Long-term research has documented the extent to which organic farming improves key indicators of soil quality, including soil organic carbon content (SOC) and particulate organic matter (POC).   An important article entitled “Total and Labile Soil Organic Matter in Organic and Conventional Farming Systems” summarizes the findings of nine long-term comparative trials assessing the impacts of conventional and organic cropping systems on soil quality.   The article appeared in the April 2006 issue of the Soil Science Society of America Journal (Marriot and Wander, 2006).

The relative importance of animal manures and legumes in building organic matter content is also analyzed in this paper.

Organic soil management improved several indicators of soil quality.  The POM-carbon and POM-nitrogen fractions in the organic systems were increased 30 to 40% — two to four times more than enrichment shown in soil organic carbon and total nitrogen.

Overall, organic management “increased SOC concentrations approximately 14% above values found in conventional systems after an average of 10 yr.”  The authors noted that these gains in soil organic carbon under organic management occurred despite the relatively heavier reliance by organic farmers on cultivation for weed control.

Applications of animal manures were found to be equally effective in building SOC as the planting of legumes in rotations and as cover crops.  Contrary to conventional wisdom, the authors concluded that particulate organic matter is in fact a significant source of mineralized nitrogen, and hence is an important pool of nutrients in support of plant development.  They also noted that POM-N is a more reliable indicator of soil quality in fields under organic management in contrast to conventionally managed soils.

The authors noted that the improved response of organic soils to tillage is likely a result of the longer and diverse crop rotations and cover cropping systems commonly used on organic farms.

Source:  “Total and Labile Soil Organic Matter in Organic and Conventional Farming Systems.”

Authors: Emily E. Marriot and Michelle M. Wander

Soil Science Society of America Journal, Vol. 70:950-959.  Online April 19, 2006.