THE CONTENTS OF THE SOIL 25 



these winds are known as "sand-storms." Most 

 shores, particularly if sandy, are much modified 

 by the action of wind. But the wind has an 

 influence upon soils even in the most protected 

 and equable regions. The atmosphere contains 

 dust, much of which is valuable plant -food. 

 This dust is transported by winds, and it finally 

 settles or is carried down by snow and rain. 

 Although the amount of dust which is deposited 

 in any given time may be slight, it is neverthe- 

 less continuous, and has an important effect 

 upon the soil. 



3. The Resources of the Soil 



41. The soil affords a root -hold for plants, 

 a place in which they can grow. It also supplies 

 the environmental conditions which roots need, 

 protection, moisture, air, agreeable temperature, 

 and other congenial surroundings. 



42. The soil is also a store -house of plant- 

 food. Roberts calculates, from many analyses, 

 that in average agricultural lands the surface 

 eight inches of soil on each acre contains over 

 3,000 pounds of nitrogen, nearly 4,000 pounds 

 of phosphoric acid, and over 17,000 pounds of 

 potash. These three elements are the ones which 

 the farmer must chiefly consider in maintaining 

 or augmenting the productive power of the land ; 



