The odor is coming from the organic material in the basement because of high relative humidity. Level the earth and cover the earth with plastic, then 4" of concrete is the best. Least money is plastic and 2-3" gravel is areas tobe walked on to protect the plastic. Also enough gravel to get an air tight seal of plastic on the earth. Monitor the relative humidity and keep it below 60% for sure or below 50% RH is ideal. This will require a dehumidifier in most parts of the country.