Soil Organic Matter Levels Under Pastures and Row Crops.

Building soil organic matter content and raising animals on pasture are key cultural practices of organic farming systems.  The amount of organic carbon sequestered under pasture versus row crops is not well documented and is an objective of this investigation.  We sampled soils of pastureland and tilled row crop fields in the Mid-Atlantic region.  Some, but not all of the farming operations were certified organic.  The soil samples were collected by sampling the surface 0 to 15 cm layer during the summer months of 2008 to 2010.  Each pasture sample was paired to a row-crop sample based on proximity and similarity of soil type.  So far 16 paired sites (pastureland versus tilled row crop fields) have been sampled.  Soil test (Walkley-Black method) results show that the average soil organic matter content level is 4.1% for pasture and only 2.4% for row crop land (statistically different at P=0.01).  The pasture soils were found to hold about 70% more organic carbon than the row crop soils.   This accumulated soil organic matter associated with pasture is also a reservoir for stored nitrogen.  Pasture forage generally includes a mix of grasses and legumes.  The legume grows on-farm nitrogen and the root density of the grass minimizes the leaching of nitrogen from the soil.  Such soil building could also be accomplished by growing cover crops.  While cover crops do feed the soil, pasture and forage crops serve to both feed the soil and at the same time produce nutrient rich food for people.  When pasture sod is eventually broken or tilled for the purpose of rotating to row crop grains, accumulated soil fertility is released.  In such rotations there is generally little or no need for purchase of off-farm nitrogen fertilizers.  The organic matter rich soil also provides drought resistance to following crops.