ORGANISMS IN THE SOIL 437 



its productiveness. It has become customary to refer to 

 the changes produced by certain forms of bacteria as their 

 function in contributing to soil productiveness. 



361. Decomposition of mineral matter. — Certain bac- 

 teria decompose some of the mineral matter of the 

 soil and render it more easily available to the plant. 

 While the nature of the processes and their extent 

 are not known, there is sufficient evidence to justify 

 the above statement. It is well known that several 

 forms of bacteria are instrumental in decomposing rock, 

 and that sulfur and iron compounds are acted upon by 

 other forms. 



To what extent the very difficultly soluble forms of 

 phosphorus, as tricalcium phosphate for example, are 

 rendered soluble and available to agricultural plants by 

 microorganisms, is a matter of great importance. The 

 extent to which the subject has been investigated is 

 rather limited, but, in the main, there is indicated a con- 

 siderable action of both bacteria and fungi on tricalcium 

 phosphate. 



362. Influence of certain bacteria and molds on the 

 solubility of phosphates. — Some very significant experi- 

 ments were performed by Stoklasa, Duchacek, and Pitra, 1 

 who found that bone meal, when brought into contact 

 with pure cultures of certain bacteria, was apparently 

 rendered soluble, the extent to which the solubility pro- 

 gressed varying with the different forms of bacteria 

 brought into contact with it. The percentage of the total 

 phosphorus in the meal that was rendered soluble was as 

 follows : — 



1 Stoklasa, J., Duchacek, F., and Pitra, J. Uber den Einfluss 

 der Bakterien auf die Knochenzersetzung. Centrlb. f. Bakt., 

 II, Band 6, Seite 526-535, 554-558. 1900. 



