42 SOILS AND FERTILIZERS 



has been kept up by the use of manures are darker 

 than unmanured soils. 18 When rich, black, prairie 

 soils lose their organic matter through improper 

 methods of cultivation, or when the organic matter 

 (humus) is extracted in chemical analysis the soils 

 become light-colored. 



The red color* of soils is imparted by ferric oxide, 

 the yellow, by smaller amounts of the same material. 

 A greenish tinge is supposed to be due to the pres- 

 ence of ferrous compounds, such soils being so close in 

 texture as to prevent the oxidizing action of the air. 

 Color may serve, to a slight extent, as an index of fer- 

 tility. Black and yellow soils are, as a rule, the most 

 productive. The main reason why black soils are so 

 generally fertile is because they contain a higher per 

 cent, of nitrogen. Black soils are occasionally unpro- 

 ductive because of the presence of compounds injurious 

 to vegetation. 



43. Odor and Taste of Soils. Soils containing 

 liberal amounts of organic matter have characteristic 

 odors. The odoriferous properties of a soil are due to 

 the presence of aromatic bodies produced by the de- 

 composition of organic matter. In cultivated soils 

 these bodies have a neutral reaction. Poorly drained 

 peaty soils give off volatile acid compounds when 

 dried. The amount of aromatic compounds in soils 

 is very small. 



The taste of soils varies with the chemical compo- 

 sition. Poorly drained peaty soils usually have a 

 slightly sour taste, due to the presence of organic 



