146 Bacteria in Relation to Country Life 



lands contain a relatively slight number of bacteria 

 before they are reclaimed and placed under cultivation. 

 When drained and limed, the number of bacteria 

 soon increases from a few thousands to many millions 

 per gram of soi . . 



I BACTERIA AND THE DECOMPOSITION OP SOIL-HUMUS 



The dark-colored humus substances in the soil, the 

 extensive deposits of peat in certain localities, and the 

 beds of bituminous and anthracite coal have a common 

 origin. They are all derived largely from atmospheric 

 air. 



The atmosphere that surrounds our earth is a mixture 

 of transparent gases, two of them, nitrogen and oxygen, 

 being present in large proportions. A third, carbon 

 dioxid, is present only in a small proportion, three of 

 four parts in ten thousand parts of dry air. It is believed 

 that at an earlier period in the history of our earth the 

 atmosphere contained a larger proportion of carbon 

 dioxid. Owing to the latter, and to an abundance of 

 moisture, the plants of that period grew more luxu- 

 riously and accumulated an enormous amount of vege- 

 table matter which was transformed, in the course of 

 many centuries, into bituminous and anthracite coal. 



The growth of plants in our own day, while not so 

 luxuriant as that in the carboniferous era, still takes 

 place in accordance with the same laws. The colorless 

 gas, carbon dioxid, is decomposed by green plants into 

 its constituent parts, carbon and oxygen. The carbon is 

 utilized by the plants for the building of their tissues. 



