LESSON IV 

 THE SOIL 



How soil is made. By the action of air and water, 

 and heat and cold, the solid rock of the earth's surface 

 decays and breaks up into fine particles. The layer 

 of fine mineral particles is called soil. But in addition 

 to the rock particles, soil contains water, air, minute 

 living forms, and decaying plant and animal matter. 

 All of these things are necessary to plant growth. 



Water in the soil. When soil is saturated, all the 

 spaces between the soil grains are filled with water and 

 the soil contains no air. Part of the water will drain 

 away and air will flow in to take its place. The water 

 which drains off from a saturated soil is called free 

 water. The water remaining in the soil exists as thin 

 sheets or films around the soil particles. This is called 

 film water. In this water is dissolved the food for 

 the growing plants. 



Air in the soil. - - The roots of growing plants need 

 air. The useful bacteria in the soil also use air. There- 

 fore it is necessary that there be a free movement of 

 air in the pores of the soil. Too much water in the 

 soil prevents the air from reaching the plant roots, 

 and the crops do not thrive. If the soil remains satu- 

 rated too long, the plants die for lack of soil air. 



