Tossing a match to improve planting conditions by eliminating field stubble may seem like an inexpensive and convenient tool for many farmers, and in the short-run it likely serves a purpose.

Burning residue may make fieldwork and planting easier, but the fire it ignites comes with a hidden long-term price tag – one that detracts significantly from the farm’s bottom line.

A study released by Utah State University 10 years ago, drawing on 30 years of research, pointed to a steady reduction in soil health with annual stubble burning, a practice that ultimately reduces soil productivity to a point no amount of increased mineral fertilizer additions will compensate for the loss.

USU literature based on those studies says long-term burning practices also significantly affects total carbon and nitrogen pools. The authors note an improved C:N ratio under residue retention increases and maintains higher microbial activity, which ensure more rapid organic matter decomposition and nutrient release in the soil.

Burning also decreases readily-assimilated carbon sources for microbes, decreases soil ammonium levels and available soil phosphorus, in addition to leaving the soil prone to wind and water erosion. Valuable plant nutrients become an expensive plume of smoke and must be replaced with the addition of commercial or natural fertilizer.

With corn stover, Iowa State University estimates the nutrient removal per ton of feedstock at about 6 lb./A for phosphorus, and 25 lb./A for potassium. With an example price of $0.38 per lbs[J1] . P2O5, and $0.24 per lbs. K2O, the per ton cost of removal – through a harvester or fire – would be $2.24 for P2O5  and $6 for K2O. 

ISU’s 14-year estimates of corn vegetation per ton nutrient composition at plant maturity in pounds are as follows[J2] :

  • Carbon                   840
  • Nitrogen                   12
  • Phosphorus               3
  • Potassium                22
  • Calcium                      9
  • Magnesium                6
  • Sulfur                          1
  • Zinc                       0.03
  • Manganese           0.069
  • Copper                  0.015
  • Boron                    0.015
  • Iron                       0.281

 

For winter wheat, the figures are similar, says Oklahoma State University precision nutrient specialist Brian Arnall.

“We estimate a ton of wheat straw to contain 800 lbs. of carbon, 20 lbs. of nitrogen, 2 lbs. of sulfur, 3 lbs. of phosphorus, and 30 lbs. of potassium,” he explains, noting the ‘burned and done’ expense of torching wheat stubble over the years. Also, as growers seek to improve soil organic carbon content, stubble fire is extremely counter-productive to those efforts as well.

“Ninety percent of carbon in standing wheat stubble is lost when that stubble is burned,” he says. 

Still, after-harvest burning continues across the Corn Belt and Great Plains, many times in the name of insect and weed control.

USU researchers say in some instances late-season stubble fires tend to lower populations for species such as black grass bugs and concede that might be a special case for burning residue.

For future weed suppression, however, long-time retired OSU soil scientist Jim Stiegler often cited research indicating fires on standing residue usually never burned hot enough, or long enough to show a significant reduction in field weed pressure.