164 FIRST YEAR COURSE IN GENERAL SCIENCE 



other chemically and carbon dioxide is set free. While this 

 gas is rising through the dough, cooking stiffens the material 

 and it remains porous, making biscuit, cake, and pastry 

 "light." 



184. Fertilizers. Plants, during their growth, are con- 

 tinually removing from the soil certain salts which are 

 needed for their food. In forests and uncultivated lands, 

 the plants die and decay in the ground, and thus all the salts 

 are returned to the soil. But on farms the plants are re- 

 moved when they are grown, and are used as food for animals 

 or men. Moreover, rain dissolves some of the salts of the 

 soil and carries them away to sea. In these ways, the salts 

 needed for plant growth are continually removed from the 

 soil and are not returned to it. 



It is easy to see that if this loss continued, the fertility of 

 the soil would soon be exhausted, and it would be impossible 

 to grow crops. The loss may be made good, however, by 

 the use of fertilizers. These are combinations of salts to 

 replace the material taken from the soil. 



Fertilizers may be mineral or organic. A mineral fertili- 

 zer is one which is obtained directly from rocks or earthy 

 deposits. An organic fertilizer is decaying vegetable or ani- 

 mal matter. Calcium phosphate, from the great beds of 

 fossil bones near the coast in North and South Carolina, has 

 long been a source of phosphates for fertilizers. Guano (the 

 excrement of various kinds of marine birds) is found in ex- 

 tensive beds on islands off the coast of Peru and Chili. 

 Guano contains phosphates and nitrogen compounds, both 

 of which are used as foods for plants. 



185. The Science of Agriculture. The composition of 

 soils is learned by analysis made in the laboratories and fields 

 of agricultural colleges and of the Unites States Department 

 of Agriculture. By analysis it is possible to determine also 

 what compounds are needed in the soil for certain crops. 

 Grass, corn, and potatoes each has its own needs, and the 



