SUMMARY 279 



plaster. It contains much calcium sulphate, or gypsum. 

 Sometimes the name is used for gypsum itself. 



Nitrogen is obtained for fertilizers in a number of forms. 

 Farmyard manure is of course the most common source 

 of nitrogen on the farm. Two commercial sources in 

 this country are cottonseed meal and fish-scrap meal; 

 the latter is about 8% nitrogen and 6% phosphorus. 

 The waste obtained in slaughterhouses (tankage), con- 

 sisting of blood and of scraps of meat and bone, is very 

 rich in nitrogen and is converted into fertilizer. The 

 value of such nitrogen compounds depends on the fact 

 that nitrifying bacteria (cf. 324) change much of the 

 nitrogen into nitrates (cf. 299) for the use of food plants. 



If a soil becomes acid, or "sour," it needs a basic substance (cf. 

 220) to make it "sweet." Lime and limestone are used for this 

 purpose. These substances also have important effects on the struc- 

 ture of soil (cf. 295). A very sandy soil is not fertile, because its 

 particles are too far apart, and do not retain water. Lime and lime- 

 stone help to cement the separate grains together (cf. 285), forming a 

 less porous soil. While they make a sandy soil less porous, lime and 

 limestone make a clayey soil more porous; they separate the particles 

 of clay, which cohere too closely, by forming a " nucleus," or center, 

 to which the clay particles can adhere. The soil particles are thus 

 made larger, and the soil more porous. 



303. Summary. The earth's crust consists of mantle rock and bed 

 rock. Exposed bed rock is an outcrop. Soil is the portion of the 

 mantle that is used for growing crops. 



Common rocks are sandstone, limestone, shale, granite, and con- 

 glomerate. 



Rocks are stratified, unstratified, and metamorphic. 



Stratified rocks are also called sedimentary, or aqueous, rocks. 

 They were formed, as deposits, under water. 



Unstratified rocks are also called igneous rocks. They are found 



