zoo NOT ALTOGETHER ABOUT PLANTS 



that rocks and, consequently, soils are made up of dit- 

 ferent kinds of substances. Thus sand, though composed 

 of grains all about the same size, may contain many dif- 

 ferent substances. These substances of which rock and the 

 soil made from rock are composed are called minerals. 

 Thus clay is largely composed of the mineral called feldspar, 

 while sand is chiefly made up of quartz. There are many 

 hundreds of different kinds of minerals. When you hear 

 of the mineral constituents of soil you will understand that 

 the words refer to those substances in the soil which come 

 from rock. These substances are very important to plant 

 life. They furnish many elements which are essential to 

 it. From what you have learned about molecules, you 

 should understand that each kind of mineral is composed 

 of the same kind of molecules. 



Different kinds of rock do not produce soil of equal fer- 

 tility. Limestone and sandstone are two of the common 

 forms of rock. Soils derived from limestone " are famous 

 for fertility, as in the blue-grass region of Kentucky and 

 the prairies of Texas" (Dryer's High School Geography). 

 Very much of the soil of the North-Central United States 

 is largely composed of material deposited by the melting 

 of the great ice-sheet which once covered these regions. 

 Regions once covered by ice-sheets are said to have been 

 glaciated, i.e. covered by a great glacier. Glacial soils 

 contain a great variety of minerals, and are usually fertile. 

 The glaciated region of the United States is one of the 

 greatest food-producing areas in the world. The most 

 fertile of all soils are the kinds which are alluvial. Alluvial 

 soils are those which are deposited by streams, especially 

 at flood time, upon the floors of broad valleys, i.e. upon 

 the flood- plains. You have heard of " rich bottom lands." 



