TRANSFORMATION OF PHOSPHORUS BY SOIL MICROBES 173 



microbial metabolism. In fact, most of the nitrogen trans- 

 formations have very close counterparts in changes of sulfur 

 compounds. 



Transformation of Phosphorus by Soil Microbes. — Phos- 

 phorus is usually added to the soil in the form of various plant and 

 animal residues (in stable and green manures), in certain organic 

 fertilizers (tankage, bone meal, guano), and in various inorganic 

 fertilizers. Phosphorus is present in the latter either as insoluble 

 rock phosphate, the chief constituent of which is Ca3(P04)2, or 

 as superphosphate, which consists of rock phosphate previously 

 treated with sulfuric acid to make it soluble, or as various other 

 basic phosphates included in slag. 



In the plant and animal residues, as well as in stable and green 

 manures, phosphorus is present in the form of such compounds as 

 phytin, phospholipids, of which lecithin is the best known repre- 

 sentative, and nucleoproteins. Before the phosphorus can be 

 utilized again by higher plants, these organic complexes have to be 

 decomposed by various soil organisms. These processes of decom- 

 position can be illustrated somewhat as follows : 



C6H24027P(5 + 6O2 = 6H3PO4 + 6CO2 + 3H2O 



Phytic 

 acid 



The phytic acid originates in the decomposition of phytin. 

 Lecithin contains both phosphorus and nitrogen; it is first hydro- 

 lyzed to cholin (a nitrogen complex), glycerophosphoric acid, 

 and various fatty acids. Nucleoproteins are compounds of 

 one or more protein molecules with nucleic acid; the latter con- 

 tains the phosphorus. Nucleic acids themselves are also very 

 complex in composition, such as C3GH48O30N14P4. On decompo- 

 sition, they give, in addition to phosphoric acid, various carbo- 

 hydrates and organic bases (adenine, guanine, cytosine). The 

 decomposition processes of some of these substances are frequently 

 very complicated, involving a number of reactions, but they all 

 lead to the formation of phosphoric acid. 



The phosphorus in organic combination may make up from 20 

 to 35 per cent of the total phosphorus of many soils. Organic 

 phosphorus may also compose a large portion of the phosphorus 

 occurring in solution in soils. In a study of twenty-one soils of 

 Southern and Central United States, the soluble organic phos- 



