842 



JOURNAL OF FORESTRY 



of sodium hydroxide, using phenolphtalein as an indicatoj. The quan- 

 tity of the alkah required to neutralize the soil extract gives, when re- 

 duced to the basis of normal alkali per cubic centimeter of extract, the 

 acidity of the soil in terms of a normal solution. 



The decomposed humus was found by this method (and also by the 

 Truog method) to be neutral; the raw humus showed an acidity of 

 .002 normal and the mineral soil an acidity of .00017 normal. These 

 acidities are weak when compared with those which Coville found in 

 soils favorable to blueberry growth. 



Fig. I. — Influence of soil on the growth of red 

 neutral; the raw humus had an acidity of .002 norm 

 acid by the Truog method; the mineral soil had ai 

 method, and medium acidity by the Truog method. 

 based are given on each graph. 



ipruce (Picca ruhens^. Mild humus was 



1 by the Coville method, and was strongly 



acidity of .00017 normal by the Coville 



Number of trees on which averages were 



The Truog method measures the hydrogen sulphide liberated by the 

 soil acid when the acid combines with zinc in a solution of zinc sul- 

 phide. Calcium chloride is added to the solution to make the test more 

 sensitive. The quantity of hydrogen sulphide is determined approxi- 

 mately by the darkening of lead acetate paper placed over the mouth 

 of the flask in which the soil is boiled with the zinc sulphide and cal- 

 cium chloride. Different colors in the lead-acetate paper express dif- 

 ferent degrees of acidity, according to a standard color chart. Five 

 shades are used, representing very slight, medium, strong, and very 

 strong acidity, the very slight being a faint yellow and the very strong 

 nearly black. The test possesses the advantage of simplicity and ra- 



