Corn Roots,Wet Soils, And Nitrogen
URBANA, ILL.
The June weather pattern
in Illinois was variable,
and the month is ending
with rainfall totals ranging
from a little less than normal
in parts of western Illinois to
nearly double the normal
amounts, with some totals as
high as 7 to 8 inches, in parts
of southeastern and northern Illinois.
While getting rainfall in June is certainly
preferable to getting little or none as happened
in Illinois in 2012, standing water and wet soils
can badly damage a rapidly-growing corn crop.
In June of 2010, 2 to 4 inches of rain fell during
the fourth week of June over most of central
and northern Illinois. With the crop planted
early and developing rapidly under high temperatures,
standing water resulted in serious
and irreparable damage to root systems. This
lowered yields in low-lying fields and parts of
fields, even where rains fell later in the season.
When soils remain saturated for more than a
day or two, the lack of oxygen causes nutrient
uptake to slow quickly, and root tips start to die
off. It helps that temperatures have not been
above normal; cooler water carries more dissolved
oxygen, and also slows growth and nutrient
uptake. Also, plants during vegetative
growth have much better ability to grow back
damaged root systems once soils drain than do
plants during or after pollination.
These factors, along with the very good crop
color (which indicates good root activity and adequate
supplies of soil N) before the rains in late
June, point to good chances for recovery of crop
yield potential in fields and parts of fields where
the water is no longer standing. In the short
run, plants may lose some of their green color
before roots are fully functional again, but this
will likely be a temporary condition. While many
worry that any stress during mid-vegetative
growth will lower yield potential, there’s not
much evidence that a few days with reduced
photosynthetic rates has much effect on yields,
at least if this occurs more than a week before
tasseling.
Regardless of how quickly the crop returns to
normal after an event like temporary flooding,
questions will remain about how standing water
might affect the amount of nitrogen left in the
soil to meet the needs of the crop. Warm, saturated
soils lose nitrogen (as gas back into the
air) through the process of denitrification. We
do not think that such losses have been very
large in most fields, given the temperatures and
the fact that most flooding was temporary. In
better-drained fields, denitrification would be
less, but percolating water has probably moved
some of the nitrate-nitrogen deeper, perhaps
below the root system or into tiles lines.
In central Illinois we have accumulated about
1,100 growing degree days (GDDs) since May 1,
and about 930 GDDs since May 15. By the time
corn accumulates 1,000 GDD, reaching about
stage V13, it has accumulated about 20 percent
of its dry weight and about 40 percent its season-
long nitrogen accumulation (Abendroth et
al., 2011) During this period the crop takes up
3 to 3.5 lb of N per acre per day, and by the time
of pollination, it will have taken up about 60
percent of its nitrogen and produced about 40
percent of its dry weight.
At the time the crop reaches stage V13 (about
head-high), it still has to take up 110 to 120 lb
of N, and in years when June is wet, a common
question is whether or not the crop might run
out of nitrogen, leaving the crop short. While the
need for 20 or more lb of N per week would seem
to raise the possibility of a shortage, the production
of plant-available N from soil organic
matter through the process of mineralization is
also at its maximum rate in mid-season.
For a crop with a good root system growing in
a soil with 3 percent organic matter, mineralization
at mid-season likely provides at least
half the N needed by the crop on a daily basis.
This means that normal amounts of fertilizer N,
even if there has been some loss, should be adequate
to supply the crop.
Though we could measure soil N present or
apply urea by air on the wetter field or parts of
fields where the crop shows deficiency, it would
seem prudent to wait to see if the crop recovers
its green color before going to this expense. The
loss of crop color in wet soils is due mostly to
loss of root function, and roots will need to recover
before the canopy does. Even without
adding more N, odds are good that the crop will
recover and thrive in the coming weeks, providing
the weather remains favorable.
Reference:
Abendroth, L.J., R.W. Elmore, M.J. Boyer, and
S.K. Marley. 2011. Corn growth and development.
PMR 1009, Iowa State University Extension,
Ames, Iowa. Δ