98 NOT ALTOGETHER ABOUT PLANTS 



Mechanical mixtures of solids are quite familiar. Some 

 grocers have been said to make them carefully out of 

 sand and sugar. Before we had a pure food law, careful 

 mechanical mixtures were also made of brick dust and red 

 pepper, and of coal dust and black pepper. 



34 A. Soil. — A certain mechanical mixture made by 

 nature is of extreme importance to plant life and to our life. 

 It is the soil. Our lives depend upon the lives of plants 

 and, to an equal extent, the lives of plants depend upon the 

 soil. To understand the nature of plant life we must under- 

 stand the nature of the soil in which plants grow and from 

 which they draw much of their sustenance. 



In dry soil the molecules of many different substances lie 

 mixed mechanically together. No law makes them tend to 

 separate. Of course, when the soil is moist, as it usually 

 is, many kinds of molecules dissolve in the water. Some 

 dissolve readily, others dissolve quite slowly. Each kind 

 has its own dissolving rate, but all these dissolved mole- 

 cules, as you have seen, tend to become evenly distributed 

 as far as they can travel through the water. Then, if the 

 soil dries out, these molecules are deposited as solids again 

 wherever they happen to be after their journey in solu- 

 tion. So after all, though it is proper to speak of dry soil 

 as a mechanical mixture, we must remember that solution 

 has had much to do with the way in which, the molecules 

 are distributed in it. 



A . Origin and Kinds of Soil. — In order to understand 

 how soil has been formed, we must think of the earth as it 

 was before there was any soil. It is believed that at one 

 time water and rock formed the entire surface of the 



