
root nitrate levels and determined that
disease pressure significantly increases
root nitrate risk.
" All of these nitrate levels were not
at threshold for concern, but in a year
of higher disease pressure there's more
risk of increased nitrate accumulation
in the roots, " Hayden said. " If it's a
high disease situation, and you go in
and apply nitrogen late in the season
trying to address that, you're likely to
increase the nitrate level in the roots. "
The study also evaluated the
potential for two alternative nitrogen
fertilizer sources to reduce nitrate
risks. In both 2017 and 2018, a lateseason,
foliar nitrogen application
had lower levels of root nitrate than a
standard, urea topdress application.
" The goal here was to get the
nitrogen into the shoot and not the
roots, " Hayden said.
Soil-applied nitrogen applied in
a slow-release formulation also had
lower levels of root nitrate in both
years. However, these lower levels of
root nitrate are most likely due to lower
nitrogen availability or lower plant
uptake, and not currently recommended
as cost-effective alternatives to urea topdress
applications.
Sound nitrogen management is
important when growing a long
duration crop like carrots on sandy
soils. Production goals must be
balanced with the risks of excessive
nitrates at harvest.
Zack Hayden, vegetable soil and nutrient
management specialist in Michigan
State University's Department of
Horticulture, said moving a late-season
nitrogen application in carrots up a little
earlier or harvesting a little later will
both reduce root nitrate risk at harvest.
Photo: Dean Peterson
" It's important to apply the
recommended rates of nitrogen and
spread the applications out, " Hayden
said. " In the late season, that last topdress
application in September - close
to harvest - is the one to be more
concerned about. " VGN
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February 2019

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