Here on the farm we have 13 acres under cultivation but in actuality only 12 of them are used for vegetable production. The 13th acre, at the far end of the farm is in permanent fallow. This means that rather than grow crops for harvest we plant cover crops year round. One of the main purposes of this field is to serve as a carbon sequestration sink and to be an example of how cover crops can be used as a form of climate change mitigation.
In recent years there has been much research done on the potential for high biomass cover crops to sequester carbon in the soil and remove CO2 from the atmosphere. Through photosynthesis plants use light energy to convert CO2 and water into oxygen (released back into the air) and a simple sugar (which nourishes the plant and is often stored in root systems). When the plant stores carbon in the form of this sugar the carbon has been drawn out of the atmosphere and is sequestered in the leaves and roots and eventually the soil where it is in a stabler form.
In order to maximize the carbon sequestration potential of our 13th field we plant sorghum Sudan grass in the summer. Growing to almost 10 feet tall (see picture above of our crew standing in the field), this species can store large amounts of carbon in leaves and roots. Some estimates suggest cover crops have a potential to sequester 1 to 3 tons of CO2 per acre and due to its size it is likely that Sorghum Sudan grass is towards the higher end of that estimate.
Sorghum Sudan grass is only a summer crop and therefore we need to ensure that the carbon stored in the soil is not released after we mow it down in August. In order to accomplish this goal we minimize the time the field is bare and aim to seed the winter cover crop of oats, vetch, and clover just a few weeks after turning the remains of the Sudan grass back into the soil. The winter cover crops then take up the carbon in the soil for their own use and it is not released back into the atmosphere as CO2.
Of course one other benefit of this 13th acre is that it provides a bit of farm fun at the end of August when we need to knock it down for winter. It is so tall that we actually can’t mow it until we drive the tractor through it to flatten the tall grass down to ground level. The photos show our view of the Sudan grass as we prepare the field to transition to winter cover crops.