38 THE PRINCIPLES OF AGRICULTURE. 



of particles of quartz rock, Avould be of little value for 

 aii'ricultural purposes. It can furnish no food for the 

 support of plants. It is generally, however, mixed with 

 other substances which give it some fertility. Sandy soil 

 has but little power to retain moisture. Rain water 

 rcadilv sonks through it and runs away. Such land suf- 

 fers severely from a drought. It is, furthermore, unaljle 

 to retain for any length of time the supplies of jjlant 

 food which are formed in it or added to it. These sub- 

 stances are washed out as water passes through it. It is 

 called light soil, and has the advantage of being easy to 

 till. It becomes dry and warm enough for cultivation 

 earlier in the spring than other kinds of soil. 



2. Gravi'Jli/. — Gravel is like sand, except that the 

 rocks of Avhich it is composed have not been ground 

 so fine. Gravelly soil is largely composed of rocks 

 ground to various degrees of fineness. It has the same 

 general properties as sandy soil. When nearly pure, 

 it is even less valuable than the latter for agricultural 

 purposes. 



3. Clayey. — Clay consists of certain kinds of de- 

 composed rock. Pure clay, of itself, contains but little 

 j)lant food, but it possesses in a remarkable degree the 

 pi-o])erty of absorljing and retaining other sul (stances 

 which tend to render it fertile. Fertilizei's which have 

 l)een ai)plied to clayey soil are retained for a long time, 

 until withdrawn by growing crops. 



Water leacbing through such soil is found to come 

 away as pure as when it enters, washing away no valu- 

 able substances. 



It is called heavy soil. Water passes through it very 

 slowly, so that it cannot be tilled until late in the spring, 

 or for a long time after heavy rains. If handled and 



