Ten fundamentals about bioenergy: Part 5 corn crop residue is beneficial for soil organic matter
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Fundamental #5: Keeping some crop residue in the field has a high agronomic, environmental and economic value.
This is the fifth in a series of articles on bioenergy. This issue’s installment addresses the importance corn crop residue plays in maintaining soil organic matter (SOM) levels. Corn crop residue, a.k.a. corn stover, has been identified as an important feedstock for future cellulosic ethanol production. While corn stover will likely be an important revenue source for farmers as a bioenergy feedstock in the not-too-distant future, it is important to understand the agronomic and environmental value associated with keeping some corn stover in the field.
In Michigan, our soils were predominately formed on the surface of a hundred feet or so of sand, silt, clay, gravel and boulders (glacial drift) which was deposited by glaciers. Because of our latitude, the soils formed primarily under tree cover over a period of 10,000 years since the last glacier retreated back to Canada. Soils formed under tree dominated landscapes are generally referred to as “alfisol” soils and they are characteristically shallower in topsoil depth, lower in organic matter, and nutrient poor relative to “mollisol” soils which form under prairie grasslands in the area now known as the Central Corn Belt. This is important to Northern Corn Belt growers because our inherently low organic matter soils require inputs of carbon from sources such as crop residue to maintain SOM at current levels.
How much corn stover can we afford to sell?
Of course the answer to that question depends upon many farm specific economic and agronomic factors, but we can make a simple estimate based on maintaining current soil organic matter levels. We know that on average due to microbial decay, we need to replenish about 2.2 percent of our total soil organic matter annually. In a two million pound acre furrow slice, a soil with 2.5 percent soil organic matter will lose (2,000,000 lb soil x 2.5 percent SOM) x (2.2 percent decay) equals 1,100 lb SOM annually. We also know that it takes about 6 lb of corn stover to replace 1 lb of SOM, so, (1100 lb SOM decay x 6 lb corn stover) equals 6,600 lb crop residue needed to be returned to soil. However, in addition to above ground crop residue, there is also carbon available from root residue. In round numbers, a corn (150 bu/A provides " 4450) rotation will supply an average of about 9,335 lb/A/yr crop residue. Therefore, in a corn soybean rotation, we can spare (9,335 lb/A/yr crop residue – 6,600 lb crop residue needed) equaling 2,735 lb of crop residue annually, or (2,735 lb x 2 year) equaling 5,470 lb crop residue removed during the “corn” year of the corn-soybean rotation.
Each ton of corn stover also contains about 22 lb N, 8 lb P2O5, and 32 lb K2O. The value of these nutrients should factor into a grower’s calculations in pricing corn stover. Other agronomic values associated with SOM include increased water permeability, improved nutrient cycling, quicker spring warm-up due to dark color, and, increased water holding capacity. Many environmental advantages are also associated with leaving some crop residue on your soil, particularly in reducing erosion. Finally, cultural management practices such as manure applications and the use of cover crops can successfully be used to augment carbon lost with crop residue removal.
Read part 1, part 2, part 3, part 4, part 6, part 7, part 8, part 9, part 10 of this series.
