656 PRINCIPLES OF SOIL MICROBIOLOGY 



pensions, and, after a certain period of incubation, the amounts of car- 

 bon dioxide formed from 1,100 gm. of soil and the phosphorus brought 

 into solution were determined, 34 as shown in table 72. 



On inoculating sterilized soil with pure cultures of bacteria, Sewerin 

 obtained a gain of 14 per cent of P2O5 soluble in acetic acid for Azoto- 

 bacter -f- Bacterium sp.; a gain of 12.9 per cent for Bac. mesentericus vul- 

 gatus; a gain of 8.0 per cent for Bad. radicicola + Azotobacter; a loss of 

 5.8 per cent for Bad. fluorescens liquefaciens; and a loss of 12.6 per cent 

 for unsterilized soil. No correlation was found between the bacterial 

 population of the soil and the soluble P 2 5 , and none between the latter 

 and the energy of decomposition of the soil organic matter. 



When manure is added to the soil, the rapidly growing bacteria cause 

 a definite decrease in the water-soluble phosphorus of the manure, 

 and transform it into organic phosphorus. This is eventually re- 

 leased in an available form as a result of the action of the bacteria on 

 the dead microbial cells, after the available energy had been used up. 35 

 The addition of green manure and stable manure to citrus soils in Cali- 

 fornia was found to bring about a measurable increase in solubilty of 

 phosphorus, calcium, magnesium and iron. 36 



The addition of carbohydrates to the soil brings about an increase 

 in the number of microorganisms and a diminution in the amount of 

 phosphoric acid soluble in 2 per cent acetic acid. 37 The amount of 

 soluble phosphate in the soil was found to depend not so much upon 

 the numbers of microorganisms as upon their kind. As to the influence 

 of season of year, an increase in soluble phosphate is usually found in 

 the spring and fall and a decrease in the summer as a result of the 

 activities of microorganisms. 



The amount of available phosphorus in the soil will thus depend on 

 the total phosphorus in the soil, nature of the phosphorus compounds, 

 soil reaction, presence of available energy and nitrogen, and kind of 

 microorganisms. A bacteriological method for determining the amount 

 of available phosphorus in the soil is described elsewhere (p. 577). 



Transformation of insoluble phosphates by inorganic and organic acids 

 formed by microorganisms. Inorganic acids are formed in the soil by 



35 Tottingham, W. E., and Hoffmann, C. Nature of changes in the solubility 

 and availability of phosphorus in fermenting mixtures. Wis. Agr. Exp. Sta. 

 Res. Bui., 29: 273-321. 1913. 



36 Jensen, C. A. Effect of decomposing organic matter on the solubility of 

 certain inorganic constituents of the soil. Jour. Agr. Res., 9: 253-268. 1917. 



17 Bazarevski, 1916 (p. 653). 



