38 THE PRINCIPLES OF AGRICULTURE. 



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

 agricultural 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 

 readily soaks through it and runs away. Such land suf- 

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

 to retain for any length of time the supplies of plant 

 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. G-ravelly. Gravel is like sand, except that the 

 rocks of which 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 

 plant food, but it possesses in a remarkable degree the 

 property of absorbing and retaining other substances 

 which tend to render it fertile. Fertilizers which have 

 been applied to clayey soil are retained for a long time, 

 until withdrawn by growing crops. 



Water leaching 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 



