184 THE CARBON, NITROGEN, AND PHOSPHORUS CYCLES 



story the quantity of combined nitrogen in the world would remain 

 constant. But it is not there are many leaks in the cycle. Some 

 of the plants and animals may be burned with the liberation of free 

 nitrogen. Millions of pounds of it reach sewers, and from here 

 rivers, lakes, and oceans. In time this is broken down and the 

 nitrates so formed are reduced by denitrifying bacteria with the 

 liberation of gaseous nitrogen. The processes of decay continually 

 going on may also liberate free nitrogen. Furthermore, millions of 

 pounds of nitrogen are returned to the air by explosives. So the 

 combined nitrogen would continue to grow less were it not that other 

 factors are at work in nature causing it to combine. Every flash 

 of lightning causes some nitrogen to combine as oxids, but the 

 quantity of combined nitrogen thus formed is relatively insignificant. 

 The major factors are biological. There are within the soil two 

 great groups of bacteria which possess the power of fixing nitrogen. 

 The first the non-symbiotic nitrogen-fixing organisms living free 

 in the soil are able, with the energy they obtain from the oxidation 

 of organic carbon, to build up complex organic nitrogen compounds. 

 There are two groups of these organisms the aerobic and the anae- 

 robic, the first being the more important. The other class of 

 nitrogen-fixers is the symbiotic; these live in conjunction with 

 legumes and obtain from them carbonaceous material, and in return 

 give combined nitrogen. In either case the combined nitrogen 

 becomes available for higher plants. Then it again starts. on its 

 journey through the living and the dead. 



The Sulphur Cycle. Sulphur is an essential element for all plants and 

 animals, but the quantity required for normal growth and develop- 

 ment is relatively small even when compared with the small per- 

 centage found in soil. It occurs in the soil as organic and inorganic 

 sulphur. The former is derived from the plant and animal residues. 

 These are acted upon by microorganisms with the liberation of 

 hydrogen sulphid, sulphur dioxid, and sulphates. Some of the hydro- 

 gen sulphid is carried into the ocean or soil by the first rain; some of it 

 reacts upon the iron silicates of the soil and forms pyrite or marca- 

 site, but most of it is oxidized by bacteria with the formation of 

 sulphates. The sulphur dioxid is also further oxidized to sulphates, 

 when they are again taken up by plants and start anew upon their 

 wonderful journey through bacteria, higher plants, and animals. 



The Phosphorus Cycle. Phosphorus occurs in the soil in the form 

 of calcium, aluminum, and iron phosphate, also as organic phos- 

 phorus. It is also found in places as huge deposits of rock phos- 

 phate. It is an integral part of every living plant and animal cell. 

 In these it occurs in two forms organic and inorganic. The organic 

 phosphorus occurs in the nucleo-proteins, phospho-proteins, and 

 phospho-lipins. 



The mineral phosphates of the soil are rendered soluble through 



