Soil organic matter is considered to be a major contributing factor to the productivity of organic farming systems. However, interactive effects of organic matter input and tillage on soil organic carbon and nitrogen pools are not clearly understood. This study was conducted in a semi-arid ecosystem of southern New Mexico with the objective of determining the influence of amount of time under organic farming on carbon accumulation in the soil. We selected four farms: three of them were under organic farming for past three, six and nine years, and the fourth under the conventional farming system. Soil samples were collected in triplicate to a depth of 100 cm in such as way that each sample consists of 3 sub-samples. These samples were air-dried, finely ground (<250 um) and were analyzed for total C, N and H by dry combustion with a Thermo Electron CHNS-O analyzer. Inorganic C was determined by decomposing carbonates in a sealed serum bottle with 1M HCl and measuring the evolved carbon dioxide by gas chromatography. Organic C was computed as the difference between total and inorganic C.