March 2019 - 13

Late-season nitrogen use may not help asparagus
By Dean Peterson
VGN Correspondent
Changes in production systems including
new asparagus varieties, the expansion
of overhead irrigation and shifting
markets are increasing grower interest
in higher nitrogen fertilizer rates on
asparagus. There's also interest in splitting
applications with some nitrogen applied
later in the season during fern growth to
promote fern health and photosynthesis.
However, first-year results from new
research at Michigan State University (MSU)
hasn't found a benefit from the practices.
" These are very early results from a
long-term study on a perennial crop, "
said Zack Hayden, vegetable soil and
nutrient management specialist in MSU's
Department of Horticulture. " We don't
change our recommendations to growers
on one year's results. "
While it's too early to draw definitive
conclusions from this study, the initial results
align with findings from previous research
on nitrogen rates on asparagus in Michigan
and other states. Hayden spoke on the topic
of asparagus fertility at the recent Great Lakes
Fruit, Vegetable & Farm Market EXPO in
Grand Rapids, Michigan.
The new research is an on-farm trial
that began in 2017 near Hart, Michigan.
Researchers evaluated nitrogen rates of
50, 80, 110 and 140 pounds of actual
nitrogen applied per acre. Researchers also
evaluated two additional treatments in
which a portion of total nitrogen - either
20 or 40 pounds of nitrogen per acre - was
applied during fern growth in late July.
The site was a three-year-old stand of the
asparagus cultivar Millennium. The site
was a coarse-textured soil with center pivot
irrigation. All nitrogen applications were
topdressed urea.
The majority of the nitrogen in the
asparagus spears comes from nitrogen
stored in the crown and roots and up to
90 percent of the nitrogen in the fern is
remobilized to the crown and roots when
the fern is fully senesced.
In this study, researchers found no clear
differences between any of the nitrogen
treatments on the fern nitrogen content or on
fern biomass during the summer.
Despite this, researchers only found
low levels of inorganic nitrogen in the soil
across all treatments later in the summer.
This suggests effective scavenging for
the added nitrogen in the high nitrogen
treatments since excess nitrogen may be
stored in the large root systems of asparagus
for future growth. This is true of even the
newer asparagus varieties thought to have
somewhat smaller root systems. The low
levels of inorganic nitrogen could also have
been from greater nitrogen losses.
One of the few effects observed in the first
season was an increase in the stem count
at midseason for all nitrogen rates higher
than 50 pounds of nitrogen per acre. " Many
of these were stems that shriveled and died
off early, " Hayden said. " They likely didn't
represent a benefit to the plant. " They might
have even been a drain on the carbohydrate
reserves of the roots.
In this on-farm trial, the effects of the
different nitrogen rates on first season spear
yields were evaluated in the spring of 2018.
This was done by counting and measuring
the diameter of all previously harvested
spears (as stubble) at weekly intervals, in
addition to counting the spear weights
from one harvest per week.
The higher or split nitrogen treatments
didn't have any benefit on the total number
of harvested spears, the average diameter of
harvested spears, or weights measured in
individual spear harvests.
" We really haven't yet seen any
production advantages to applying
more nitrogen than MSU's standard
recommendations, " Hayden said.
For established asparagus plantings,
MSU's standard recommendation is to
apply some nitrogen preharvest and the
rest postharvest before fern growth. The
total amount should not exceed 80 pounds
of actual nitrogen per acre.
For new plantings, MSU's
recommendation is for 50 pounds of
actual nitrogen per acre broadcast and
incorporated before setting out the crowns,
followed by another 50 pounds of nitrogen
after the fern growth is six inches tall.
" If there isn't a benefit from higher rates
and split applications, that will mean lower
fertilizer and application costs, " Hayden said.
On the coarser-textured soils asparagus is
grown on, it will also mean a lower potential
for nitrogen leaching and the associated
environmental risks. VGN
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VGN | MARCH 2019 | 13
Zach Hayden in an Oceana County asparagus
field in Michigan. Photo: Gary Pullano

March 2019

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