266 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



bacteria and fungi themselves. Its color probably is in part 

 determined by the pigments produced by the bacteria and 

 the molds. 



THE CYCLE OF PHOSPHORUS 



Phosphates are quite essential to the development of most 

 higher plants. They are usually present in soil in the form 

 of tricalcium phosphate which is quite insoluble. It must 

 be converted into soluble phosphate to be taken up by plant 

 roots and combined into organic compounds such as the 

 phosphoproteins, nucleic acid, etc. The solution of the 

 insoluble phosphate in soil is probably in large part the 

 result of the action of organic acid and the carbon dioxide 

 produced as the result of the growth of microorganisms and 

 of the formation of nitric acid from ammonia. There seems 

 to be good evidence that rapid nitrification in the soil brings 

 about a considerable solution of the insoluble phosphates, 

 rendering them available for plant growth. 



Phosphates also have a relationship to the activity of soil 

 microorganisms in their direct stimulating effect upon 

 growth. Many species of bacteria bring about fermentative 

 changes much more rapidly in the presence of phosphates 

 than in their absence. This is most certainly true with the 

 bacteria which are capable of fixing atmospheric nitrogen. 



THE CYCLE OF SULPHUR 



Sulphur in nature is found in its free form, as sulphates, 

 sulphides and in organic compounds, particularly in cer- 

 tain classes of proteins and in certain essential oils, such 

 as mustard oil. Microorganisms are active in bringing 

 about the various transformations of sulphur from one 

 condition to another. Protein compounds are broken down 

 into amino acids, particularly the amino acid cystein, and 

 from this is formed hydrogen sulphide. This may be 

 oxidized to sulphates. Sulphates are taken up by the plant 



