Fertility and Nutrients
Crop residues – the big picture
By Treena Hein
In deciding whether or not to remove crop residues, particularly corn stalks, farmers need to consider the big picture, says Joel Bagg, forage specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs. “There is always more interest in crop residues when forage is scarce,” he says. “It comes down to what’s available, but farmers need to remember that residues aren’t free."
”The first thing livestock farmers turn to for extra feed, says Bagg, is corn silage. Corn stalks are also an option, but rather than baling them, farmers can avoid these costs and allow the cattle to graze. “The limitation on that, however, is that these corn fields are not generally fenced, but you could use temporary fencing or inexpensive fencing,” says Bagg. However, he adds a cautionary note, “Everyone’s always looking for feed, but if you can’t recover your phosphorus (P) and potassium (K) and organic matter in terms having to add extra fertilizer, you’re better to leave residues in the field.”
Dr. Jim Camberato, associate professor in the Agronomy Department at Purdue University in West Lafayette Indiana, agrees. “The amount and value of P and K in stover clearly should be considered a cost of stover baling. Other essential plant nutrients, such as nitrogen, calcium, magnesium, sulfur and the micronutrients are also taken away when stover is harvested and should be figured into the nutrient cost of stover baling, especially in low nutrient soils and with long-term stover removal.”
Camberato says corn stover contains about 3.6 lb P2O5 and 20 lb K2O on a per ton basis (based on stover concentrations of 0.18 percent P2O5 and 0.99 percent K2O). The 2.5 tons per acre removed from a field producing 150 bushels per acre contains about nine pounds of P2O5 and 50 pounds of K2O. He calculates the current value of these nutrients, using a price of $0.75 US per pound of P2O5 or K2O to be about $44 US per acre or $18 US per ton of stover.
Camberato’s colleague Dr. Tony Vyn says the greatest nutritional values of crop residue, especially with corn, are right after harvest. Soluble sugars will be reduced in precipitation as well as K, with K being one of the nutrients lost the fastest. “In terms of total nutrient return to the soil,” says Vyn, “harvesting later is best.”
A look beyond feed to fuel
Nutrient value aside, crop residues are also important to leave on the field in terms of their capacity to reduce soil erosion and replenish soil organic matter. “Corn stover removal eventually will lead to reduced soil quality through a reduction in soil organic matter,” says Camberato. “Compaction during stover removal is another factor that may reduce soil quality and should be minimized.”
Vyn says the amount of residue needed for soil quality and erosion control depends on the soil quality that already exists, and how susceptible the field is to erosion. There are currently investigations occurring in the states of Indiana, Iowa and Illinois into the impact of removing 50 and 100 per cent of residue, but Vyn says there are no conclusive findings yet. However, he believes it is safe to say “Fifty per cent removal of stover would give adequate erosion protection in no till or strip till, but you are removing a food source for soil microbes." Vyn adds, “I have much less concern about farmers taking residue from the fields of a cow-calf operation because they’re returning nutrients to the soil via manure. In a cash crop situation, the concerns are more serious.”
There is also research occurring at Iowa State University, says Vyn, into harvesting the top half of the corn stalks at the same time as the grain is threshed. “This is an interesting approach as it is all harvested at once, saving energy,” says Vyn. “In addition, the top half is least likely to have soil splashed on it.”
This is important, Vyn says, because some American electricity generation plants are considering the use of crop residue biomass to generate electricity by burning it along with coal, and soil is an undesirable addition. “The plants are being mandated to derive 8-15 percent of total generation from renewable sources,” says Vyn. “It varies by state as to when that mandate becomes law. They are also looking at wind, solar and hydro-electric possibilities.”
In Ontario, “The Ontario Power Authority estimates that an additional 450 MW of energy could be produced by biomass projects in the province by 2027, five times the current and committed capacity,” as stated on the Ontario government’s website. Baled corn stalks could be used, says Bagg, in cellulosic ethanol production in the future, “if there is a breakthrough and it’s worth the cost of harvest and transportation. There are a lot of economic unknowns with that.”