72 THE EEACTIONS OCCURRING IN SOILS. IV. 



4. Carbon dioxide is more abundant in summer and autumn 



than at other periods of the year. 



In addition to the gases existing in the gaseous state in the 

 interstices of the soil, considerable quantities are present in an 

 absorbed condition. The various constituents of a soil possess 

 very different powers of absorbing gases and vapours from the 

 air. The following table" gives the maximum amount of 

 water vapour, ammonia, and carbon dioxide which can be 

 absorbed by 100 grammes of the various soil constituents at 

 and from an atmosphere containing the maximum amount 

 of the aqueous vapour or from the pure gas, ammonia, or car- 

 bon dioxide : 



If the soil be water-logged the decomposition of the organic 

 matter proceeds in a different way, and marsh gas, CH 4 , free 

 nitrogen, and other gases are evolved. Such decomposition 

 rarely occurs in a cultivated and properly drained soil. 



The Water in a Soil. The water in a soil is present to a 

 great extent as a liquid film enveloping the particles composing 

 the soil. This liquid film contains the soluble matter of the 

 soil and its enclosed air. It consists, therefore, of a highly 

 dilute solution of a large number of compounds. In a fertile 

 soil it contains all the constituents of plant food (since they 

 can only be assimilated by the plant in the soluble formf) and 

 generally other substances non-essential to plants, e.g., sodium 

 and silicic acid. 



The water retains the sodium chloride and sulphate origi- 

 nally present in the rain; but inasmuch as considerable evapo- 



* Von Dobeneck, quoted by Wiley, Agric. Analysis (1894), Vol. I., 290. 

 t Or in some instances after being rendered soluble by the acid liquid secreted by 

 the roots. 



