WOOD SOIL CONTACT CULTURE TECHNIQUE 



121 



A) Ca(NO,)j-4HjO . 207 



MgS04-7H,0 1 18.8 



KH.PO4 83.6 



B) ZnS04 0. 176 



MnS04-7H..O 0.572 



Boric Acid 704 



AU(SO0. : 0. 176 



C) FeS04 5.0 



Note: The salts in A, B, and C were dissolved separately and then the three parts 



mixed. 



The sticks were removed and examined at intervals of three, twelve, and 

 fifteen months. The percentage failure, as determined by the ease with 

 which the specimen could be broken, is shown in the following: 



At the end of 12 months the greatest number of failures due to decay 

 was observed in the plot treated with the acid salt (aluminum sulfate). 

 Colorimetric determinations of the pH of the soil showed it to be between 

 5.8 to 6.0 for the soil treated with the acid salt and about 6.6 to 6.8 for the 

 control plot. This experiment needs to be repeated for confirmation of 

 results, but the present indications are that the acid soil was much more 

 favorable for decay than the soil in the control plot. Limed and fertilized 

 soils gave results comparable to those of the control plot, with an indication 

 that the fertilizer increased the decay. The complete disintegration of 

 the sticks in the acid treated soil was particularly noticeable, whereas the 

 sticks in the soil treated with manure were intact, though easily broken. 

 The plot treated with the nutrient solution showed the same rate of decay 

 as the manure plot at the end of the twelve-month period. Ten pounds 

 of aluminum sulfate were then added to the nutrient plot, and three months 

 later all the sticks were completely disintegrated. The rapid disintegration 

 of the. sticks in the plot treated with the acid salt points to the importance 

 of further work on the effect of the pH on the rate of decay. 



Comparison of Soil Technique With Other Toxicity Assays 



In the selection of a method for testing relative toxicity of chemicals to 

 microorganisms, the rapidity of test, the standardization of the medium. 



