CHAPTER III 

 THE SOIL AND ITS IMPORTANCE TO PLANTS 



It is impossible to study the plant as a living organism without 

 an understanding of the surroundings or environment in which it 

 grows. For most of the Seed Plants, the physical environ- 

 ment may be divided roughly into the soil and the atmosphere. 

 Of these two, the soil is much the more complex — physically, 

 chemically, and biologically. This fact, together with the 

 profound effect produced upon the plant by changes within it, 



Fig. 11. — Composition of soil. Graph showing the percentage composition, 

 by volume, of a rich, loam soil. 



warrants us in devoting to the soil a brief preliminary discussion 

 before we consider in detail the structures and functions of the 

 plant itself. 



The soil has three main uses in the plant's economy: It 

 provides an anchorage and support whereby the plant may be 

 held firmly in position ; it furnishes the supply of water which the 

 plant uses, and it contributes certain mineral salts essential to the 

 plant's successful activity. 



23 



