THE SOIL AND ITS IMPORTANCE TO PLANTS 35 



all of the sciences, but we still lack a precise knowledge of many 

 of its aspects and fail to understand clearly the manner in which 

 it affects the life of plants growing in it. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



53. Through what processes does bare rock gradually become covered 

 with soil? 



54. Is soil always formed by the rock lying directly under it? What 

 was the origin of the soil in this region? 



55. In what various ways is soil being impoverished or "used up?" 



56. In what various ways is soil being replenished? 



57. What is the surface of a bit of gravel which is in the form of a 

 solid cube, the sides of which are 1 cm. long? What would be the 

 surface of this bit of gravel if the cube were cut up into smaUer cubes with 

 sides of 1 mm. each? What conclusion can you draw from this as to 

 the importance of the size of soil-particles? 



58. What disadvantages to plants are there in a soil where the 

 particles are very small? What disadvantages where they are large 

 and coarse? 



59. Why are soils in which the particles are small usually more pro- 

 ductive than soils in which the particles are large? 



60. It is important to have the soil very loose and "mehow" if such 

 "root-crops" as potatoes, carrots, beets, and turnips are to yield heavily. 

 Explain. 



61. The surface of land after plowing is a few inches higher than it is 

 before plowing. Why? 



62. In what respect is the cutting off of forests bad for the soil? 



63. Is a gentle shower or a hard, beating rain better for the soil and 

 for plants growing thereon? Explain. 



64. It is harmful to the soil to till (or "work") it immediately after a 

 rain or while the soil is very wet. Why? 



65. Why is extensive removal of forests in a region often followed by 

 floods there? 



66. What various means of soil treatment can you suggest for insuring 

 that the largest possible proportion of the rainfall is utilized by plants 

 growing on this soil? 



