When it comes to factors that affect nutrient planning, the carbon penalty is a big reason every field or soil management zone needs its own fertility plan.
“The carbon penalty occurs because crop residue on the surface or shallowly incorporated causes microbe populations to explode,” explains Ken Ferrie, Farm Journal field agronomist.
“The microbes consume soil nutrients, making them unavailable to plants. Those immobilized nutrients are released later in the season, after the microbes die and decompose, but you must ensure plants don’t go hungry in the meantime.”
Spring immobilization ties up many nutrients, primarily nitrogen, phosphorus and sulfur.
“The presence of the carbon penalty, and its size, depend on many factors. Your challenge is to weigh all those factors and apply enough nutrients to prevent crop stress, without applying extra that may be lost to water supplies,” he adds.
By focusing only on immobilization, Ferrie notes, we oversimplify the situation: “In reality, immobilization and its opposite, mineralization — when nutrients become available again — occur simultaneously. But at various times we have either net immobilization or net mineralization. An effective 4R fertility program takes both processes into account.”
Here are some factors to plan for to cope with the carbon penalty:
1. Type and quantity of carbon.
“The higher the carbon-to-nitrogen ratio, the more immobilization occurs,” Ferrie says. “Corn stalks have a higher carbon-to-nitrogen ratio than soybeans. Some cover crops have higher carbon-to-nitrogen ratios than others.”
2. Crop rotation.
Carbon penalties are higher in corn following corn and corn behind wheat where the straw wasn’t baled, compared to corn following soybeans or silage,” Ferrie says.
3. Location of the residue.
“Shallowly incorporated residue decomposes faster than residue on the surface, so there is more nutrient immobilization,” Ferrie explains. “Conversely, residue plowed deep into the soil, beneath the aerobic zone, doesn’t contribute to immobilization because the organisms that decompose it need oxygen to live.”
4. Type of tillage.
“When you bury residue at the depth where fence posts rot off, you put all the carbon within access of decomposing microbes,” Ferrie says. “Immobilization will increase dramatically when the soil warms up and the organisms become active.”
5. Timing of tillage.
“If you till residue in the fall while the soil is still warm, some carbon will decompose in the fall, creating less carbon penalty in the spring,” Ferrie says.
6. Weather.
“As we move north, due to later harvesting and cooler climate, many fields are chiseled after the soil cools down in the fall or in the early spring,” Ferrie says. “This creates some of the harshest carbon penalties. The same thing can happen farther south if we have dry fall weather — we need moisture in the soil to drive decomposition.”
Some rules of thumb to start your planning process: “In central Illinois, Farm Journal studies show that corn following soybeans needs at least 60 lb. per acre of surface-applied nitrogen to pay the carbon penalty. If we chisel corn residue and then plant corn again, we need at least 100 lb. per acre — and I’ve seen spring-chiseled corn stalks take up 150 lb. of nitrogen,” Ferrie says.
“Farther north, with less time for residue to decompose in the fall, I’d expect the minimum rate to go up.”
Keeping the total rate of nitrogen the same, this Farm Journal study compared a one-and-done treatment — all fertilizer sidedressed between vegetative stages V6 and V7 — with the 4R program (right source, rate, time and place) involving multiple applications. The one-and-done approach ignores the carbon penalty, while the 4R method addresses it. As the graph shows, across three hybrids and three tillage systems, paying the carbon penalty increased yield in every instance except one (Hybrid 2 in spring chiseling, where yield was identical).
“The amount of the yield increase from managing the carbon penalty varies with tillage system and hybrid,” explains Ken Ferrie, Farm Journal field agronomist. “It is highest with spring tillage because that system produces the greatest carbon penalty. That’s because very little surface residue decomposes in the fall; but spring chiseling buries it in the zone where soil organisms are most active, causing it to be immobilized and temporarily unavailable.”
