88 FIRST YEAR SCIENCE 



bunches of particles that may still cling together. Why was it neces- 

 sary to soak the soil so long? Draw the shapes of a few of the par- 

 ticles. Describe the composition of the soil you have examined. 



If we examine most soils with a microscope, we shall 

 find that they are composed, as was seen in Experiment 48, 

 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 

 there are many more, dependent upon the fineness; and 

 in a gram of clay there are several billion particles. Ag- 

 ricultural soils, intermediate between 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 entirely from the rocks, only 

 those minerals which were present in the underlying rock 

 can be 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 

 substances if the soils are to support plants. The 

 substances needed in most abundance are nitrogen, 

 phosphoric acid, potash and lime. Practically all soils 

 except the quartz sands contain more or less of these 

 substances. 



The chemical make-up of the soil is, however, only one 

 of the qualities necessary for it to support plant life. It 

 must contain water. Plants require a very great deal of 

 water. Yet few plants absorb the proper amount of water 

 if they are submerged in it, or even if their roots aTe sub- 

 merged. They must have the soil only partly saturated 

 with water. 



