COLOR OF SOILS 



42. Organic Matter and Iron Compounds. — The 



principal materials which impart color to soils are or- 

 ganic matter and iron compounds. Soils containing 

 large amounts of organic matter are dark-colored. 

 A union of the decaying organic matter and the 

 mineral matter of the soil also produces compounds, 

 brown or black in color. When moist, many soils are 

 darker than when dry, and soils in which the organic 

 matter has been kept up by the use of manures are 

 darker than unmanured soils. 17 When rich, black, 

 prairie soils lose their organic matter through im- 

 proper 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. 

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

 ence of ferrous compounds, such soils being so close in 

 texture as to exclude the oxidizing action of the air. 

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

 tive. Colcr may serve, to a slight extent, as an index 

 of fertility. 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. 



