108 FIRST YEAR SCIENCE 



The soil comes chiefly from the decaying and weather- 

 ing of rocks. It is divided into local or sedentary soil, 

 that which is formed from the rocks directly beneath it, 

 and transported soil, that which is generally brought down 

 and deposited by water, ice and wind. Soils are classed 

 roughly as gravel, sand, clay, and loam. 



The fertility of the soil depends largely upon composi- 

 tion, air and water, ventilation and drainage. Fertile soil 

 must contain nitrogen, potash and lime. The roots of 

 plants must have air to breathe, and water must dissolve 

 the nourishing substances in the soil and bring them to 

 the roots to be absorbed. For this reason the soil should 

 not be packed hard like clay, nor should it be loose like 

 coarse gravel, as clay does not let the water soak through 

 readily, while gravel lets it seep down too fast. 



To maintain its fertility, the soil must be frequently cul- 

 tivated to decrease seepage and evaporation. It must be 

 supplied with fresh nourishment by manures or other 

 fertilizers. In some districts where the rain supply is 

 inadequate, irrigation and dry farming are practiced. 

 Different soils are suited to different crops. Without 

 the fertility of the soil, life on the earth would cease. 



QUESTIONS 



For what would you look if trying to determine whether a land 

 surface had ever been under the sea ? 



What are the characteristic differences between land and water 

 surfaces and between the conditions to which the animals and plants 

 of each are subjected ? 



What reasons can you suggest for likening the earth to a water 

 engine ? 



To what great classes do the rocks in your neighborhood belong ? 



What examples of rock weathering have you ever seen ? Describe. 



Is the soil in your neighborhood local or transported ? Does its 

 character vary much in different places? Does the fertility vary? 



