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Nutrient management following the 2009 corn crop

A large portion of the 2009 Manitoba corn crop was not harvested for a number of reasons linked to the cool growing season. Such factors causing growers to till down corn included immaturity, excessive kernel moisture and excessive mould growth on the cob. Questions now arise whether this mass of material should cause changes in fertility management for 2010 crops, particularly nitrogen (N) needs. The following article relies upon a number of information sources to offer suggestions for growers and agronomists.


April 30, 2010
By John Heard

Topics

A large portion of the 2009 Manitoba corn crop was not harvested for a
number of reasons linked to the cool growing season.

wtcm-13-23--corn-harvest-img_1401

Unharvested corn will cause some nutrient management challenges. (Photo courtesy of John Heard)


Such factors
causing growers to till down corn included immaturity, excessive kernel
moisture and excessive mould growth on the cob. Questions now arise
whether this mass of material should cause changes in fertility
management for 2010 crops, particularly nitrogen (N) needs. The
following article relies upon a number of information sources to offer
suggestions for growers and agronomists.

First of all, the yield potential of the 2009 Manitoba corn crop was
high. Demonstration plots I maintained at the University of Manitoba
farm in Carman peaked at 200 bu/ac and growers were routinely
harvesting 120+ bu/ac crops, albeit at high moisture. High yields of
kernel corn mean a corresponding high yield of stalk, leaves and cobs.
The breakdown of these crop components depends upon their volume and
“decomposition rate,” which is dictated both by physical structure and
chemical makeup. In 2003, the Manitoba corn growers sponsored a study
where I measured nutrient uptake by corn throughout the season. The
grain yield was high and individual corn components were weighed and
analyzed chemically (Table 1).

Table 1.  Crop biomass and composition at harvest (2003)
picture-12
(Heard and Hay, 2006)

Typically, if the C:N ratio exceeds 20-25:1, then nitrogen is consumed
by soil microbes to decompose the residue.  This process is called
immobilization. This nitrogen may come from the soil or from applied
fertilizer if it is not managed optimally. One might then suspect from
the above numbers that the stalk and cob will therefore cause the most
immobilization, due to their high C:N ratio and high mass volumes.
Immobilization is not simultaneous, but sequential in the crop.
Actually the initial immobilization will be from the kernels that have
been returned, since they are in a physical form and sugar content that
are most vulnerable to microbial breakdown. Because the C:N ratio is
rather low for the kernels, this decomposition will be rapid, and that
N liberated back to the soil for use by the crop or further
immobilization to breakdown remaining plant components.

This sequential breakdown of corn components was wonderfully described
by Quebec researchers (Burgess et al. 2002. SSAJ 66:1350-1358).
Previous researchers often ground plant material to dust and then
showed large and almost immediate immobilization of high levels of
nitrogen. But these researchers left plant parts largely intact (4-6”
long pieces), placed them on the surface, at two- or six-inch depths
and measured decomposition for two years.  Indeed cobs, husks and
stalks all immobilized N at some point. However, N immobilization was
counterbalanced by simultaneous N release from other residues. No net
immobilization was observed for the residues overall. In fact, the cobs
immobilized little N at all due to their very slow decomposition.
Likewise Manitoba growers often observe old cobs for two to three years
following a corn harvest. So I would expect our 2009 corn crop to
decompose in the order of kernels followed by leaves and lastly by
stalk and cobs.

So our corn crop does have potential to immobilize N, but this is
spread over a rather large time frame and growers may manage to
minimize any impact.


Other factors, other influences

Straw and fertilizer management influences the amount of
immobilization. In our two-day Manitoba Soil Fertility Workshops, we
always refer to the classic Manitoba crop residue and N placement study
reported in 1981 (Table 2).

From this table it is clear that the greatest immobilization is when
both residue and fertilizer are incorporated together (three times as
much N ended up in the residue than in the crop). In such a situation
one may certainly need to increase N rates since the crop will be
standing in line behind the microbes at the dinner (N) table. However,
if the grower were to separate the straw from the nitrogen, either by
leaving the crop residue on the surface or sub-surface banding the N
(or both), the nitrogen efficiency would increase dramatically. Banding
N efficiently separates the nitrogen from the crop residue.

As shown in Table 2, one might also consider the removal of the
residue; however this also has fertility implications.  Stover from a
100 bu/ac corn crop contains about 50 lbs of N, 20 lbs of P2O5, and 100
lbs of K2O per acre (from Manitoba’s Soil Fertility Guide). When
stubble is left in the field after maturity, a large portion of this K
may leach out into the soil and be left for following crops. However,
removal of the remainder will remove much of the nitrogen, phosphorus
and other nutrients. Now some growers may feel they need to burn the
residue in order to create a suitable seedbed for 2010 crops. Our
burning studies of cereal straw show that all the N and a quarter to a
third of the P and K are lost through burning (Heard et al., 2001). So
considerable amounts of N and other nutrients can be lost permanently;
however burning is seldom complete in cornfields.

Table 2.  The fate of applied nitrogen based on straw and fertilizer placement.
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* Incorp = incorporated into the soil.  (Tomar and Soper, 1981)

In some traditional corn growing areas, nitrogen was routinely spread
on corn in the fall to promote microbial breakdown of corn stalks. I
remember an elderly Ontario fertilizer dealer fondly reminiscing about
these good old days, which came to an abrupt end in Ontario with the
release of research findings that if corn was adequately fertilized to
optimize yield potential, then sufficient N remains in the plant for
orderly decomposition. So the message is that one should not short the
nitrogen to the growing crop. That nitrogen applied to the growing crop
is far more profitable to the grower than making the application to its
straw.

If growers are unable to manage straw and N optimally, they may
experience temporary immobilization this spring and a shortage to their
crop. New guidelines from North Dakota State University for fertilizing
spring wheat consider supplementing more N to the crop if the previous
straw return has been excessive. For each ton per acre of straw
returned above normal amounts, 30 lbs of N per acre is suggested.
Again, this N is not applied to the previous straw to promote breakdown
but as a supplement for the current crop.

In summary, growers will want to manage their “crops after 2009 corn.” Growers should consider:

Planting N fixing soybeans on those fields where immobilization may be expected.
Banding nitrogen fertilizer away from crop residue to minimize immobilization 
Using the new Manitoba Nitrogen Rate Calculator. This bases N rates
according to crop and fertilizer prices and with current prices these
application rates are generally high for wheat, barley and canola. 
This calculator is found at
http://www.gov.mb.ca/agriculture/financial/farm/nitrogencalc.html 
Monitoring their crop for any N shortages and being prepared for a
supplemental application.  A “planned overlap” or an N-rich strip will
show if N is limiting growth.
Fully fertilizing the 2010 corn crop, knowing this ultimately aids in stover decomposition
Planning to be pleasantly surprised in the future.  Reluctantly
returning all this N in an unharvested corn crop usually pays off as
soil N release in years when yield potential is high.  An example is
the yields of cereals and canola in the years since working down much
of the 2004 crop.

* John Heard is Soil Fertility Specialist, with Manitoba Agriculture,
Food and Rural Initiatives. Reprinted with the permission of Manitoba
Agriculture, Food and Rural Initiatives.