660 PRINCIPLES OF SOIL MICROBIOLOGY 



and iron phosphates in addition to calcium as shown by the following 

 analysis of Florida rock: 



per cent -per cent 



P 2 6 35.52 A1 2 3 1.03 



CaO 49.92 Si0 2 3.57 



MgO 0.46 C0 2 1.64 



Fe 2 3 0.98 S0 3 0.21 



H 2 2.67 



The reactions involved in the conversion of the rock phosphate (in- 

 soluble tricalcium phosphate) into soluble forms (di- and monocalcium 

 phosphate and phosphoric acid) by means of acid belongs to the type 

 of reactions of heterogeneous systems. The rock phosphate minerals 

 have no definite composition and the products formed are not always 

 the same. The following factors control the reaction: 42 (1) concentra- 

 tion of the reacting mass; (2) temperature of the reacting medium; 

 (3) the amount of contact of the reacting substances; (4) the speed 

 of diffusion of the reacting substances; and (5) catalytic agents. 

 In addition to these, other factors are of importance, including 

 the chemical composition and the physical properties of the solid 

 phase. These have a tremendous influence on the speed of the reac- 

 tion and they are the least known, since the chemical makeup of the 

 rock phosphate is still obscure. 



Schloesing 43 showed in 1898 that the quantity of phosphoric acid 

 dissolved in normal soils is a result of an equilibrium of very complex 

 chemical processes tending, on the one hand, to take this acid out of 

 solution and, on the other, to bring it into solution. It may only be 

 added that these chemical processes are brought about by the activi- 

 ties of microorganisms, and whatever will influence these activities 

 will also influence the amount of phosphorus available in the soil at 

 any given time. 



Transformation of potassium in the soil by microorganisms. Potas- 

 sium is present in the soil in the form of organic compounds and in the 

 form of various zeolitic and non-zeolitic silicates. The potassium 

 added to the soil is either in a soluble inorganic form, an insoluble inor- 

 ganic form (marl) , or an insoluble organic form (manures) . The K 2 

 content of fresh manure ranges from 0.288 to 0.504 per cent. 44 The ash 



< 2 Kazakov, A. V. Moskau Inst. Agron., 9: 21^5, 57-68. 1913. 

 43 Schloesing fils, Th. Etude sur l'acide phosphorique dissout par les eaux 

 du sol. Compt. Rend. Acad. Sci., July 25, 1898. 

 "Thome, 1914 (p. 429). 



