38 BOTANY: PRINCIPLES AND PROBLEMS 



102. Why are wood ashes so valuable as fertilizer? 



103. Why do sewers often become clogged with roots? 



104. What two important contributions to human welfare are made 

 by the bacteria of decay? 



105. What various organisms living in the soil may be harmful to 

 plants? 



106. What is the chief importance of angleworms in the growth of 

 plants? 



107. Plants native to the woods will often fail to thrive when trans- 

 planted to a garden, even though the soil there is rich and the conditions 

 of shade and temperature are much like those in the forest. Can you 

 suggest a reason for this difficulty? 



108. At what point in the nitrogen cycle, and in what form, is loss of 

 available nitrogen most apt to occur? 



109. Manure left freely exposed to the air will lose much of its fertiliz- 

 ing value. Why? 



110. For many plants, rather old manure is better than that which 

 is absolutely fresh. What reason can you suggest for this? 



111. Plants which have very deep roots (such as certain weeds and 

 cover-crop plants) are sometimes of advantage to crops which are 

 subsequently grown on the soil. Why? 



112. A "cover-crop" is a crop (such as rye) planted in late summer or 

 early fall which grows up enough to cover the ground before winter. 

 What are the advantages of planting such a crop? 



113. Why is flood-plain or "river-bottom" soil usually very produc- 

 tive? 



114. In what ways may the productivity of a soil be diminished other 

 than by the removal of crops grown upon it? 



115. It has long been recognized that land which is left uncropped 

 or "fallow" for a few years will prove to be more productive after- 

 ward. How do you explain this? 



116. In what ways is the productivity of a soil maintained in nature, 

 before it becomes the seat of agriculture? 



117. What other functions may the addition of fertilizer to the soil 

 perform aside from that of providing nutrient materials for plants? 



