CHAPTER III 

 THE SOIL. 



Soil is the layer of disintegrated rock, mixed with the remains 

 of plants, which covers a large portion of the land. It also con- 

 tains living organisms of various kinds and variable quantities 

 of water and air. The depth of soils varies greatly, being usually 

 from six to twelve inches, and sometimes as great as several feet. 

 Beneath it is the subsoil which differs from the upper layer in 

 containing less organic matter. The line of demarcation can 

 often be distinctly seen in deep trenches by the difference in color, 

 the subsoil being generally of lighter color, and gradually grad- 

 ing into the dark color of the upper soil. 



Soils consist largely of disintegrated rock fragments and de- 

 pend for their chemical nature mainly upon the character of the 

 rocks beneath. The rocks have been classified by geologists ac- 

 cording to their origin into three classes : 



(1) Igneous rocks are those which resulted from the cooling 

 of intensely heated matter. The granites represent this type. 



(2) Sedimentary rocks are those resulting from the settling 

 out of particles suspended in water. Limestones are examples 

 of this type. 



(3) Metamorpkic rocks are those which have been changed in 

 character since their deposition. The conversion of limestone 

 into marble by pressure and heat is an illustration of this type. 



These rocks must have contained all of the mineral or ash ele- 

 ments of plant food as no other source for them is conceivable. 



Rocks are rarely homogeneous, that is, alike in all parts but 

 are generally made up of several components mingled together, 

 often lying side by side as separate crystals. These components, 

 which have a more or less definite composition, are called min- 

 erals. Distinctly separate minerals are more frequently to bo 

 seen in the igneous rocks. A piece of granite will readily show 

 that it is made up of several distinct minerals. 



