SULPHUR BACTERIA 277 



nitrogen as a gas. Obviously, from the point of view of 

 conserving soil fertility, they are undesirable. Fortu- 

 nately they thrive best in wet and poorly aerated soils 

 and do not do much damage in properly cultivated soils. 

 Drainage, aeration, and addition of lime to neutralize the 

 acids are the usual means of holding them in check as 

 well as promoting the activity of the nitrifying organisms. 



Sulphur Bacteria. Sulphur is taken from the soil 

 in solution as a compound of sulphuric acid (H2SO4) and 

 built into the proteins and some other compounds of 

 plants and animals. In putrefaction it is commonly liber- 

 ated as hydrogen sulphide (H2S) gas, familiar as the 

 odor of over-ripe eggs. In this form it is usable by types 

 of soil and water bacteria which oxidize the hydrogen to 

 form water and store the sulphur in their bodies. When 

 the supply of hydrogen sulphide runs short they oxidize 

 this stored sulphur to form sulphuric acid and its 

 sulphates. 



The Carbon Cycle. — It is clear from what has been 

 said in this and preceding chapters that almost the only 

 source of the carbon of organic carbon compounds is the 

 CO2 of the air. This is combined with H2O by green 

 plants to form foods and fuels. These in turn are used 

 by nearly all living things as their source of energy. In 

 their metabolism they oxidize carbohydrates and fatty 

 foods to CO2 and H2O. Thus the carbon withdrawn from 

 the air as CO2 by the green plants in the process of photo- 

 synthesis is eventually returned to it when the food is 

 oxidized by the protoplasm of some animal or plant. 

 Fuels are also oxidized in stoves and furnaces, and yield 

 CO2 and H2O. The same carbon has been in this man- 

 ner used over and over again by successive generations of 

 organisms (Fig. 64). ^ 



The Nitrogen CycLE, like that of carbon, permits the 

 same nitrogen to be used repeatedly. Starting with the 

 abundant supply in the atmosphere, it is first made avail- 

 able by nitrogen-fixing organisms. Accumulating in the 



