310 MAN'S USE AND CONSERVATION OF SOILS 



If we examine most soils with a microscope, we shall 

 find that they are composed, as was seen in Experiment 90, 

 of many different kinds of material. Some of these mate- 

 rials dissolve slowly in water and thus furnish food for 

 plants; others are insoluble. 



In different soils the particles vary greatly in size as 

 well as in composition. In gravel the particles are large 

 and in a gram's weight there would be but few ; in sands 



RELATIVE SIZES OF SOIL PARTICLES 

 From left to right : clay, silt, sand, gravel. 



there are many more, dependent upon the fineness ; in silt 

 particles are still smaller ; and in a gram of clay there are 

 several billion particles. Agricultural soils, intermediate be- 

 tween sand and clay, are usually called loams. There are 

 sandy loams and clayey loams, with many intermediate 

 varieties. As the mineral part of the soil is derived en- 

 tirely from the rocks, only those minerals which were present 

 in the underlying rock can IDC present in sedentary soils, 

 whereas in transported soils the, underlying rock has had 

 no influence upon the soil. 



The minerals composing the soil must furnish certain 



