It has been a dry winter and several fires have moved accidentally into cornstalk fields in recent weeks. Regis Voss, retired Iowa State University soil fertility extension specialist, wrote an article for the May 14, 1993 ICM newsletter on mineral element losses when cornstalks burn. Following is that article.
"What is lost? The mineral elements such as phosphorus and potassium will be in the ashes. Most of the nitrogen can be lost and you can assign a dollar value to this loss. How much nitrogen is lost? You can base an estimate on some assumptions. In the mid-range of corn yields, the dry matter weight of stalks, leaves, and so on, is approximately equal to the dry matter weight of grain. The only measure available is some estimate of corn grain yield. The nitrogen concentration in corn plant residue without the grain is about one percent.
"An example is 140-bushel corn grain yield. Take 140 x 56 to equal 7, 840 pounds. Take 7,840 x 0.845 to equal 6,625 pounds. (This has corrected the plant residue to a dry matter basis because the grain yield was a 56-pound bushel at 15.5 percent moisture.) Take 6,625 x 1 percent N to equal 66 pounds of nitrogen. Assign some price per pound of nitrogen and this would be the estimated value lost.
"You may wish to assign a value for loss of organic matter. The effect on organic matter of removing all crop residue versus leaving it for a one-year period cannot be detected in an organic matter analysis. By using some standard values for what ends up in the soil organic matter, the addition of the above example of 6,625 pounds of cornstalk residue would increase soil organic matter by 0.056 percent. There are no assigned dollar values for organic matter, but we know it has some value. We usually suggest a dollar an acre for this loss."
A few additional considerations. The estimate of N loss can be adjusted up or down for yields higher or lower than the example calculation in the 1993 article. This is easily done from the assumptions that cornstalk residue contains approximately 1 percent N and that the residue is approximately equal to the grain dry matter produced. This adjustment will not be exact, but it should provide a reasonable estimate for production fields with adequate fertility.
If a burnt field is in a corn-soybean rotation, then it is possible that the N loss would not have a significant influence on the subsequent soybean crop. The N lost essentially is not important because soybeans symbiotically fix N and can compensate easily for less available soil inorganic-N.
Sulfur (S) contained in the cornstalk residue will be affected in the same manner as N. The amount would be approximately 15 lb S/acre. However, assigning a value to the S is questionable because there is S deposited in precipitation, and for Iowa soils, organic matter mineralization supplies considerable plant-available S. Also, sulfate-S is easily leached from soil; therefore, not all the S from residue would remain crop available.
Assigning a value to lost organic matter is difficult. There are no universally assigned values for crop residue or soil organic matter. And compared with organic matter levels in soils, the amount that would normally remain after residue decomposition from one crop is very small (long term, approximately 10 percent of carbon in crop residue will remain as soil organic matter). With the advent of carbon trading, cornstalk consumption for energy production, and cornstalk bedding for swine production, perhaps values will become standardized for organic carbon. Loss of surface residue may be a short-term issue for erosion control.
We know from laboratory procedures that dry ashing at high temperatures is an accepted method for determination of many mineral elements, such as phosphorus (P) and potassium (K). Therefore, these elements will remain in the ash. If ash is blown from the field, then losses would occur (someone else will ultimately benefit when the ash reaches the ground). It would be difficult, however, to determine the extent of this loss. Also, weathering of the residue since last fall would normally cause much of the K, and some N and P, to be moved out of the residue and into the soil where it is not subject to off-field loss.
This article originally appeared on pages 30-31 of the IC-484 (4) -- April 10, 2000 issue.