THE SOIL AND ITS IMPORTANCE TO PLANTS 37 



85. After sowing seed for certain crops, a farmer often rolls the soil 

 with a heavy roller. What is gained by this? 



86. "Why is irrigation necessary in some regions l)ut not in others? 



87. What is the l)est time of day to water a garden? Why? 



88. Why is it necessary to drain wet land before ordinary crops can 

 be grown thereon? 



89. In what ways is the air in the soil constantly being changed and 

 renewed? 



90. Of what use is the air in the soil in addition to providing oxygen for 

 plant roots? 



91. Give two reasons why the formation of a "crust" on the soil 

 surface is harmful to plants. 



92. Is it better to water house phmts frequently antl lightly or infre- 

 quently and heavily? Why? 



93. How is the supi)ly of humus maintained in soils which are not 

 under cultivation? 



94. Dark-colored soil is usually richer than light-colored soil. 

 Explain. 



95. Why is soil from low land usually darker in color than soil from 

 a side hill? 



96. The continual addition to the soil of no other fertilizer than 

 the ordinary "commercial" fertilizers is generally found to be harm- 

 ful. Why? 



97. What conditions are there under which the addition of humus to 

 the soil might be actually injurious to plants? 



98. How would you prove that a particular element is, or is not, essen- 

 tial to plant life? 



99. Most commercial fertilizers contain nitrates, phosphoric acid, 

 and potash. Why? 



100. A good crop of corn, wheat or potatoes removes about 50 pounds 

 of nitrogen per acre from the soil. Nitrogen composes about four-fifths 

 of the weight of the atmosphere. At this rate of removal by crops, 

 how many years would the nitrogen in the air over an acre last, provided 

 that it could be made available to the plants growing there? 



101. Name several ways in which chemical substances essential to 

 plant growth are wasted in modern civilization. What methods can 

 you suggest to prevent some of this waste? 



