102 BACTERIA AND SOIL FERTILITY 



either case the combined nitrogen becomes available for higher 

 plants. Then it again starts on its journey through the living and 

 the dead. 



The Sulfur Cycle. — Sulfur is an essential element for all 

 plants and animals, but the quantity required for normal growth 

 and development is relatively small even when compared with the 

 small percentage found in soil. It occurs in the soil as organic 

 and inorganic sulfur. The former is derived from the plant 

 and animal residues. These are acted upon by microorganisms 

 with the liberation of hydrogen sulfide, sulfur dioxid, and sul- 

 fates. Some of the hydrogen sulfide is carried into the ocean or 

 soil by the first rainj some of it reacts upon the iron silicates of 

 the soil and forms pyrite or marcasite, but most of it is oxidized 

 by bacteria with the formation of sulfates. The sulfur dioxid 

 is also further oxidized to sulfates, 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 

 phosphorus. It is also found in places as huge deposits of rock 

 phosphate. 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 phospholipins. 



The mineral phosphates of the soil are rendered soluble through 

 bacterial activity, as outlined previously. This is taken up by the 

 living plant and deposited either as organic or inorganic phos- 

 phorus compounds within the plant tissues. The plant tissues, if 

 eaten by animals, yield phosphorus to the animal to be laid down 

 in the body of the animal as organic or inorganic compounds. 

 The excreta of animals always contain phosphorus in both organic 

 and inorganic forms. The inorganic phosphorus is readily utilized 

 by plants and again starts on its cycle. However, the organic and 

 animal residues must be mineralized by bacteria before they can 

 be utilized again by plants. Microorganisms split off the carbon- 

 aceous material and the phosphorus is liberated mainly in the 



