Soils managed with synthetic pesticides and fertilizers hold 28.6% less organic carbon than organic
"In soils under organic management, the SOC [soil organic carbon] stocks averaged 37.4 tonnes C ha-1, in comparison to 26.7 tonnes C ha-1 under non-organic management."
Calculation: 37.4-26.7=10.7/37.4=.286, or 28.6% less SOC.
Andreas Gattinger et al., “Soil Carbon Sequestration of Organic Crop and Livestock Systems and Potential for Accreditation by Carbon Markets,” in Food and Agriculture Organization of the United Nations, Organic Agriculture And Climate Change Mitigation: A Report of the Round Table on Organic Agriculture and Climate Change, (Rome: FAO, 2011), 16, accessed March 19, 2015, http://www.fao.org/docrep/015/i2537e/i2537e00.pdf