80 



ELEMENTS OF AGRICULTURE 



ture of many different compounds. But there are two 

 compounds which are common to all soils and which 

 serve as a basis for their classification. These two 

 compounds are sand and clay. 



81. Sand. — The element silicon, when combined with 

 oxvgen, forms a hard compound, which we know as 

 quartz rock. Quartz often forms small crystals, and in 

 this form makes up a large part of many kinds of rocks, 

 such as granite, sandstone, and some forms of limestone. 

 When these rocks decay, the quartz particles being very 

 hard and insoluble, remain behind and form what is 

 known as sand. Besides quartz grains, sand contains 

 fragments of other minerals, such as mica, particles of 

 iron, and lime. Sand is seldom, if ever, made up ex- 

 clusively of quartz. Silica, as quartz is called, makes 

 up more than one-half of the dry land. It is found in 

 many rocks, and makes up a large part of most soils. It 

 is one of the most abundant substances on earth. Silica 

 is almost insoluble in water; the sand of river and creek 

 beds being practically unaffected by the running water. 

 Water runs through sand easity, and after being wet the 

 sand dries out quite rapidly. Sand has no tendency to 

 become sticky, and though wet sand may stick to one's 

 clothes or person, it is easily brushed off. Sand absorbs 

 much of the sun's heat, as any one who has walked 

 through warm sand with his bare feet can testify. Sand 

 alone makes a poor soil for growing plants; besides sup- 

 plying little food itself, it holds little or no moisture 

 and becomes very hot when exposed to the sun. 



82. Clay. — This substance in its properties is very 

 different from sand, though it is what chemists call a 



