THE SOIL AND ITS IMPORTANCE TO PLANTS 30 



118. State at least three advantages which are gained by plowing 

 the soil. 



119. Cultivating the soil around growing crop plants (by hoeing or 

 otherwise) is not necessary or advantageous after the crop has covered 

 the ground fairly well. Explain. 



120. How can soil in a state of nature, entirely without cultivation, 

 support plant life so well? 



121. Give several reasons for the desirability of keeping the soil 

 around a young fruit tree well "mulched." 



122. Vegetable growers often sterilize by steam, formalin or other 

 means, the soil in the greenhouse beds where they are to grow their 

 young plants for later transplanting outside. What advantages has 

 this process? What disadvantages would it have if applied in the 

 field? 



123. Farmers sometimes build a big bonfire over the spot where they 

 are going to start their young plants of cabbage, tomato, and similar 

 crops. What two advantages are gained by this? 



REFERENCE PROBLEMS 



12. What is meant by tillage? 



13. What is irrigation and how is it practiced? 



14. Soils which are poorly aerated are apt to be "sour." Explain. 



15. Distinguish between animal manures, green manures, and com- 

 mercial fertilizers, and state the advantages in the use of each. 



16. What is the chemical formula of atmospheric nitrogen? of ammonia? 

 of potassium nitrate? of some complex nitrogenous compound? 



17. May organic substances dissolved in the soil solution enter directly 

 into ordinary green plants and be used by them as food ? Explain. 



18. Give an example of a crop plant which thrives best on alkaline soil; 

 one that thrives best on acid soil. Which of these two soil types do most 

 crop plants favor? 



19. About how many bacteria are usually present in 1 c.c. of rich soil? 



20. What is "rotation of crops " and what is its value? 



21. Give the derivation of each of the following terms and explain in what 

 way it is an appropriate one: 



Capillarity Humus Mycorrhiza 



