FERTILE SOILS 313 



livery tube just passing through the other hole. See that there is 

 no air space between the soil and the stopper. The soil in the bottle 

 should be as hard packed -as it was originally in the ground. If 

 necessary, push a wire down through the neck of the funnel so as 

 to free all hard-packed particles of soil in it. 



Connect the delivery tube with a bottle full of water standing 

 inverted on the shelf of a pneumatic trough. Pour water into 

 the funnel until it is full, and keep 

 it full during the rest of the experi- 

 ment. Allow the apparatus thus 

 arranged (Figure 94) to stand for 

 some hours. Air will collect in the 

 bottle over the pneumatic trough. 

 Where did it come from? When the 

 soil in the bottle has become entirely FIGUBE 94 



saturated with water, roughly com- 

 pare the amount of air collected with the volume of the bottle 

 containing the soil. What part of this soil's volume is the collected 

 air? 



We have seen by this experiment that soil contains air 

 as well as water. Air is needed if plants are to flourish; 

 and it is necessary that soil air be changed frequently, just 

 as it is necessary that air in living rooms be changed if 

 people are to flourish. The soil must be ventilated. Plant 

 roots must have air to breathe. 



Fertile Soils. Rock disintegration does not furnish 

 all the complex materials needed for the growth of agricul- 

 tural plants. Only the lower orders of plants, such as 

 lichens, can grow on soil as at first formed. 



A fertile soil is the product of ages of plant and animal 

 life, labor, and decay. One of the most important plant- 

 foods that is furnished by these means is nitrogen. It is 

 an element that enters into the structure of every living thing. 

 Practically all the nitrogen compounds in the earth's soil 



