SOIL AND ITS ORIGIN 279 



to grind rocks slowly into soil but also in carrying the soil 

 from one place to another. One of the chief agents in 

 making the soil in much of the United States was the gla- 

 ciers, of which you have learned in your geography. These 

 great bodies of ice came moving down over the country 

 from the north, grinding up the rocks and soil, carrying 

 vast masses of this ground-up material long distances, and 

 finally depositing them far from the places where they were 

 ground up. 



Plants help also in breaking up rock. They do this in 

 two ways; by their roots penetrating rocks and soil, and by 

 an acid that they give out from their roots which acts 

 chemically upon the soil, dissolving it. 



Besides all this broken-up rock, which is inorganic 

 matter, the soil, as we know it, has the humus or organic 

 matter in it. This is the part of the soil that gives it its 

 black color. It is, as we shall find out later, a most im- 

 portant part of the soil. Bacteria are constantly at work 

 on the humus in the soil, making new compounds, setting 

 free elements, and making the soil finer. So we can think 

 of the soil not as made, but as being made. It is con- 

 stantly changing, not only physically but chemically. It 

 is losing some things and gaining some, and where it is 

 under cultivation the gain and loss depend largely upon the 

 methods used in cultivating it. 



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