by Leah Ten Napel
Once the combine leaves the field, it’s easy to think that the soybean crop is finished. However, the residue left behind continues to interact with the soil throughout the fall and winter and can affect conditions for the next growing season.
How that residue is distributed and managed can influence soil protection, nutrient cycling and the conditions into which next year’s crop will be planted.
Here are five things to remember:
1. Residue management begins with the combine. Management starts before any tillage pass is made. The way residue leaves the back of the combine can have a major impact on what happens in the field afterward.
Soybean plants produce less residue than corn, and soybean residue generally breaks down more quickly. However, the residue that remains still needs to be distributed as evenly as possible across the width of the combine header.
Paying close attention to residue distribution is important. That’s because areas with heavier residue can remain cooler and wetter in the spring, potentially affecting tillage, planter performance, seed placement and emergence.
A quick look behind the combine can tell you a lot. Is residue spread evenly across the full header width, or is it concentrated in a narrow band? Are there areas with excessive residue? Are soybean stems and other material being distributed uniformly?
Taking the time to make combine adjustments during harvest can help create more uniform field conditions for the following crop.
2. Soybean residue isn't all that's left behind. Once soybean residue is on the soil surface, it becomes part of the soil system. Residue can help protect the soil from wind and water erosion, reduce the impact of rainfall on the soil surface, and contribute organic material as it decomposes.
Soybean residue also plays a role in nutrient cycling. During the growing season, soybeans take up nutrients from the soil and incorporate them into plant tissue. After harvest, nutrients remaining in the stems, leaves and other plant material are gradually released as microorganisms break down the residue.
Some nutrients, such as potassium, are relatively mobile and can be released from plant residue fairly quickly, while other nutrients are tied up in organic compounds and become available more slowly as decomposition continues.
Because soybean residue has a relatively low carbon-to-nitrogen ratio compared with corn residue, it generally breaks down more quickly. The rate of decomposition still will depend on factors such as temperature, moisture and how much residue is in contact with the soil. As the residue breaks down, nutrients are returned to the soil and become part of the nutrient cycle available to future crops.
While this nutrient cycling is an important part of the system, the nutrients returned from soybean residue should not be viewed as a replacement for a sound fertility program. Soil testing and crop-specific nutrient recommendations should continue to guide fertility decisions.
3. The amount and distribution of soybean residue can influence fall tillage decisions. While tillage may be part of a grower's overall production system, residue management alone does not necessarily mean more tillage is needed.
The decision to till should consider the specific goals and conditions of the field rather than assuming residue automatically needs to be incorporated. Soil type, slope, erosion risk and crop rotation all play a role in determining the best management approach.
In some fields, maintaining residue on the soil surface may provide valuable protection throughout the winter. In others, a specific tillage operation may be needed to address a known field or planting concern. The important thing is to understand what the management pass is accomplishing before making it.
4. Soybean residue can impact next spring’s planting choices. One of the best reasons to pay attention to soybean residue this fall is that it can affect next year's planting conditions. Uneven residue distribution can create differences in soil temperature and moisture across the field. Heavy bands of residue may dry and warm more slowly, while areas with little residue may be more exposed to erosion and weather.
Uniform residue distribution helps create more consistent conditions across the field. This is especially important in reduced-tillage and no-till systems.
5. Take a second look at soybean residue before leaving the field. Harvest is a busy time, but it’s also one of the best opportunities to evaluate a field before next year's crop is planted. As soybean harvest progresses, take a few minutes to look beyond the yield monitor.
Look at how evenly residue is distributed. Identify areas with excessive residue or areas where little residue remains. Consider erosion risk and soil conditions.
Make notes about anything that may need attention before spring. A good soybean harvest is about more than getting bushels in the bin; it’s also about leaving the field in a condition that sets up the next crop for success.
Ten Napel is a field agronomist for Iowa State University Extension and Outreach.