72 



SOIL GASES SOIL WATEE 



3. In general, the amount of carbon dioxide increases with the 

 depth (up to 50 or 60 cm.) from which the sample is collected. This 

 is due not to diminished production near the surface, but to more 

 rapid diffusion there. 



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 inter- 

 stices 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 table l 

 below 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 carbon 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 culti- 

 vated 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 of its en- 

 closed air. It consists, therefore, of a highly dilute solution of a large 

 number of compounds. In a fertile soil it contains all the constitu- 

 ents of plant food (since they can only be assimilated by the plant in 

 the soluble form) 2 and generally other substances non-essential to 

 plants, e.g., sodium and silicic acid. 



The water retains the sodium chloride and sulphate originally 

 present in the rain ; but inasmuch as considerable evaporation always 

 takes place, these substances become more concentrated. 



The composition of the water present in a soil can be deduced 

 from analyses of drainage water, though doubtless the former is richer 

 in dissolved substances. 



Many analyses of drainage water have been made at Eothamsted 

 by Warington, 3 and at Grignon by Deherain. At Kothamsted the average 

 rainfall and drainage through 5 ft. of bare soil were as follows : 



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



2 Or in some instances perhaps after being rendered soluble by the acid liquid 

 secreted by the roots. 



3 Jour. Chem. Soc., 1887, Trans., 500. 



