Si 



may serve, to a slight extent, as an index of fertility. 

 Black and yellow soils are, as a rule, the most produc- 

 tive, although occasionally black soils are unproductive 

 because of the presence of acid compounds injurious to 

 vegetation. The main reason why black soils are so 

 generally fertile is because they contain a high per cent 

 of humus and nitrogen. 



49. Odor and Taste of Soils. Soils containing liberal 

 amounts of organic matter have characteristic odors due 

 to the presence of aromatic bodies produced by the 

 decomposition of organic matter. In cultivated soils 

 these have a neutral reaction. The amount of aromatic 

 compounds in soils is very small. Poorly drained peaty 

 soils give off volatile acid compounds when dried. 



The taste of soils varies with the chemical composi- 

 tion. Peaty soils usually have a slightly sour taste, due 

 to the presence of organic acids. Alkaline soils have 

 variable tastes according to the prevailing alkaline com- 

 pound. The taste of a soil frequently reveals a fault, 

 as acidity or alkalinity. 



50. Power of Soils to absorb Gases. All soils pos- 

 sess, to a variable extent, the power of absorbing gases. 

 When decomposing animal or vegetable matter is mixed 

 with soil, the gaseous products given off are absorbed. 

 Absorption is the result of both chemical and physical 

 action. The chemical changes which occur, as the fixa- 

 tion of ammonia, are considered in the chapter on fixa- 



