296 THE PRINCIPLES OF AGRICULTURE 



Why is soil useful to plant life? How much plant-food may an 

 acre of land contain? What is available plant- food? What is 

 potential plant- food? Is all of the plant-food in common soils 

 available? How does nature restore or maintain the fertility of 

 soils? How do man's operations differ from Nature's in this 

 regard? Are all plant-food materials equally useful to all plants? 

 What effect has deep-rooting on the soil, and on the amount of 

 plant-food that the plant obtains? Why are fertilizers useful? 

 What is the reason for their application? 



Let the circle or pupils read paragraph 48 in concert. 



Chapter 2 



In this chapter we discuss the texture and 

 structure of the soil. We shall find that the 

 condition of the soil is as important as its com- 

 position. Farmers have always known this, but 

 it is only recently that we have found out the 

 underlying reasons why. The subject of "soil 

 physics" has now come to be of first importance. 



What are the two general offices of soil so far as the growing 

 of plants is concerned? May a soil that is rich in all the plant- 

 foods still be unadapted to the growing of crops? Why cannot 

 crops grow on rock? Why not on very hard clay? What i.s meant 

 by the "texture" of the soil? By the "structure"? What is the 

 "physical condition" of the soil? In what language does the 

 farmer express a good physical condition? What words does he 

 use to express a poor physical condition? 



Name the reasons why good structure is important (52). 

 Where do the roots feed? What relation has the size of soil par- 

 ticles to the amount of available plant-food? Illustrate this by 

 breaking up a cube of sugar or a lump of chalk. Mathematically 

 this could be best illustrated by cutting up a cube of wax. In 

 what way, then, may the fining of soil be said to increase its pro- 



