A one-time plowing may not hurt carbon credits

COSHOCTON, Ohio – A one-time tillage will not cause great soil carbon loss, even though major damage is caused to soil structure.
That’s the finding of Lloyd Owens, a soil scientist with the USDA’s Agricultural Research Service in Coshocton, Ohio, after a study comparing soil carbon in the top foot of soil under a meadow with the carbon level in soil under cornfields with various levels of tillage.
Owens found that it takes a few years of continual annual plowing before carbon losses become noticeable in fields previously unplowed for years.
Slug control. One reason why farmers in the northern Midwest might plow is to help control a heavy infestation of slugs. It was just such a slug infestation that inspired Owens to study the effects of plowing on soil carbon.
He found that for the first year of plowing land, the upper 8 inches – the plow layer – held about 19 tons of carbon per acre, the same as was retained in the meadow and untilled cornfields.
But after a few years of annual plowing, the loss became noticeable, and, by the fifth year, the soil carbon level fell to 15 tons an acre.
Down side. Although there was no noticeable loss of stored soil carbon in the first year of plowing, there were other trade-offs, Owen said.
That’s because just one pass with a plow greatly damages soil structure, which increases erosion and reduces water infiltration and soil health.
Also, a single plowing moves more than half the organic matter in a field a little deeper underground, away from the surface where it is needed to hold water and nutrients.