Additional research showed that the fastest rate of soil OM accumulation occurs on the pasture-based dairies between two and six years after converting from row crops. Carbon (C) in the top 12 inches of soil (OM is about 58 percent C) rose by approximately 3.6 tons of C per acre per year (Figure 2). Incidentally, this rate of soil OM buildup is among the highest rates ever recorded in any system.
In fact, if one considers that the average automobile produces 1.5 tons of carbon per year, the average 500-acre, pasture-based dairy farm in Georgia is sequestering the annual carbon emissions of over 1,200 vehicles. According to EPA estimates, 5.6 tons of CO2 per year x CO2 is about 27 percent C. In other studies, prediction models developed by USDA’s Agricultural Research Service and refined for Georgia forages and conditions showed that pasture-based dairying in the Southeast has a carbon footprint similar to the freestall dairies in this region (on per unit of milk produced basis).
Won’t occur overnight

Soil organic matter is comprised of many components.
It is worth noting that Rome wasn’t built in a day and neither will soil OM. The soil OM on the pasture-based dairies we studied did not show much gain in the first one to two years following conversion. This is probably the result of a lag in getting the population of soil microbes and earthworms built up.
Additionally, it is unclear if that high rate of OM buildup can continue. In some of our older pasture-based dairies, the soil seems to have stabilized at 3 to 4 percent OM, indicating that soil OM levels will eventually plateau.
In addition to continuing to monitor soil OM levels, this research has now moved into trying to determine which part of the forage system contributes the most to this change in soil OM. The preliminary results seem to indicate that the roots and root exudates are the major sources of soil OM improvements. These results support the findings of a consortium of American and European scientists in a recent review in the journal Nature. Their report conclusively showed that roots and root exudates are the primary source of soil OM buildup, disproving the long-held dogma that crop residues and biomass on the soil surface are the primary sources of soil OM buildup.