Oxygen per cent. 



Carbonic Acid per cent 



20031 



1019 



17-9 



376 



698 DR C. HUNTER STEWART ON THE 



oxygen in the atmosphere, then putrefaction is going on in the soil. Under the action of 

 anaerobic organisms the organic matter itself supplies the oxygen necessary for oxidising 

 its own carbon. 



Such excess of carbonic acid has generally been found at considerable depths, and in 

 late autumn, when the deeper layers of the soil have their maximum temperature. 

 Jn the deeper layers of an impure and rather impermeable soil, putrefaction is 

 common if the suitable conditions of warmth and moisture are present : in an impure and 

 permeable soil, on the other hand, there would be simply oxidation. Ammonia and 

 carburetted hydrogen are present in very small quantity, but the amount is said to 

 increase with the depth. The most of the ammonia formed in the soil is absorbed by 

 the humus. The amount of these two gases is so small and so variable, and their estima- 

 tion, moreover, so laborious, for practical purposes the determination of the carbonic acid 

 is the best index to the amount of organic decomposition taking place. 



The following are taken from Fodor's exhaustive work, Luft Boden und Wasser ; — 



1 Metre deep, . 

 4 Metre deep, . 



The amount of carbonic acid in the air of a given soil depends on (l) the amount of 

 organic matter, (2) permeability, (3) depth, (4) temperature and moisture. Pettenkofer 

 found that the ground-air from the Desert of Sahara had the same composition as the 

 atmosphere, the conditions for the development of carbonic acid being absent. But no 

 reliable conclusions can be drawn as to the purity of a soil from the carbonic acid estima- 

 tion unless its permeability be known. If the carbonic acid be determined in two soils, 

 similar as regards organic impurity, temperature, and moisture, but of different permea- 

 bilities, the less permeable will contain more carbonic acid than the more permeable, 

 because in the former the air is pre vented from so easily diffusing vertically into the 

 atmosphere, and horizontally into the adjacent soil. As regards temperature, the amount 

 of carbonic acid is least in late winter and spring, and goes on increasing, reaching its 

 maximum in summer and early autumn, and declining again to its minimum in winter. 

 Unfortunately, I had no means of determining the temperature of the soil at the time 

 that this work was in progress. In the curves appended will be found one of the mean 

 temperature of the week, derived from the averages of the maximum and minimum 

 temperature, and the temperature at 9 a.m. and 9 p.m. These show that at a depth of 

 3 feet the carbonic acid curve follows the temperature curve, but, on an average, about 

 3 weeks later. This, probably, may be taken as the time required for a variation of 

 atmospheric temperature to be propagated to the depth of 3 feet, and to influence the 

 rate of decomposition there. Fodor found the rainfall to exert a great influence, 

 especially if rain followed a prolonged drought in the summer. As this investigation 

 extends only over fourteen months, it does not permit of any conclusions on this point. 



