Factors to consider before burning wheat residue
Following wheat harvest there are some producers who might be thinking about burning their wheat stubble. Reasons some producers consider this practice is to control volunteer plants, weeds, and certain diseases, among other reasons. While burning is inexpensive, producers should understand the true value of residue ahead of time. Some of the information below comes from K-State Extension publication MF-2604, The Value of Crop Residue.
There are four main factors to consider.
Loss of nutrients
The products of burned wheat stubble are gases and ash. Nutrients such as nitrogen (N) and sulfur (S) are largely combustion products, while phosphorus (P) and potassium (K) remain in the ash. When residue is burned, about one-third to one-half of the N and S will combust. The nutrients in the ash may remain for use by the plants, if it doesn’t blow away first. Therefore, instead of cycling these important plant nutrients back into the soil, they can essentially become air pollutants when the residue is burned.
Amounts of nutrients remaining in wheat stubble (assuming 50 bu/ac yield)
Pounds present in 5,000 lbs of wheat straw
Protection from soil erosion
Bare soil is subject to wind and water erosion. Without residue, the soil will receive the full impact of raindrops, thus increasing the amount of soil particles that may become detached during a rainfall event. Bare, tilled soils can lose up to 30 tons per acre topsoil annually. In no-till or CRP systems where residue is left, annual soil losses are often less than 1 ton per acre. The detachment of soil particles can lead to crusting of the soil surface, which then contributes to greater amounts of sediment-laden runoff, and thus, reduced water infiltration and drier soils.
Leaving residue on the field also increases surface roughness, which decreases the risk of both wind and water erosion. Most agricultural soils in Kansas have a “T” value, or tolerable amount of soil loss, of between 4 and 5 tons per acre per year, which is about equal to the thickness of a dime. To prevent water erosion, 30% ground cover or greater may be needed to reduce water erosion to “T” or less, especially in fields without erosion-control structures such as terraces.
Standing stubble is more effective at preventing wind erosion than flat stubble. Several accidental residue burns have occurred in 2012 and 2013, with devastating wind erosion events that happen over and over again until a new ground cover is established. Once a field begins to erode, it is extremely difficult to stop. During extended droughts the soil profile gets dried out and not even emergency tillage is effective at stopping the wind erosion. Losing topsoil degrades soil productivity, and the long-term effect of this loss is not easy to quantify.
Moisture infiltration rates and conservation
Wheat residue enhances soil moisture by increasing rainfall infiltration into the soil and by reducing evaporation.Residues physically protect the soil surface and keep it receptive to water movement into and through the soil surface. Without physical protection, water and soil will run off the surface more quickly.
Ponded infiltration rates were measured at Hesston in September 2007. Very low infiltration rates (1.9 mm/hr) were observed for continuous winter wheat in which the residue was burned each year prior to disking and planting the following crop. In contrast, high infiltration rates (13.3 mm/hr) were observed for a no-till wheat/grain sorghum rotation.
Another way residue increases soil moisture is by reducing evaporation rates. Residue blocks solar radiation from the sun and keeps the soil surface cooler by several degrees in the summer. Evaporation rates can decline dramatically when the soil is protected with residue. Research from dryland experiments has shown that crop residues are worth 2 to 4 inches of water annually in the central Great Plains states.
Soil quality concerns
Over time, the continued burning of cropland could significantly degrade soil organic matter levels. By continually burning residue, soil organic matter is not allowed to rebuild. Soil organic matter is beneficial for plant growth as it contributes to water holding capacity and cation exchange capacity. Soil organic matter binds soil particles into aggregates, which increases porosity and soil structure and thus, increases water infiltration and decreases the potential for soil erosion. One burn, however, will not significantly reduce the organic matter content of a soil (unless the field erodes, as discussed above).
Considerations for fields that will be burned
If producers do choose to burn their wheat stubble, timing is important. It’s best to burn as late as possible, close to the time when the next crop is planted. This minimizes the time that the field will be without residue cover and vulnerable to erosion. If weed pressure is significant, producers should control the weeds before burning to prevent them from producing seed.
Before choosing to burn residue, producers should check with the USDA Natural Resources Conservation Service and/or the Farm Service Agency to find out if this will affect their compliance in any conservation programs.
For more information, see:
* Efficient crop water use in Kansas, MF3066, available at: http://www.ksre.ksu.edu/bookstore/pubs/mf3066.pdf
* Emergency wind erosion control, MF2206, available at: http://www.ksre.ksu.edu/bookstore/pubs/MF2206.pdf
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