No. 16 Article 4/July 22, 2011

Should I Do Something to Get Rid of Wheat Stubble?

Crop residue is useful in protecting soil from wind and water erosion, protecting soil moisture from evaporation, stabilizing moisture and temperature conditions in the soil surface, increasing microbial activity in the soil, and providing a good source of nutrients for the next crop and carbon for the soil. On the other hand, crop residue can make it challenging to plant a new crop, increase the potential for soilborne diseases, and potentially tie up nutrients temporarily for the following crop. Farmers who harvested wheat are now deciding what to do with the leftover straw. Some have baled it, while others want to keep the entire residue in the field. For those who baled, I recently described how to estimate the nutrient value of that straw.

Here I will address options for managing fields where the entire residue was left behind. The ideal situation is to leave enough residue on the soil surface to protect against erosion but allow planting and other operations to take place without interference. Unless you are prepared to plant double-crop of soybean or a cover crop, leaving residue on the soil surface will result in very little trouble for planting operations next spring, so I would leave it in place and not worry about trying to speed up its decomposition. If you plan to till the soil, I recommend waiting until fall to reduce the time that the soil has less residue cover.

If crop residue is buried by tillage, burned, or decomposes early, the chances increase that it won't provide sufficient soil surface cover to protect against erosion in late fall and the following spring. Since wheat is harvested in the summer, its residue has a long time to break down and decompose by natural processes and leave the soil surface with relatively little residue by the time corn and soybean planters start to roll next growing season. Breakdown and decomposition occur through physical, chemical, and biological processes. Natural physical breakdown happens through wind, freeze-thaw and wet-dry cycles, and animals. Of course, physical breakdown can be speeded by tillage or chopping, which exposes more residue surface area to water and microorganisms. Chemical breakdown is mostly the result of water washing carbon and nutrients out of the residue. In the article about straw nutrient value, I noted that rain can very quickly leach a substantial amount of potassium from wheat straw.

As important as physical and chemical processes might be, biological processes (microbial decomposition) are far more important in the reduction of crop residue. Keep in mind that not all wheat residue is created equal. The ratio of carbon to nitrogen (C:N) is a very important component that can affect the rate of residue decomposition by microorganisms. The higher the ratio, the more time it will take soil microorganisms to decompose residue. Leaf blades typically have the lowest C:N ratio (typically below 40:1) and are the first to be decomposed; leaf sheaths and chaff are next, and the most persistent material is the stems, which typically have a ratio above 80:1. A rough estimate of proportions of total residue is that leaf blades comprise no more than 15%, sheaths and chaff about 55% to 60%, and chaff about 35% to 40%.

The typical value used to determine whether microorganisms need additional nitrogen to break down residue is a C:N ratio of 25:1. Wheat materials have a higher ratio, so soil microorganisms will require additional nitrogen to decompose them. Microorganisms use nitrogen available in the soil to break down the residue. In doing so, they temporarily tie up nitrogen, but that nitrogen plus the nitrogen present in the wheat stubble will become available for the next crop. Some people like to apply nitrogen to help speed the process of decomposition. A common rate is 10 to 15 lb of nitrogen per ton of straw.

Since a wheat field produces about 2.5 tons of residue, that means applying 25 to 40 lb of nitrogen per acre. In my view, that is unnecessary for several reasons: it speeds decomposition, potentially leaving less residue cover on the soil surface; it costs more because of the additional nitrogen fertilizer; and it may increase the chance for nitrogen loss to the environment since nitrogen is being added long before the next crop will be ready to use it. As I mentioned, microorganisms will use nitrogen already present in the soil, and in doing so they will transform inorganic nitrogen into organic nitrogen that is not susceptible to leaching or denitrification. Although it is not very likely, if by the time the next planting season arrives a lot of residue remains, with nitrogen tied up in it, you can apply nitrogen then. That will provide enough to get the corn crop off to a good start and greatly lessen the chance for nitrogen loss, and the nitrogen present in the residue will become available in time to be used by next year's crop.--Fabián G. Fernández

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