A FERTILE SOIL 



rain and warm air, and also upon density and color. 

 The denser or more compact soil and the darker soil 

 having greater power to absorb heat. 



The compactness of the soil which gives it greater 

 powers to absorb heat weakens its powers to hold it, 

 because the compactness allows more rapid conduc- 

 tion of heat to the surface, where it is lost by radia- 

 tion. 



The more moisture a soil holds, the weaker is its 

 heat-holding power, because the heat is used in 

 warming and evaporating water from the surface 

 of the soil. 



These important properties or conditions of moist- 

 ure, heat and air, are, as we have seen, dependent 

 on soil texture and color, which in turn are depen- 

 dent upon the relative amounts of sand, clay and 

 humus in the soil. We are able to control soil tex- 

 ture and therefore these physical properties to a 

 certain degree by means of tillage and the addition 

 of organic matter or humus (see Chapter IV). 



BIOLOGICAL PROPERTIES OF A FERTILE SOIL 



Biology is the story or science of life ; and the 

 biological properties of the soil have to do with liv- 

 ing organisms in the soil. 



The soil of every fertile field is full of very small 

 or microscopic plants called bacteria or germs. They 

 are said to be microscopic because they are so small 

 that they cannot be seen without the aid of a pow- 

 erful magnifying glass or microscope. They are so 



