98 THE PRINCIPLES OF AGRICULTURE. 



Of what is phosphoric acid composed '? In what forms is it to be 

 found ? Of what special use is it to plants ? Why is new land 

 generally well supplied with potash ? 



What do artificial fertilizers usually contain V From what sources is 

 nitrogen obtained for these fertilizers ? Name the different sources 

 of phosphoric acid. How is potash obtained ? 



What is o-uano ? What other substances are sometimes useful as fer- 

 tilizers besides those mentioned ? In what form is lime found ? 

 How is quicklime produced ? What is air-slaked lime ? In what 

 three ways does lime benefit soil ? 



What is marl ? Give the history of its formation. Of what use is it 

 as a fertilizer ? 



What is gypsum? How does it benefit soil? Is salt a fertilizer, 

 and how ? 



What points should be understood in order to make a proper use 

 of fertilizers ? How may farmers learn what fertilizers contain ? 

 How can they learn what the soil requires ? Why does one crop 

 need a different fertilizer from another ? Why must soil contain 

 more fertilizing material than the crop uses ? Would the surplus 

 be wasted in a clayey soil ? 



Why should chemical fertilizers be placed near the surface of the 

 soil ? Why should they not be allowed to come in contact with 

 seeds and roots ? Why should but little be used in the hill ? 



Why is the manure heap sometimes called the farmer's gold mine ? 

 In what three respects does stable manure differ from commercial 

 fertilizers ? Why is it a complete fertilizer ? 



What part of the elements of plants is retained in digestion? Of 

 what advantage is the bulky nature of manure ? How does it ren- 

 der land more moist in a dry time ? What part of the value of 

 manure may be obtained in the first crop ? What are the three 

 sources of loss to which stable manure is subject? 



What is fermentation ? How is the heat produced? What substan- 

 ces are lost by fermentation ? Why is fermentation desirable in a 

 compost heap ? In what ways may it be partly prevented ? 



What is the chief advantage of allowing pigs to run upon the manure 

 heap ? What is the value of the liquid parts of manure ? What 

 element gives them their chief value ? What are the benefits of 

 using absorbents in the stable ? 



Of what special benefit is gypsum in manure ? How can there be any 



