Often farming exhausts the organic material in the soil and so can add to the amount of carbon dioxide in the atmosphere. But if you farm so that the amount of organic material in the soil increases each year then you are trapping carbon in the soil. Just how much carbon we can trap at low cost by changing agricultural practices I don't know but I imagine costs would be low in some areas and for some crops. We need a system so farmers can be paid for trapping carbon no matter where in the world they live. We will also need some form of inspection to ensure carbon is being trapped and to prevent corruption.