98 BACTERIA AND SOIL FERTILITY 



breaking down, bacteria plan a major part. The higher plants 

 build up the carbon and nitrogen into complex organic com- 

 pounds. The same end is also accomplished to a lesser degree by 

 the animals which, however, mainly act as decomposers of organic 

 matter, but the master analysts are the bacteria which are con- 

 tinually resolving into simple and often elementary constituents 

 all plant and animal debris. Were this not true, all the carbon 

 and combined nitrogen of the world would soon become locked 

 up in the dead bodies of animals j plants would starve and die, 

 and animals likewise would become extinct. Therefore, bacteria 

 are the link between the living and the dead. The absence of 

 bacteria is incompatible with life on this earth, or, as stated by 

 Pasteur, *'They are the important, almost the only, agents of uni- 

 versal hygiene. They clear away more quickly than the dogs of 

 Constantinople or the wild beasts of the desert, the remains of all 

 that has had life; they protect the living against the dead; they 

 do more; if there are still living beings, if, since the hundreds of 

 centuries the world has been inhabited, life continues, it is to 

 them we owe it." 



The Carbon Cycle. — Carbon occurs free in the earth as coal 

 to the extent of over £ve hundred billion tons. Chemically com- 

 bined, it is found in far larger quantities in limestone, chalk, 

 marble, and dolomite — -rocks which form such a considerable 

 portion of the surface of the earth. According to Pettenkoffer, a 

 man weighing 154 pounds contains 26.4 pounds of carbon. No 

 less than two hundred and fifty-seven million tons' weight of it is, 

 therefore, stored up in the bodies of men and women living upon 

 the earth at the present time, to say nothing of the far greater 

 quantities occurring in the tissues of trees, plants, and lower 

 animals. 



Carbon dioxid occurs in the atmosphere to the extent of three 

 parts in 10,000. This is the equivalent of six hundred billion tons 

 of carbon. Moreover, the ocean is a vast reservoir of carbon 

 dioxid which is partly in solution and partly combined. Between 

 the surface of the sea and the atmosphere there is a continual 

 interchange, each at times losing and at times gaining the gas. 



