SYCAMORE, Ill. — Building soil organic matter is important to farmers for several reasons, including sequestering carbon and increasing crop yields.
There are different types of carbon in soil, said Matt Ruark, professor of soil fertility and nutrient management at the University of Wisconsin.
“There is a passive pool, which is carbon, that will sit there for a long period of time and a slow pool that will turn over in decades,” said Ruark during the Illinois Crop Management Conference presented by University of Illinois Extension.
“The active pool of carbon is the stuff that rapidly decomposes and rapidly cycles as the carbon is consumed by microorganisms, but in that process there is also a release of nitrogen,” he said.
The university professor talked about a study that evaluated the impact of biologically active soils on crop yields.
“The soil that had greater biological activity had greater yields, but that effect is not near as strong as the effect of fertilizer,” he said. “In general with a 20% increase in soil health metric, we see a 5% increase in yield.”
Other research work evaluated the Haney Test.
“This shows if you have greater biological activity, you can apply less nitrogen, but there’s a lot of variation,” Ruark said.
“The problem with nitrogen cycling is it’s complicated, so we’re not ready to put this in recommendations,” he said. “But we’re going to continue to work on it.”
According to a global meta analysis on corn and wheat production, Ruark said, researchers found yield gains occurred with soil organic matter increases up to 4%.
“We do a lot of on-farm soil work, so we did a survey across Wisconsin and into Minnesota to get all the farm records for organic matter levels and crop yields, as well as other information about soil practices, tillage, crop rotation, cover crops, manure application and how much fertilizer was used,” he said.
All this information was entered into a model.
“The No. 1 most important variable was pH when it comes to yield,” Ruark said.
“We see a clear relationship between increasing organic matter and yield, but we still seen an increase when we fertilize,” he said. “Increasing organic matter from 3% to 4% on average, we see a 5-bushel increase in yield and a 15-bushel yield increase as you go from 2% to 3% organic matter.”
Ruark is interested in expanding the data set to learn more such as how the clay content of the soil fits into the relationship.
Researchers in Iowa conducted a study at four locations that included fields that were continuous corn or a corn-soybean rotation for 14 to 15 years.
“They measured the total carbon in the upper six inches of the soil at the beginning and end of the experiments,” Ruark said.
“In the continuous corn system, as long as they applied the optimal amount nitrogen, over time they built organic matter in the soil,” he said. “For the corn-soybean system there was a slight effect, but there was no real gain in organic matter, so the message is nitrogen matters as long as you’re getting carbon from it.”
Historically, all of the sites in the study were a corn-soybean rotation.
“Switching from a corn-soybean rotation to continuous corn increases the total amount of carbon,” Ruark said. “What matters is where you’re starting from — if you’re shifting to a system to add more carbon to the soil, that will probably lead to more carbon in your soil.”
Studies that compared adding manure or synthetic nitrogen to fields showed if no nitrogen was applied to the fields, that tended to decrease the carbon in the soil, Ruark said.
“Adding synthetic nitrogen increases the carbon a little bit,” he said. “But adding organic nitrogen has bigger gains, on average up to 40%, so where we can fit manure into our cropping systems, it is really beneficial for carbon in our soil and soil health.”
Cover crops are another option for building organic matter in soils.
A study in Ontario included nine years of cover crops that were planted between late July and early September. Oats, radishes and an oat/radish mixture with cereal rye were planted in the fields.
“Over time, they saw an increase in carbon, especially with the radish and cereal rye,” Ruark said.
“If you’re interested in building carbon with cover cropping, it’s a biomass game, so you need to plant the cover crops every year,” he said. “Adding more carbon to your soil over a decade will move the needle on the organic matter.”
Once the carbon is in the soil, Ruark said, it is important to protect it.
“We want our soils to better aggregated so it sticks together,” he said. “Then the carbon will be trapped and protected in the aggregate and less likely to be consumed by the microorganisms.”