14 SCIENCE AND SOIL 



It may well be stated here Jthat plants secure their supply of 

 both carbon and oxygen from the carbon dioxid of the air. The .01 

 per cent of carbon (C = 12) shown in the table is equivalent to 

 nearly .04 per cent of carbon dioxid (CO 2 = 44). The hydrogen of 

 plants is taken from the water absorbed by the roots. The corn 

 plant secures its supply of nitrogen from the " trace " contained 

 in the earth's crust, which, however, amounts to about .25 per cent, 

 in the tilled stratum of a good soil. Under proper conditions 

 legume plants secure more or less of their nitrogen from the air. 

 The remaining six essential elements are secured only from the soil 

 by all plants. 



Of the atmosphere, ocean, and solid crust (ten miles deep), 

 the solid crust constitutes about 93 per cent of the whole ; while 

 the entire atmosphere amounts to only .03 per cent. These addi- 

 tional facts make possible a mathematical comparison between the 

 supply and crop requirements of carbon and oxygen (in CO 2 ) 

 and nitrogen in the air, and emphasize the importance of the 

 carbon cycle and of the circulation of some other elements, all of 

 which is more fully discussed and explained in the following pages. 



A ready working knowledge, sufficient for everyday use, lies at 

 the basis of success in every industry and profession. It is worth 

 while to have in mind a few fundamental facts relating to the seven- 

 teen elements named in Table 2, which constitute more than 99 

 per cent of earth, sea, and air, and of all plants and animals. 

 Nothing can be made of nothing. 



Compounds consist of two or more elements, and the molecule 

 of a compound must contain two or more atoms. If one knows the 

 valence of the elements, he is then in control of much information 

 of very great value in relation to compounds. Valence is the key 

 to the understanding of compounds and chemical reactions. Table 

 2 gives this information for the very important elements. 



Three of these elements hydrogen, potassium, and sodium 

 have only one bond, or hand, for each atom (H , Na , and 

 K ); while chlorin (Cl) may use i, 3, 5, or 7 bonds. 



Three other elements have only two bonds for each atom 

 (O = , Mg = , and Ca=), these elements being strictly bivalent. 

 Sulfur sometimes uses only two bonds (in H 2 S and CS 2 ), but may 

 use four or six. 



