178 MINERALIZATION AND SOLVENT BACTERIA 



In a later work, Severin obtained different results. He used three 

 soils one sterile, a second sterilized and inoculated with pure 

 cultures of Azotobacter, and a third sterilized and inoculated with 

 cultures of Ps. radicicola and Azotobacter. The solubility of the 

 phosphorus increased 8 to 14 per cent, over that in the sterile soil. 

 The acid-producing organisms, due to the acid secreted and their 

 intimate contact with the soil particles, possess the power of dissolv- 

 ing silicates. Moreover, arsenic greatly stimulates nitrogen fixa- 

 tion, and there is a relationship between this increased bacterial 

 activity and the form and quantity of phosphorus found in a soil. 



Although the metabolic activity of Azotobacter gives rise to large 

 quantities of phosphate solvents, yet these organisms transform 

 phosphorus into organic phosphorus compounds less rapidly than 

 do the ammonifiers. There are, however, cases in which bacterial 

 activity has decreased the water-soluble phosphorus of the soil and 

 of raw rock phosphate. This does not mean, however, that it is 

 less available, for, as pointed out by Truog, the mixing of floats with 

 manure caused an immediate decrease in the solubility of the phos- 

 phorus in 0.2 per cent, citric acid solution, yet when thoroughly 

 mixed with the feeding area of the soil its availability was increased 

 to such an extent that some species of plants were apparently able 

 to secure almost an adequate supply of phosphorus from this 

 material. The addition of manure to a soil greatly increased the 

 carbon-dioxid production, and for a short time measurably increased 

 the solvent action on floats. Where there is for a time a decrease 

 of water-soluble phosphorus in fermenting media, it is probably due 

 to the formation of phospho-proteins within the bodies of the 

 bacteria which would later be rendered soluble due either to further 

 bacterial activity or to autolytic enzymes. 



Sulphur. Sulphur is an essential element for all plants, but the 

 quantity required is relatively small and most soils contain sufficient 

 for maximum crop production. It occurs within the soil mainly as 

 sulphate or organic sulphur, and these substances are often materially 

 changed by bacterial activity. 



Bacteria act on sulphur compounds in three ways: (1) on complex- 

 organic compounds with the production of hydrogen sulphid or 

 mercaptans, (2) the oxidation of sulphur compounds occurring in the 

 soil, and (3) the oxidation of sulphur compounds, especially hydrogen 

 sulphid by the true sulphur bacteria, with the production of metallic- 

 sulphur, sulphuric acid, and eventually mineral sulphates. 



Hydrogen sulphid is produced by the majority of the common 

 laboratory forms of bacteria. Lafar states that this faculty is even 

 very common among the pathogenic bacteria and was absent in not a 

 single one of 37 species examined. Other bacteria possess the 

 power of reducing sulphates. Beijerinck found in'soil an organism 

 which he named Spirillum desulphiiricans and which Van Delden 



