190 A YEAR IN SCIENCE 



as the ice expands it eventually splits and crumbles 

 the rock into many smaller pieces. 



Kinds. Since the surface of the earth originally was 

 solid rock and water, it is evident that soil is the prod- 

 uct of decayed rock, together "with the remains of de- 

 cayed plant and animal life. 



In the weathering of rocks we find two kinds of soil 

 resulting. The fine, soft powder, or rock flour, is called 

 clay; while the hard, loose, insoluble particles form 

 sand. 



Clay. Clay, when dry, is a powdery substance ; when 

 wet, it is sticky and plastic, and easily molded. It 

 takes up water slowly and when thoroughly wet be- 

 comes compact and solid, difficult to cultivate and diffi- 

 cult for the roots of plants to penetrate. 



It is equally slow to give up the moisture once ab- 

 sorbed. When thoroughly wet it is able to hold as 

 much as 40% of its weight of water. Thus in wet 

 weather it may hold too much water for good growth, 

 while in drought it may bake and become too difficult 

 for roots of plants to penetrate. 



Air in the soil is necessary for plant life. Although 

 clay is finely powdered and affords great pore space, 

 because of the smallness of the spaces, air moves about 

 in it with difficulty, aerating it but poorly. 



Sand. Sand is made up of hard separate particles. 

 It is loose and gritty, absorbs water readily, and gives 

 it up just as readily. It often contains less than 5% 

 of water. When wet the particles of sand are some- 



