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Cobs harvested as bioenergy feedstock - Photo provided by USDA-Agricultural Research Service

Trash or Treasure?

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Renewable energy is a hot topic these days and using "trash" from agricultural fields as a way to produce energy is in the news. But as the saying goes "one man's trash is another man's treasure." For some, corn stalks, corn cobs, and other non-grain plant material may seem like trash, while to farmers, soil scientists, and soil conservationists that plant material is treasure just waiting to be turned into soil organic matter (stored carbon).

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As the renewable energy industry develops, it is important to establish crop residue harvest rates that protect the soil resource. The importance of protecting the soil against erosion from wind and water has been recognized for decades. More recently, the importance of protecting soil organic matter (humus), which is more than 50% carbon, has come to the forefront. The first estimate of harvestable crop biomass only considered erosion factors, but soil organic matter content appears to be more sensitive to biomass removal than soil erosion.

What is the value of soil organic matter? Increasing soil organic matter increases soil carbon, which reduces carbon dioxide in the atmosphere. Reducing carbon dioxide in the atmosphere reduces the risk of global climate change. Soil organic matter is what gives Minnesota and other Midwest soils their characteristic rich, dark color. Soil organic matter is directly linked to many desirable soil properties, nutrient cycling, water-holding capacity, resistance to compaction, and good tilth. These are properties that support high yield potential.

How much non-grain plant material is there in a corn field? What is the

distribution of that material in a corn stalk? Could we use that information in the existing models to predict soil erosion if corn stalk removal occurs at different heights? How does removing the non-grain plant material affect soil organic matter?

These are some questions being studied by researchers from the

Agricultural Research Service in Morris, Minn., Lincoln, Neb.; and Ames Iowa in collaboration with David Lightle of the Natural Resources Conservation Service in Lincoln, Neb. In addition to Morris, Lincoln and Ames, data on the diameter and weight distribution of individual corn stalks have been collected in St. Paul, Minn.; Fort Collins, Colo.; Florence, S.C.; Auburn, Ala,; and Mandan, N.D. The samples are also being analyzed for carbon, nitrogen, and a suite of elements to determine what effect removing "trash" might have on soil erosion and organic matter.

We found that if corn stover was harvested at ear height, about half of

the biomass would be left in the field. Cobs represent about 20% of the

corn biomass produced. Can 20%-50% of the corn biomass be removed

without affecting soil carbon? That will depend on yield, and other

management practices. Previously published estimates suggest that 2 to 4

tons per acre of stover must be kept on the land to prevent loss of soil

organic matter in corn based systems. A corn crop yielding 200 bushels

would yield less than 1 ton per acre of dry cobs and about 6 tons per

acre total stover. Soil organic matter likely could be maintained if

only cobs were harvested. However, harvesting half the corn biomass may

not be reasonable, even with high yielding corn.

Long-term studies are in progress seeking to refine the harvest

recommendation to make sure that the soil resource is protected. Tillage

practice, crop rotation and use of cover crops are among the factors

being looked at that are expected to influence residue removal

practices. Some of this work is being conducted on local farmer's

fields.

This work is part of the USDA-ARS multi-location Renewable Energy

Assessment Project (REAP) which is working to provide regional and local

harvest recommendations that protect soils' ability to provide food,

feed, fiber and fuel.

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