366 SOILS: PROPERTIES AND MANAGEMENT 



268. Absorption of water vapor and of gases by soils. 

 Hygroscopic water in soils has already been discussed (pars. 

 133, 134, 135). It need merely be remarked here that 

 there is a close relation between the absorptive power of a 

 soil for water vapor and for bases. Soils having a high 

 content of humus and composed of very fine material are 

 likely to have great absorptive properties for both vapors 

 and solutes. 



In a similar way soils absorb gases. The deodorizing 

 property of soil is well known. Decomposing organic 

 matter is rendered inoffensive by covering it with soil. 

 Gases produced in the processes of decomposition are 

 largely absorbed by the soil. The fertility of the soil 

 may be increased by the absorption of certain gases. 



269. Absorption of ammonia. Ammonia, which exists 

 in minute quantities in the air, is absorbed by soils, and 

 also when given off by decomposing organic matter in the 

 soil. As all nitrogeneous organic matter may eventually 

 form ammonia when decomposed, the ability of the soil 

 to absorb it is very important. Quartz alone will absorb 

 only a very small quantity of ammonia, while a clay soil 

 will hold practically all that is likely to be produced by 

 the decomposition of the organic matter incorporated in it. 



270. Absorption of carbon dioxide. Carbon dioxide 

 is absorbed by soils to a very considerable extent, and 

 this also adds to the productiveness of soils, since it aids 

 in their decomposition. The supply of carbon dioxide 

 comes from decomposing organic matter and from plant 

 roots. As will be explained later, the soil air always 

 contains a considerable supply of this gas, and its con- 

 densation and absorption is constantly going on. It 

 forms soluble bicarbonates with the alkalies and bases 

 of soils, producing a readily available plant-food material. 



