FUNCTIONS OF SOIL BACTERIA 



403 



that of nitrate salts, but where the supply is insufficient 

 the value of ammonium salts is relatively quite low. 



274. Functions of soil bacteria. Bacteria have 

 a part in many of the processes of the soil which 

 greatly affects 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. 



275. Decomposition of mineral matter. Certain 

 bacteria 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 decom- 

 posing rock, and that sulfur and iron compounds are 

 acted upon by other forms. Again, the much greater 

 efficiency of difficultly soluble phosphate fertilizers, 

 when used in conjunction with a quantity of organic 

 matter, is evidence of the relation of bacterial action 

 to the decomposition of mineral substances. Stocklasa 

 has shown that, when B. megatherium and B. fluor- 

 escens are added to soil fertilized with insoluble 

 phosphates, plants grown thereon take up a larger 

 amount of phosphorus than those on uninoculated 

 soils. 



Organic acids and carbon dioxid are constantly 

 produced by soil bacteria. These in soil water are 

 weak but ever-acting solvents, the effect of which 

 must in the end be considerable. It seems likely, 

 however, that there is a more direct effect of certain 



