Two of the questions in mind when the experiment was planned were these: 1. Will the aeration of a sample of soil according to the procedure noted stimulate carbon dioxide production? 2. Will the amount of carbon dioxide in the air above the soil have any effect on the production in the soil and its diffusion from the soil? The results reported in Table 1 open up some interesting questions for further study. This table shows clearly that with the accumulation and accumulation-aeration methods, no differences were observed in amounts of carbon dioxide produced when the air of the flask was changed each 24 hours, and when the carbon dioxide was allowed to accumulate from day to day until the concentration in the air above the soil reached slightly over 12%. The data secured with the aeration method indicate that had the moisture content of these soils been maintained, the amounts of carbon dioxide produced probably would have been the same as those produced by the other soils. These results indicate that the production of carbon dioxide may not be affected by concentrations of this gas up to 12% in the air above the soil and that constant aeration at 5 liters per hour did not stimulate production. This indicates three possibilities. First, that carbon dioxide may not inhibit the action of certain microorganisms which produce it when present in the air in concentrations up to 12%. As the amount of air in a loam soil at optimum moisture conditions occupies about one-fourth of the total volume, it is possible that in a normal soil the micro-organisms live and function under much higher concentrations of this gas. Second, that the inherent properties of the soil and its moisture content affect the rate of production and consequently the diffusion of this gas to a much greater extent than its concentration in the air above the soil. Third, assuming that the gas diffused quite rapidly into the air above the soil, the rate of action of the micro-organisms may not be affected by rapid diffusion of this gas. The accumulation-aeration method is by far the most desirable method of the three to use in carbon dioxide studies. The difficulty of determining accurately the percentage of carbon dioxide in high concentrations has been discussed. The data reported in the aeration study required 80 titrations, each of which presented a chance for error. In addition, the suction pump required attention several times each day to insure a uniform rate of aeration. These tests show that no differences occurred in the amounts of carbon dioxide produced when the soil was aerated constantly or intermittently or when the carbon dioxide was allowed to accumulate up to a concentration of 12% in the air above the soil.