ORGANIC ACIDS IN THE SOIL 



165 



With all of the acid substances, whether they be weak or strong, 

 their effects on mineral materials are proportional to the degrees of 

 acidity produced. Figs. 66 and 67 show this relationship. The 

 stronger acids naturally cause solvent effects in low concentra- 

 tions equal to those brought about by the weak acids in propor- 

 tionately high concentrations. 



4.0 

 Solution of Calcium 



2.0 LogCca 



Increase 



Fig. 66. — The influence of acidity created by lactic acid on solution of calcium 

 from calcium silicate (after Wright). 



Change in Soil Reaction. — Besides being important as 

 potential supplies of plant nutrients when acted upon by acid 

 substances, the phosphates, carbonates, and silicates play an 

 important role as agencies regulating the rapidity of change of 

 soil reaction. In the absence of an abundance of buffering mate- 

 rial in soils, the large amounts of acids which are produced would 

 soon change the reaction to such acidity as to make the soil a 

 medium unfit for the growth of most higher plants and of various 

 important soil organisms, especially those that are sensitive to 

 acid conditions, as Azotobacter, nitrifying bacteria, and Bad. 

 radicicola. The presence of carbonates, phosphates, and silicates 



