Download Soil organic matter (SOM) is often considered to be an important

Stubble Management: Bale, Bury or Burn
Andrew Nadler, Manitoba Agriculture, Food and Rural Initiatives, Carman, MB R0G 0J0
E-mail: [email protected]
Introduction
Stubble burning has traditionally been practiced by agricultural producers in an effort to mitigate
the negative effects of excess residues from the preceding crop. The practice is appealing for
its short-term economics and effectiveness in dealing with materials that may often be
considered as byproducts. Excess residue is often blamed for impeding tillage and seeding
operations and delaying emergence due to cooler soils. Certain parts of Southern Manitoba are
in a situation where ample moisture and heavy soils often result in high crop production,
combined with abundant residues. Some of the areas that produce the most residues are
situated to the west, and southwest of the city of Winnipeg. When burning is used extensively
as a management practice in this region, the resulting smoke can be carried by the prevailing
westerly winds directly into the largest population centre in the province.
The Crop Residue Burning Regulation
Following a damp and late harvest season in 1992 which resulted in the persistence of heavy
smoke throughout southern Manitoba and the declaration of a state-of-emergency, a Controlled
Burning Program was implemented. Starting in 1993, the Burning of Crop Residue and NonCrop Herbage Regulation was enacted based on recommendations from a diverse group of
stakeholders. The intent of the regulation was to minimize the negative effects of smoke from
crop residue burning. The first component was to prohibit nighttime burning year-round. During
the night, temperature inversions often develop whereby the air aloft is warmer than the air near
the ground. These stable atmospheric conditions prevent smoke from rising above the layer of
inversion thus causing the smoke to linger near to the ground and not disperse vertically. A
second component to the regulation applies to the typical fall harvest season. From August 1
through November 15, the burning of crop residue is prohibited unless an authorization to burn
has been issued by the province. These authorizations are provided daily and are based on the
winds and smoke dispersion characteristics of that day. Since 1993, the program has been
reasonably successful at controlling smoke from crop residue burning and reducing the number
of smoke-related problems. Farmers have had the flexibility to burn during periods when smoke
is least likely to have negative effects downwind.
The 2007 Season
The high-yielding 2007 crop resulted in above average production of crop residues. While
harvest occurred early in the season leaving ample time for fall field work, the abundance of
straw left many farmers deciding to burn. As result of the shear magnitude of burning that took
place combined with some illegal burning during times of poor atmospheric dispersion, stubble
burning smoke was the cause of numerous complaints related to air quality and highway safety.
During the month of September, several motor vehicle accidents and road closures were
attributed to smoke and on one occasion a large part of Winnipeg was engulfed in smoke for an
entire night. These events led to a temporary province-wide suspension of crop residue burning
along with severe restrictions on burning for the remainder of the season. The restrictions on
crop residue burning in Manitoba are likely to continue; therefore alternatives to burning must be
explored and incorporated into farming practices. In situations where burning is completely
necessary, farmers must do so in a manner that poses minimal risk to the public.
The Agronomics of Crop Residue
To deal with excess residue, producers often resort to one of three options: 1) Finely chop and
incorporate the residue into the soil, 2) Remove the straw from the field, or 3) Burn off the
residue. The primary issue related to residue management is the benefits associated with
organic matter. Soil organic matter (SOM) is an important indicator of soil health. Maintaining a
sufficiently high level of SOM is a primary objective of sound soil management. The amount of
organic C and N within a soil is a balance between the amount of organic matter that is being
returned to the soil versus the rate of decomposition (Malhi and Kutcher 2007). Upon
converting soils from their undisturbed natural vegetation cover to agriculture, the amount of soil
organic carbon that is lost within 20 to 50 years is between 25% and 50% (Lal 2004). Longterm continuous straw burning can cause further reductions in carbon thus adversely affecting
the physical, chemical, and biological properties of the soil (Malhi and Kutcher 2007). The
benefits of soil organic matter include increases in a soil’s available water holding capacity,
improved aggregate stability, and better nutrient use efficiency (Lal 2006). The presence of
residue on the soil surface is also effective at reducing wind erosion, particularly when the
residue is standing (Bilbro and Fryrear 1994).
The nutrients that are held within – and subsequently released from residue must also be
considered. When straw is burned or removed, few or none of these nutrients will enter the soil.
According to work done in Manitoba by Heard et al. (2006), burning of residue resulted in the
loss of over 90% of C, between 98 and 100% of N, 24% of P, 35% of K, and 75% of S. When
the residues are left in place, most of the nutrients will be returned to the soil to be used by
subsequent crops.
Despite the numerous advantages of retaining crop residues and SOM, excessive amounts of
residue can be problematic. For example, straw that is left on the soil surface may plug
conventional tillage and seeding implements. To minimize these effects, many producers have
invested in straw choppers and straw spreaders on their combines. While these tools require
additional horsepower which also consumes more fuel, choppers and spreaders have been
found to be effective at chopping and distributing the residue. More suitable tillage equipment
such as heavy harrows have also become more common on farms.
