102 THE PRINCIPLES OF SOIL MANAGEMENT 



in the soil. In the boulder clay of the glaciated sections 

 a bluish color is common, which seems to be due to the 

 presence of protoxid of iron (FeO), resulting from 

 the* great deficiency of oxygen. Where this comes 

 in contact with carbonated water, it may be changed 

 to the carbonate of iron, which is gray, and consequently 

 along the line of roots and in the bottom of ponds this 

 gray color may be found. 



Where there is an abundant supply of oxygen, the 

 iron takes on the sesquioxid (Fe 2 3 ) form, which 

 has a deep red color, typified by iron rust. Where the 

 red soil stands much in contact with water, it may 

 become yellow by the hydration of the iron (Fe 2 ;j + 

 H 2 0). In many regions a dark-colored soil is looked 

 upon as a fertile soil. This relation has developed 

 because of the association of a dark color with the 

 presence of organic matter, with all its beneficial effects, 

 while the light color indicates its absence. This relation 

 does not hold universally, but it is quite a reliable 

 guide. 



The only instances where the color of the particles 

 themselves give color to the soil is in some of the clean 

 quartz sands, where the white color of the dominant 

 mineral gives color to the mass. In some dark shaley 

 sands this same principle obtains. 



To the experienced person, the color of the soil is a 

 valuable guide to its condition and productiveness. 

 Mottled and uneven color, for example, indicates poor 

 aeration, frequently the result of deficient drainage. 



41. Physical absorption. The soil particles attract 

 and hold materials upon their surfaces. This physical 



