THE NATURE OF SOIL 29 



remove from a soil comes from the air. The air, 

 not the soil, is the greatest storehouse of fertility. 

 From the air plants get, through their leaves, three 

 other foods oxygen, hydrogen and carbon. These 

 are all gases, the latter being combined with oxy- 

 gen in the form of carbonic acid gas. The supply 

 of these plant foods is, so far as we know, in- 

 exhaustible. A friend once remarked, "That is 

 mighty lucky. I have a hard enough time now, 

 trying to supply my worn-out soil with enough 

 potasn, phosphoric acid and nitrogen to grow 

 profitable crops; yet you say these are only side 

 dishes of a plant's dinner. If I had to supply it 

 with the main dishes, or fillers, as you might call 

 these foods that it now gets from the air, I don't 

 believe I could have raised my family of six on 

 these forty rocky acres of New England soil." 



HOW WATER IS HELD IN THE SOIL 



All fertile soils contain many tons of water, which 

 is present in the soil in several forms. First, and 

 most conspicuous, is what is variously called free 

 water, ground water, standing water or bottom water. 

 This fills all the spaces between the particles up to a 

 certain height, which varies with different soils, and 

 even different parts of the same field. Free water 

 is supplied by rainfall; it frequently comes to the 

 surface as springs and is often the source of supply 

 of wells. If a hole is dug in any soil water will 

 stand in it up to a certain point, which may be 

 several inches or many feet below the surface. 

 This point is called the "water table." The 

 height of the water table may be judged in a general 

 way by the depth of surface wells, but this evidence 

 is not always reliable. It may vary at different 



