The Soil and They That Dwell Therein 275 



some carbon dioxide to the air, but in the main the over-all at- 

 mospheric concentration of this gas is not materially raised by 

 such combustion except in the immediate locality. 



CYCLES OF OTHER ELEMENTS 



A scheme similar to those outlined for nitrogen and for carbon 

 can be diagramed for each element in nature, but space will not 

 be utilized in this book to present these cycles. Other textbooks 

 can be consulted by those having a further interest in this phase of 

 science. 



Death is the fore-runner of life. Were it not for the death of 

 plants and animals, available elements would eventually become 

 depleted. Man borrows chemicals for his lifetime, and then after 

 his death these elements are released for future generations when 

 he keeps his rendezvous with the soil. 



MICROBIOLOGY OF PETROLEUM 



A relatively new phase of microbiology is that study concerned 

 with the formation and the destruction of petroleum and its prod- 

 ucts. It appears possible that the temperature and the pressure 

 required for the formation of petroleum are within the range com- 

 patible with bacterial life. There is further evidence that the 

 types of materials, including microorganisms, found on the bottom 

 of seas, resemble those substances found in deep oil wells. Even 

 a pressure of 150,000 pounds per square inch, which mav be the 

 pressure in deep oil deposits, will permit microbial activitv to 

 occur. 



Spoilage of gasoline may take place when certain bacteria begin 

 to utilize this hydrocarbon as a source of food, and a list of such 

 organisms includes some of the common soil genera.— Pseiidomonas, 

 Sarcina, Achromohacter, and Alcaligenes. Even molds and yeasts 

 are capable of attacking petroleum products as a source of carbon. 

 This knowledge is made use of by investigators searching for new 

 petroleum deposits. By inoculating a medium containing all ele- 

 ments necessary for growth except carbon with a petroleum- 



