An article at BBC News said there is three times more carbon in the soil than in the atmosphere, but that carbon is being released by deforestation and poor farming. This is both fueling climate change and reducing our ability to feed the growing population. Problems include soils being eroded, compacted by machinery, built over, or harmed by over-watering. (Does flooding count as over-watering?) You can find the original article at http://bit.ly/soil-damage.
There is some very good news about soil and carbon sequestration at The Nation. The easiest and least expensive way to sequester carbon is probably photosynthesis. Plants and microbes take carbon dioxide from the air and combine it with water to make simple sugars, with oxygen as a by-product. The sugar is used by plants to grow, and that includes putting roots into the soil. When a plant dies, the roots decompose, mostly into black carbon products that stay in the soil. The news is that this process has been shown to be able to take carbon dioxide out of the air at rates previouslly thought impossible. See http://bit.ly/The-Nation-soil.