Plant Sunflower, Save on
Fertilizer
(Reprint from the NSA Magazine)
April 2007
You might not need to apply as much nitrogen to your
sunflower crop as you think you do.
For one thing, the amount of residual soil nitrogen can
be expected to be quite high in areas with dry
conditions in 2006. This is illustrated in regional
trends for soil nitrate levels from soil samples
analyzed by Agvise Laboratories last fall. With
laboratories in Northwood,
N.D. and
Benson,
Minn., Agvise provides soil
testing and plant analysis for crop consultants,
fertilizer retailers and producers.
In the figures, Agvise Laboratories summarized the
average fall soil nitrate levels following wheat and
corn production (lb/ac 0-24� samples) in the Dakotas,
Minnesota and Manitoba. The bold values indicate the areas
where the average soil nitrate test was much higher in
2006 compared to 2005.
The comparative results are striking. For example, areas
of South Dakota that had average soil N levels of 52,
54, and 58 lb/ac following corn in 2005 had 100, 102,
and 107 lb/ac of residual soil N, respectively,
following corn in 2006. This is a tremendous amount of
residual soil N, and there will be many cases in
drought-affected areas where the amount of N needed for
the subsequent crop will be much less than a normal
application.
Following a drought, soil P and K levels don�t change
much compared to what they were the previous year. North
Dakota State University Extension soil specialist Dave
Franzen, points out that this is usually the case in
most years, due to the small amount of these nutrients
removed in a single year by even a bumper crop, but
especially so in a small, drought-affected yield.
Soil fertility levels following other crops (including
sunflower) and a snapshot of other nutrients can be
found online at
www.agvise.com. Click on �2006 Soil Test Summaries.�
See the excellent information on a variety of fertility
topics under �technical articles� as well.
John Lee, soil scientist with Agvise in Northwood,
cautions that these numbers are just big picture
snapshots to help gauge soil fertility trends. �As
averages, you can expect half of fields to be more, and
half of the fields less,� he says. While it is
interesting to look at the big picture to see the
overall trends, the only way to know for sure what
nutrients your fields need (or don�t need) is through
soil sampling analysis.
However, even a soil sampling analysis doesn�t give the
entire picture, especially if you�re not sampling
deeper. Mycogen agronomist Bruce Due reviewed Agvise
soil zone sampling results for sugarbeets in the Red River Valley
several years ago. He wanted to see how much N might be
available at deeper levels. �With beets, they test to
four feet down, because the beets need to run out of N,
otherwise they don�t put sugar on. The numbers I got
were unbelievable, 35 to 40 lbs of N, into the hundreds
of pounds of N, that sunflower can tap into.�
That much N deep in the soil profile isn�t surprising. Franzen points
out a nutrient analysis in a field near Williston, N.D.,
where N levels from 2-4 feet were over 200 lb/ac � heat and
drought resulted in high amounts of residual N left
behind in a non-leaching environment. And in the High
Plains, on fields that have been on irrigated corn over
multiple years, it�s not at all uncommon to find 200 to
400 lbs/ac of nitrate below the corn root zone,
typically three or more feet deep in the soil. As one of
the deepest rooting crops grown, sunflower will root
down and extract that residual N, provided there�s not a
compaction zone to inhibit root growth, and if subsoil
moisture is adequate to encourage root growth.
Organic matter breakdown matters
Due says more and more data is being gathered that
supports backing off from previously recommended N
rates. Part of it is better plant genetics � many crop
varieties and hybrids on the market today are better
than their predecessors, with better root systems and
improved nutrient uptake. Fertility recommendations
today are also changing to take deep soil N and the
breakdown of soil N during the growing season into
account. Further, with $400 - $500/ton fertilizer, more
attention is being given to economic optimum N rates.
Universities are adjusting their fertility
recommendations accordingly. The University of Minnesota
revised its nitrogen recommendations for corn last year
(www.extension.umn.edu/distribution/cropsystems/DC3790.html)
and the
University
of Nebraska
has done the same (http://soilfertility.unl.edu).
North Dakota State University
has revised its fertility recommendations for dry beans,
and expects to do the same for other crops as additional
site-year data to support the fertility revisions is
accumulated.
Due says a soil test in the fall or spring, and
traditional nitrogen recommendations, don�t fully
account for the biology that goes on during the growing
season. Microbial activity is breaking down organic
matter, making soil nitrates available to developing
crops. A lot of this breakdown in organic matter in May
and June becomes available and is used by late-season
crops such as corn and sunflower in July and August. The
microbial activity is greater in soils with higher
organic matter (>3%) and in areas that receive more
moisture. This helps explain why crops will overshoot
yield goals, like in 2005, when growing conditions for
row crops in the Northern Plains was ideal.
�You fertilize for 2,000 lbs, and get 3,000 lb �flowers.
How does that happen, if you only put down enough N to
meet 2,000 lbs? You don�t get good yield unless you have
good moisture. If you have good moisture, you�re getting
good organic matter breakdown,� says Due. �It�s causing
some in the industry to think that maybe the variable
rate application thinking is backward. Maybe you need
the extra fertilizer on your poor ground, and not as
much on your good ground, because if you�re getting good
moisture, there�s enough organic N becoming available in
the field to take care of extra yield needs.�
In fact, Due says too much N can be detrimental to a
crop like sunflower. �I�ve had situations where a grower
would apply enough N to plant corn, then the season gets
late, and he puts in sunflower. With good moisture and
all that N, now you have a sunflower plant that is
building a big factory � tall, big leaves. You get
25,000 plants doing that, and you get a stand that�s
dense, thick, lush, and prone to disease. So when soil
fertility tests suggest there�s enough residual soil N
to grow corn without adding much additional N, that�s
too much N for sunflower.�
He says that a number of growers with high fertility
levels, or on fields that haven�t seen a deep-rooted
crop like sunflower in four or more years, they put down
a starter fertilizer, and that�s it. �Are they going to
pull 3,000 lb �flowers? Probably not. Can they pull
2,000? Probably. With the cost of fertilizer the way it
is, a lot of growers are trying to figure out how to get
by with less, and sunflower is one crop that can
scavenge pretty well. And because it uses much of its N
later in the season, there�s a pretty good return there
in residual soil N that they can draw on that isn�t
going to show up in a soil test.� � Tracy Sayler
|