Better Farming February 2017

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50 The Business of Ontario Agriculture Better Farming February 2017
CROPS: YIELDMATTER$
(Source: Mario Tenuta, Department of Soil Science,
University of Manitoba, Soil Health Research Forum,
Guelph, Nov. 28.)
Crop residue impacts on SOM
The question becomes: how do the various crop residues
in a rotation impact the SOM content?
There are three things to understand when determin-
ing how crop residues impact SOM (or any other organic
additions).
First, there is an annual loss of SOM due to normal
microbial respiration of CO2. That annual loss ranges
from 0.5 to 5.0 per cent. To determine the total amount
of SOM in an acre we assume soil weighs 2,000,000 lbs at
a depth of 6 inches (15 cm). Therefore a soil with 3 per
cent organic matter contains 2,000,000 x 3 per cent =
60,000 lbs/ac. Applying the loss factor produces a range
of 300 lbs to 3,000 lbs. For illustration we will use a value
of 2 per cent which indicates an annual loss of 1,200 lbs/
ac of SOM.
Second, we need a way to calculate crop residue
additions, so we use harvest indexes. A harvest index is
the grain yield divided by the sum of grain yield and
above ground stover yield. For corn, soybean and wheat
that ratio is approximately 0.5. This ratio indicates for ev-
ery pound of grain produced there is a pound of stover.
In addition to above ground biomass we need to include
the root mass. For corn there is equal biomass both
below and above ground. For soybeans, there is 20 per
cent more biomass below ground and for wheat there is
60 per cent more below ground.
The third piece to understand is retention factors:
after the microbes have had their fill, only 10 to 20 per
cent of the residue is retained as permanent SOM. We
will use a middle factor of 15 per cent retention for our
example below.
Putting it all together, we’re working with a typical
corn-soybean-wheat rotation with 2,000 lbs of straw sold
off and 3 per cent organic matter soil.
As illustrated, a corn-soybean-wheat rotation produc-
es a lot of residue – over 41,000 lbs. After respiration by
soil micro-organisms, only 2,582 pounds are left to
contribute to SOM. In this case, the remaining amount
would change the soil test value theoretically by 0.13 per
cent (2,582/2,000,000 lbs x 100).
When corn yields approach 250 bushels per acre, the
net contribution to SOM jumps to 3,000 lbs/ac from
1,992 lbs/ac. Higher-yielding corn supports removal of
some stover to support the emerging bio economy
without negatively impacting SOM.
To cover the annual loss of SOM in this example
requires a breakeven addition of 8,000 lbs of crop
residue/acre/per year.
Let’s look at two years of soybeans and one year of
wheat with the same amount of straw sold (2,000 lbs):
As the above table shows, a rotation of two years of
soybeans and a year of wheat, accompanied by the sale of
2,000 lbs of straw, does not help to build SOM. As a
matter of fact, this rotation would theoretically result in a
drop in SOM by 0.01 per cent.
Crop Yield
bu/
ac
Total
residue
produced
lbs/ac
Retention
of
residues
at 15%
lbs/ac
Annual
SOM
loss at
2%
lbs/ac
Gains
(losses)
SOM
lbs/ac
Soybeans 48 6,336 950 1,200 (250)
Soybeans 42 5,544 831 1,200 (369)
Wheat 80 10,480* 1,572 1,200 372
Total 22,360 3,353 3,600 (247)
* 2,000 lbs of straw sold.
Crop Yield
bu/
ac
Total
residue
produced
lbs/ac
Retention
of
residues
at 15%
lbs/ac
Annual
SOM
loss at
2%
lbs/ac
Gains
(losses)
SOM
lbs/ac
Corn 190 21,280 3,192 1,200 1,992
Soybeans 48 6,336 950 1,200 (250)
Wheat 100 13,600* 2,040 1,200 840
Total 41,216 6,182 3,600 2,582
* 2,000 lbs of straw sold.
A corn-soybean-wheat rotation
produces a lot of residue.
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