CHAPTER II 



ORIGIN OF THE SOIL 



What the soil is. All ordinary plants are rooted in the soil, 

 which serves them as a source of food materials and affords a 

 convenient place of anchorage. The soil, however, is far from 

 being the simple collection of rock fragments that it is often 

 supposed to be. Fragments of rock are important, to be sure, 

 and in ordinary soils form from 60 to 95 per cent of their 

 weight ; but any good agricultural soil must contain, in addi- 

 tion, air, water, humus, bacteria, and similar organisms. So 

 important are all of these that none can be omitted without 

 impairing the fertility of the land. From the rock fragments 

 come all the minerals used by plants. These are steadily, 

 though very slowly, dissolved out by the soil water and carried 

 in solution into the plant. The humus, formed of decaying 

 plant and animal remains, is the principal source of the all- 

 important nitrogen ; but since the plants cannot use it as 

 humus, it must first be worked over into nitrates by the 

 bacteria. The higher plants are so dependent upon the good 

 offices of the bacteria in this respect that if the latter should 

 all disappear from the soil, crops would soon be unable to 

 grow in it. The air in the soil is required for the activities 

 of the bacteria, as well as for the respiration of the under- 

 ground parts of the plants. 



Depth of the soil. The soil ranges from a few inches to many 

 feet in depth. In humid regions that is, in regions of abun- 

 dant rainfall the decaying humus gives it a darker color and 

 enables us to distinguish it from the underlying materials ; but 

 in arid regions, where the rainfall is scanty, the humus is 



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