OS THE PRINCIPLES OF SOIL MANAGEMENT 



there are materials composed almost entirely of the 

 refractory mineral quartz. Such, for example, is the 

 barren LaFayette sand of Maryland, which contains 

 94.4 per cent of silica, and a sandstone occurring in 

 Utah contains 96.6 per cent of silica. Doubtless, dune 

 sands as rich in quartz might be found. Not all silica 

 is in the form of quartz, but it is an indication of the 

 latter. 



Fine-textured soils also exhibit much variation, but 

 do not go nearly to the extreme in silica content shown 

 by sand soils. 



It is the very exceptional soil of any grade of fineness 

 which does not contain, in its ultimate analysis, a fair 

 amount of all of the essential mineral plant-food ele- 

 ments. Other conditions must also be taken into account 

 in determining the crop value of such soil its physical 

 properties, the climate, the crop, the introduction of 

 new materials by wind, the movements of water and 

 the action of plants and animals. 



2. The Soil Mass. Physical Properties of the 

 Soil and Their Modification 



The term soil is used to designate that superficial 

 portion of the earth's surface in which plant roots 

 distribute themselves. This includes sand, gravel and 

 boulders, containing practically no available plant-food 

 material, as well as rich garden soil. 



22. Soil and subsoil. A common and natural dis- 

 tinction is made of (a) the top soil, which is called 

 "soil," and which usually extends to the depth of the 



