THE WORK OF ROOTS 107 



The experienced eye easily distinguishes between the 

 particles of hornblende (which are black) , those of mica 

 (which are brownish) and those of quartz and feldspar, 

 which are both of light color but differ in form. 



HUMUS. Examine a little leaf -mold: can you make 

 out its nature? Place some of it under the microscope: 

 place some on a red-hot shovel for some minutes : what 

 change takes place I Perform the same experiment 

 with some dark colored soil : does it change color ? The 

 simplest way to remove all the humus from a soil is to 

 burn it thoroughly ; we then find that the dark color of 

 the soil is due entirely to the humus. The fact that 

 humus burns so readily recalls the origin of peat and 

 coal, which are formed from humus: find out what you 

 can about this. Humus gives to the soil a loose, open 

 texture such as we see in leaf -mold ; it makes the land 

 "mellow" and gives it "heart"; it retains an extra- 

 ordinary amount of water (up to nearly twice its own 

 weight) and is rich in plant-food. It often accumulates 

 in forests to considerable depths. In ordinary soils hu- 

 mus is present to the depth of three or four feet (i. e., 

 the depths to which the roots ordinarily penetrate) , and 

 this part of the soil has a looser texture and darker 

 color than the underlying subsoil, which is devoid of 

 humus . 



By a proper admixture of humus, sand and clay we 

 can obtain a soil of any desired consistency or texture 

 and of any grade of fertility. The gardener does this 



