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Non harvested soybeans: Can you expect an extra N credit?

By Daniel Kaiser, Extension Soil Fertility Specialist

With the recent flooding or late season hail there may be questions on whether a credit can be taken from soybeans not harvested for the next year's crop. Soybeans are a high protein crop which means they can contain a large amount of nitrogen. Average values of nitrogen removed in soybean grain are reported at around 3.8 lbs of N per bushel (Source IPNI) for a total of 190 lbs of N in a 50 bu/ac soybean crop. In comparison corn grain would remove about 0.90 lbs of N per bushel and a total of 180 lbs of N in a 200 bu/ac crop.  Can all of this nitrogen be counted on if the soybeans cannot be harvested and are plowed under if they cannot be harvested?

In theory if the soybeans are plowed under there should be additional nitrogen available for the next year's crop due to the unharvested grain.  However, if all of the N in the grain is tied up in protein or other organic compounds it must be broken down to either ammonium or nitrate to be available for the next crop.  This breakdown takes time therefore it may be better to plow the crop under sooner in the fall to allow for decomposition of the grain.  Since the source of N is subject to microbial decomposition some of the nitrogen will be tied up in microbial biomass, but this amount will likely be low due to the large amount of nitrogen in the grain.  An article written by Dr. John Sawyer at Iowa State University suggests that as much as 80% of the N could potentially be available (Click here for a copy of that article).  However, little to no field research data has looked at this particular subject.

A research project studied the effects of unharvested soybeans in 2007 on the yield of corn in 2008.  A field near the Zumbro river was studied that had flooded and the soybeans on part of the field buried in silt deposited by the water.  Half of the field was harvested for grain while the other half was too difficult to harvest.  Identical nitrogen rate trials were established close to the boundary in the field where the soybeans had and had not been harvested and both areas were tilled in the fall.  Reported past yield from the field were near 50 bu/ac of soybeans therefore an average of nearly 190 lbs of N should have been added to the soil in the soybean grain assuming average nutrient uptake values.   

The data collected in 2008 shows that nitrogen was needed to maximize yields when both the soybeans were and were not harvested.  The graph at the right shows the data collected from the two studies.  In both studies maximum corn yield was nearly 230 bu/ac.  When the soybeans were harvested it took about 106 lbs of N to achieve maximum yield.  When the soybeans were not harvested yields were increased until 67 lbs of N.  At this field it appeared that the N credit from the unharvested beans were only 40 lbs of N and not the total amount that could have been contained in the soybean grain.  It should be cautioned that this is only one trial one year, but it may provide evidence that a 1:1 N credit should not be taken when soybean grain is returned to the field while a small N credit may be expected from the unharvested grain.

What should be done if you are unsure about the amount of N to credit from unharvested soybeans?

  • Decide whether the field areas are large enough that it is worth taking an additional N credit.  If areas are relatively small (<1 acre) it may not be worth crediting extra N due to uncertainty of availability.
  • If field areas are large a small N credit may be warranted but do not assume all N taken up from the current soybean crop will be available for the next crop.
  • If significant decomposition results, a 2' soil N test taken in the spring may give an indication of the additional N available for the next years crop in areas where this test is recommended.  Since the N will need to be mineralized all the N may not show up in a sample taken early in the season.
  • If a credit is taken where the soil N test is not recommended, evaluate the field for potential N deficiencies early enough in the growing season in order to apply N as a side-dress if a deficiency is likely.
  • A small starter rate of N could be applied if none is broadcast pre-plant in order to supply crops with N early in the season if conditions favor low mineralization or to potential uncertainties in availability.
In addition to N remember that phosphorus, potassium, sulfur, and any micronutrients contained in the grain will be returned to the soil.  Subsequent soil testing will pick up any changes resulting from these additions.  Similar availability concerns should be considered for phosphorus or sulfur but any potassium in the grain released during decomposition should be readily available for the crop.

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