One of the key elements to dealing with excess residue is to consider the amount of straw that
is being produced by the crop and whether this amount may be reduced. Farmers must
consider that some crops, such as wheat and oats produce an abundance of non-readily
digestible residue (Table 1). Other crops, such as canola and peas produce large quantities of
residue that break down rapidly in the soil. Efforts to reduce the overall amount of residue
should focus on rotations that allow adequate time between major straw-producing crops.
Table 1. Amount of straw produced from various crops (source: MAFRI).
Grain
Pounds of straw / bushel of grain
Canola
110
Wheat
100
Peas
100
Flax
70
Barley
48
The rate of decomposition of organic material is largely dependant upon the type of residue, soil
properties, and environmental factors. The carbon/nitrogen (C/N) ratio represents the amount
of carbon compared to the amount of nitrogen within a material. The active fraction of SOM with
a low carbon/nitrogen ratio provides food for soil organisms, enhances nutrient cycling, and
improves the structural stability of the soil. The passive fraction, which persists for hundreds to
thousands of years, enhances a soil’s ability to retain nutrients and water.
Soil microbes, which normally require about eight parts of organic carbon for every one part of
nitrogen, will respond to new organic material that is added to the soil. If this material has a C/N
ratio greater than 25, the microbial demand for N (NH4+, NO3-) will restrict the availability of N to
higher plants (Figure 1). This nitrate depression period persists until the C/N ratio of the organic
material decreases to approximately 20/1 as carbon is gradually removed by respiration. At this
point, both the levels of soil humus and nitrogen would be higher than they previously were.
Table 2 provides some typical C/N ratios of several common organic materials.
Figure 1. Example of nitrate depression period.
Table 2. Typical C/N ratios of selected organic materials (Source: Brady & Weil 1996).
Organic Material
%C
%N
C/N
Spruce sawdust
50
0.05
600/1
Wheat & oat straw
38
0.5
80/1
Corn stover
40
0.7
60/1
Mature alfalfa hay
40
1.8
25/1
Rotted barnyard manure
41
2.1
20/1
Young alfalfa/clover
43
3
13/1
Fungi
50
5
10/1
Bacteria
50
10
5/1
The rate of residue breakdown is a function of the microbial activity within the soil which
requires suitable temperature and moisture conditions. Microbes are most active when soils are
at or near field capacity. As the soil dries, the amount of activity decreases. Warm soils also
promote decomposition where optimum activity occurs at 32°C with declining activity above and
below this temperature (Steiner et al. 1997). At temperatures between 5°C and 10°C, microbes
become relatively inactive (Rabenhorst 2005). While heavier textured soils often retain more
water than coarser soils, they also tend to remain cooler, particularly when the soils are poorly
drained in the spring after the snow melts. This can slow the rate of decomposition resulting in
more residues being present at the time of spring seeding. Figure 2 shows the soil
temperatures at a weather station located near Starbuck from April 1 though October 31, 2007.
In this case, the soil temperatures rise above 5°C in early April and remain above that
temperature until after October 31. A late harvest can limit the amount of time available for
microbial breakdown of organic matter prior to winter freeze-up. Upon spring thaw and after
seeding, the nitrate depression period could correspond with crop emergence, allowing very
little N to be available for plant uptake. For this reason, soil fertility should be observed closely
and additional N should be applied if necessary.
Figure 2. Daily maximum soil temperature at Starbuck weather station – 2007 season.
Conclusion
There is no single solution to dealing with excess crop residue. Every cropping system is
unique with regards to soil, climate, and cultural practices. A solution that works in one part of
the province may not work elsewhere. While organic matter is a valuable resource that should
be returned to the soil, certain cropping practices or unique situations make this option difficult.
The air quality and safety issues associated with crop residue burning make it a less desirable
option. If the residue cannot be retained, it should ideally be used for other purposes such as
fibre or energy. In these cases, Geography will dictate the distance to potential markets which
will determine whether the transportation of straw is economical. In the near future, it is hoped
that new markets for straw will emerge and the dependency upon burning will decrease.
References
Bilbro, J.D. and Fryrear, D.W. 1994. Wind erosion losses as related to plant silhouette and soil
cover. Agron. J. 86: 550-553.
Brady, N.C. and Weil, R.R. 1996. The nature and properties of soils, 11th edition. Prentice
Hall, New Jersey, USA.
Heard, J., Cavers, C., and Adrian, G. 2006. Up in smoke – nutrient loss with straw burning.
Better crops. 90: 10-11.
Lal, R. 2004. Soil carbon sequestration impacts on global climate change and food security.
Science. 305: 1623-1627.
Lal, R. 2006. Carbon management in agricultural soils. Mitigation and adaptation strategies for
global change. 12: 303-322.
Malhi, S.S. and Kutcher, H.R. 2007. Small grains stubble burning and tillage effects on soil
organic C and N, and aggregation in northeastern Saskatchewan. Soil & Tillage Res.
94: 353-361.
Rabenhorst, M.C. 2005. Biologic zero: A soil temperature concept. Wetlands. 25: 616-621.
Steiner, J.L., Schomberg, H.H. and Unger, P.W. 1997. Predicting crop residue decomposition
and cover for wind erosion simulation. Proceedings of Wind erosion: An international
symposium. June 3-5, 1997. Manhattan, Kansas, USA.