Rocks and Soil 307 



usually most needed are nitrogen, phosphorus and potassium. The 

 fertilizers used should contain nitrates, phosphates and potash. 

 Farm manure is a good fertilizer when used in sufficient quantity. 

 It contains nitrogen, potash, some phosphoric acid and other 

 ingredients removed from the soil by growing crops. However, 

 it lacks enough phosphoric acid, and should be reinforced with 

 a commercial fertilizer, such as bone meal, acid, or rock phos- 

 phate. Wood ashes are sometimes spread on the soil to provide 

 potash. 



Rotation of Crops. By rotation of crops is meant the rais- 

 ing of a series of different crops on the same plot of land in 

 different years. It is claimed that this plan aids in the control 

 of weeds, fungi and insects, conserves the fertility of the soil by 

 keeping up the humus supply, prevents infection by germs that 

 are harmful to plant life, and permits the alternation of deep- 

 rooted and shallow-rooted crops. For example, the raising of crops 

 of corn on soil infested with the potato-scab deprives this fungus 

 of its food. Certain crops add nutrients to the soil. Thus legumin- 

 ous plants, such as clover and peas, are able to take nitrogen from 

 the air and restore it to a soil from which it has been removed by 

 other plants. 



SUMMARY 



According to their origin, rocks are classified as igneous, sedi- 

 mentary and metamorphic. 



Igneous rocks are rocks which have cooled from a molten mass. 

 Granite is an example. 



Sedimentary rocks are formed by material deposited in layers. 

 Sandstone is an example. 



Metamorphic rocks are sedimentary rocks whose forms have 

 been changed by heat and extreme pressure. Marble is an example. 

 The principal rocks used for building stones are granite, marble, 

 sandstone, and limestone. 



Cement is a modern building material, made largely from 

 limestone. Concrete is a mixture of cement, sand, and stone. 



Most metals are extracted from rock-like masses, called ores, 

 by crushing and heating. 



