Carbon's impact on soil moisture and crop performance

Building soil carbon is key to Michael Thompson’s diverse operation near Almena, Kansas, where he farms in partnership with his father, Richard, and his brother, Brian. The partners no-till primarily corn, soybeans, and winter wheat and graze cattle on cover crops.

While the carbon marketplace offers helpful financial incentives for building soil carbon, Michael Thompson values carbon most for its contributions to the soil’s moisture-holding capacity.

“Carbon holds water, so when we get heavy rain events, the more carbon in the soil, the more water-holding capacity the soil will have,” he says.

Of course, as plants receive more water, they grow more. This creates more top growth to harvest and provides more soil cover. Roots and surface residue feed soil biology, and the symbiotic relationship forms organic matter, which is primarily made up of carbon. Thus, the regenerative circle in the soil continues:

• More carbon
• More water
• More plants
• More carbon

A resilient cropping system results, with soil and plants able to better withstand both dry and wet conditions. Because of this, profitability stays on an even keel, not peaking in the good years only to crash in the drought years.

“We aim for an even stream of profitability, where we’re profitable every year,” says Thompson.

How it Started

The Thompsons began building their present farming system in 2000, when they started no-tilling.

“We had always been a minimum-till operation, but we did fall tillage before planting wheat,” he says. “We had erosion on our rolling hills. The soil would wash or blow away from the slopes and hilltops.”

After switching to no-till, the biggest challenge they faced in making the transition was “getting the fertilization levels right,” he says. “With soil cover left on the surface, some nutrients get tied up. Until the nutrients started cycling, which takes time, we found that during that transition period we had to increase the nitrogen we applied.”

They added cover crops to the system after observing that no-till alone was not eliminating compaction in the soil. “We believed we needed to add cover crops to help the water to infiltrate better,” says Thompson.

They were also seeing herbicide-resistant weeds in their cropping system. Adding cover crops seemed like a good way to control the weeds.

They envisioned developing a cover crop system that would benefit crops and soil but also provide grazing for cattle. They hoped the grazing of the cover crops would potentially give grass pastures a rest as well as reduce the need to feed harvested hay to cattle in the winter.

In 2009, they experimented with a full-season grazing cover crop. In spring, they planted corn and sorghum-sudangrass on a small acreage next to a grass pasture holding 24 heifers. The Thompsons grazed the cattle on both the pasture and the cover crop through the fall and into February.

That was the start of their present practice of incorporating grazed cover crops into the cropping system. They typically plant the full-season cover crops in spring following a crop of milo or corn.

“The species we plant depend on moisture availability,” he says. “We plant both warm-season and cool-season cover crop mixes. We often plant oats, peas, and clovers. We include red clover in the mix and sweet clover on degraded soil. We also often include forage sorghum, forage soybean, barley, a brassica, and a broadleaf like sunflower.”

Grazing Cover Crops

Deciding when to graze the cover crop and for how long depends on growing conditions.

“If we have planted a cool-season cover and have had good growing conditions, we can start grazing the cover crop in May, after the oats have nearly reached the boot stage,” says Thompson. “But sometimes we’ll defer grazing if it’s dry, and the cover crop is not thick and robust.”

By cross fencing fields to hold cattle on smaller parcels, they can concentrate beneficial manure and urine in small areas of the field and better manage the amount of forage cattle consume and trample.

“Our goal is to leave a lot of residue on the surface after grazing,” he says.

To further diversify their grazing options, the Thompsons sometimes seed winter wheat, triticale, or rye as dual-purpose crops in the fall.

“We can harvest these crops as cash crops or graze them,” says Thompson. “They green up earlier than our pasture grass; rye and triticale will green up as early as February and March.”

Expanding grazing across fields also benefits subsequent cash crops. “We always plant corn and soybeans on fields that have been grazed in the previous growing season,” says Thompson. “We have some fields that don’t get grazed because of logistics, and we can see the difference in crops on those fields compared to the crops on the fields that we do graze. The crops on the previously grazed fields are more resilient, and we get more efficient use of the purchased fertilizers that we apply. We’re using cattle as a soil management tool.”

Soil Health Perks

Improved soil health shows that the grazing of the cattle on crops and cover crops— along with practicing no-till—is paying off. Wind erosion is gone and runoff from slopes has reduced, and in some places eliminated, the ditching previously created by water running off fields.

“Soil organic matter is climbing,” Thompson adds. “When we first started no-tilling, our organic matter was at 1%. Now it’s about 2%.”

Carbon, of course, increases along with the increases in organic matter. Thompson says this is evident by the darkening of the soil at deeper depths than previously.

“On Haney soil tests we use measurements of water-extractable soil carbon as a scorecard to see how we’re doing,” he says. “We find that carbon ebbs and flows through dry and wet times. In wetter times there is more carbon in the soil, but in drier times the carbon is not as high. As long as we get some moisture and have green plants growing, we will have carbon in the soil.”