By Greg LaBarge, CCA, Ohio State University Extension
A dry fall has led to an increased number of field fires. Farmers have asked a few questions about how a field fire impacts nutrients. A quick review of several Extension resources gives us helpful information. There are two things to consider in assessing the actual losses. One, how completely did the fire consume the residue? Second, what is the coverage area? The highest losses will be when the residue is absent.
What nutrients are lost?
Nitrogen and sulfur are volatilized and lost when residue is burned.
Our other macronutrients, phosphorus (P) and potassium (K) loss, generally have minimal losses. P and K will remain in the ash, and losses are related to any ash blown offsite.
How much nutrient is lost per acre?
The amount of nutrients lost is related to the amount of residue per acre and the nutrient content of the residue.
The amount of stover produced is related to grain production. For corn, multiply the field yield in bushels at 15% moisture by 47.6 to estimate the stover, according to R. Gelderman from South Dakota State University. For soybeans, multiply the field yield in bushels at 13% moisture by 52.2.
The amount of nitrogen in corn stover is 0.60%, and for soybean stover, 0.58%. The amount of sulfur in corn stover is 0.08%, and in soybean, 0.12%.
For example, a 180-bushel corn crop would have (180 * 47.6) 8,600 pounds of stover. Therefore, the N content would be (8,600 * 0.006) 52 pounds of N and (8,600 * 0.0008) 7 pounds of sulfur. The estimated nutrient value of $52 per acre for the N and $3 per acre for the sulfur, based on 2023 fertilizer prices. Remember, these values would be a situation where the residue burned entirely, and only ash remains.
What about organic matter effects?
This is a difficult question since converting residue into organic matter is a continual soil process. The impact of a single year without residue added is challenging to evaluate with our current test. Sawyer estimated where crop residue was burned a value of $1 per acre for organic matter loss in Iowa. Other research has estimated a range of $2 in tilled and $4 in no-till based on long-term tillage studies.
One potential corrective action is to plant a cover crop. The cover can replace some lost residue and protect a field from erosion.