952 EXPERIMENT STATION RECORD. 



suj^plyiiig the oxygen necessary for the respiration of the plant roots 

 wlieu the sujiply from the atmosphere is cut oft' by excessive moisture. 

 The hygroscopic water, which occurs to a considerable extent in many 

 soils, is also of comparatively slight importance for vegetation, because 

 the plants wilt before the quantity of water has fallen sufficiently to 

 make it possible for an absorption of water vai)or to take place at all. 



THE COMPOSITION OF THE SOIL ATMOSPHERE. 



The air inclosed in the soil differs from atmospheric air principally in 

 containing more carbonic acid, less oxygen, and more moisture. From 

 the well-established fact that the soil atmosphere becomes richer in car- 

 bonic acid iu proportion to the diminution of its oxygen, in such away 

 that the volume of carbonic acid and oxygen together remains constant, 

 we must infer that the carbonic acid is the product of processes of oxi- 

 dation. Of the latter the most important are those connected with the 

 decomposition of organic substances in the soil. 



The quantity of carbonic acid in the soil air, which is always greatei 

 than that in the atmospheric air, sometimes several hundred times as 

 great, is subject to great variations, which are due (1) to a variation in 

 the quantity of organic matter in the soil, (2) to tiuctuation in the 

 rapidity of decomposition, and (3) to differences in the processes by 

 which the carbonic acid can leave the soil. 



Concerning the first of these reasons, investigations have shown that 

 the quantity of free carbonic acid is proportional to that of the organic 

 matter only when the latter is small, but that the ratio of carbonic acid 

 to organic matter diminishes when the latter is great. The quantity of 

 carbonic acid may remain constant when that of the organic matter 

 varies, since a high proportion of carbonic acid in the soil air dimin- 

 ishes the activity of the lower organisms taking part in the processes 

 of decay, so that with an increase of organic matter the conditions for 

 decay become less favorable. 



The degree of decomposition of the organic substances, and conse- 

 quently the proportion of carbonic acid, depends principally upon the 

 access of air, moisture, and warmth. The quantity of free carbonic 

 acid increases, other conditions being the same, with the supply of oxy- 

 gen, up to a certain low limit. This increase, however, is not directly 

 proportional, but is progressive at first, and after passing a certain 

 limit is less than that of the supjily of oxygen. The decomposition of 

 organic matter, and hence the production of carbonic acid, rises and 

 falls with the quantity of water in the soil, so long as the access of 

 air is not prevented beyond a certain limit. With an increase in tem- 

 perature up to about 50° C. the intensity of the decomposition of 

 organic matter and of the production of carbonic acid increases. The 

 resultant action of the above-mentioned factors depends on the fact 

 that the extent of the production of carbonic acid is determined by the 

 minimum factor. From this it follows tliat the quantity of free carbonic 

 acid, the proportion of organic matter remaining constant, is very 



