Organic matter – the organic component of soil – is key to soil health. Organic matter serves as a reservoir of nutrients for crops, provides soil aggregation, increases nutrient exchange, retains moisture, reduces compaction, reduces surface crusting, and increases water infiltration into the soil. And organic matter is closely related to soil organic carbon, the carbon stored in organic matter. Soils with high levels of organic matter have higher levels of carbon, and consequently also benefit the climate by “sequestering” carbon that otherwise would be in the atmosphere.
In the rain-fed croplands of the Pacific Northwest, wheat-based agriculture has historically mined carbon out of the soil. Near Pendleton, winter wheat grown every other year depleted soil organic carbon up to 63% over 80 years of cultivation. Re-building soil carbon is thus an important task for supporting continued agricultural productivity across the region.
But rebuilding soil carbon isn’t easy in the dry rain-fed croplands of the inland Pacific Northwest, especially in the driest areas, which receive average precipitation ranging from as little as six inches a year up to 12 inches a year. In these areas, farmers have generally planted winter wheat every other year. In the fallow year, when no crops are grown, the soils store moisture that is used for the next crop. Given that the primary carbon inputs to soils are in the form of crop residues, it’s very difficult to build or even maintain soil carbon in a winter wheat-fallow system, even if you reduce or eliminate tillage.
What to do under such challenging conditions? Bill Jepsen, who has been a longtime pioneering dryland grower in Oregon, uses “flex cropping” to try to overcome these challenges, plus an added challenge of shallow soils that limit the amount of water that can be stored during the fallow year. When the land receives sufficient winter precipitation, Jepsen replaces fallow with spring wheat and barley, or occasionally re-crops winter wheat without fallow. This benefits the farm by increasing the number of cropped acres on which they can make a profit. And at the same time, it provides a range of benefits to the soil, and to their management of grassy weeds.
You can watch a video of Jepsen describing his system below, or read the full case study to find out more. This case study is one of a number that showcase innovative farmers who are implementing strategies to enhance their resilience in the face of climate change. Other completed videos and case studies can be accessed on the REACCH project website.
 Ghimire, R., S. Machado, S., and K. Rhinhart. 2015. Long-Term Crop Residue and Nitrogen Management Effects on Soil Profile Carbon and Nitrogen in Wheat-Fallow Systems. Agronomy Journal 107(6): 2230-2240.