174 TRANSFORMATION OF MINERAL SUBSTANCES 



phorus (calculated as P2O5) was present in 0.176 parts per million 

 of soil solution, while the inorganic phosphorus was found in only 

 0.034 parts per million. 



The microbial cells which are constantly formed in the soil are 

 also very rich in phosphoric acid ; the ash of the microbes frequently 

 contains 50 per cent or more phosphate, calculated as P2O5. 

 Largely as a result of this consumption of phosphorus by microbes 

 and its use in cell synthesis, the organic matter of the soil contains 

 a definite amount of phosphorus. The ratio of the carbon content 

 of the soil to the organic phosphorus content is more or less con- 

 stant, just as is the case in the ratio of the carbon to nitrogen. 

 When the organic matter of the soil or that added to the soil is 

 decomposed, there is a continuous liberation of phosphorus in an 

 inorganic form, and the more rapidly the organic matter is decom- 

 posed the more rapidly the phosphorus is Hberated. In the pres- 

 ence of an abundance of available organic food material containing 

 little or no phosphorus, the microbes will reassimilate part of this 

 phosphorus in the synthesis of their cell substance, and a deficiency 

 of phosphorus available for plant growth may be created. The 

 amount of food material used by microorganisms for growth, the 

 amount of microbial cell substance synthesized from the food, and 

 the quantities of both nitrogen and phosphorus which are assimi- 

 lated in these processes are all in definite ratios to one another. 

 In other words, a definite amount of microbial cell substance is 

 formed per unit of organic food material consumed, and quite 

 definite quantities of nitrogen and phosphorus are built into these 

 cells. 



Various fungi and bacteria are capable of liberating phosphorus 

 from organic complexes in an inorganic form. Some bacteria 

 are particularly active in this connection since they are 

 capable of attacking the phosphorus-bearing complexes more 

 readily. 



Processes similar to those causing reduction of nitrates and 

 sulfates may be concerned in the reduction of phosphates. Nitrates 

 are quite readily reduced; sulfates, with somewhat more difficulty; 

 and phosphates, least readily of all. Under anaerobic conditions, 

 if organic food materials are available, if phosphates are present 

 in abundance and if the necessary bacteria are active, phosphates 

 may be reduced to phosphite (H3PO3), hypophosphite (H3PO2), 

 and phosphine (PH3). The reactions may be illustrated in a gen- 



