98 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 6 
tion, measured by hydrogen-ion, with reference to the hydrogen-ion 
content of water asa unit. To transpose Ps values (which are hydro- 
gen-ion exponents, not concentrations, though often erroneously so 
termed) into specific acidities, subtract from 7, and raise 10 to the 
power thus indicated. The great advantage of this method of state- 
ment is the ease with which the relative acidities of different solutions 
can be appreciated. For example, the Ps values of two of the soil 
extracts discussed later are 5.8 and 5.5; which of these is the more 
acid, and how much more? Considerable calculation would be neces- 
sary in order to answer these questions. When these values are 
subtracted from 7 they give 1.2 and 1.5 respectively; and raising 10 
to these powers with the aid of a table of logarithms yields the specific 
acidities 15.9 and 31.6. It then takes but a glance to see that the 
second solution is practically twice as acid as the first. The use of 
this method is recommended to writers who desire to enable their 
readers to appreciate relative values of acidity with a minimum of 
effort. 
RELATIONS TO BE EXPECTED 
Is any correlation between specific acidity and lime-requirement 
to be expected? Let us consider what these two quantities represent. 
The writer’s views as to their significance have already been published 
in another connection‘ but they would seem to require re-statement 
here in a somewhat different form. In the determination of lime- 
requirement, lime water is added to the soil until the mixture shows a 
neutral or somewhat alkaline reaction. The calcium hydroxide is 
used up in decomposing any aluminium or iron salts present; and in 
neutralizing soluble acids, (the hydrogen-ion of which is the source 
of specific acidity) insoluble acids or acid salts, and any hydrogen- 
ion which may exist adsorbed on the soil colloids,® (the place of which 
may then be taken by calcium-ion). 
As decomposable aluminium or iron salts, (such as the chlorides, 
sulfates or citrates) or of insoluble acids (e.g. dihydroxystearic) 
have never been demonstrated to be present in normal soils in more 
than minute amounts, the lime used up by them must be inconsider- 
able. The amount of lime needed to neutralize the soluble acid repre- 
sented by a specific acidity of even 1000 (Px 4) has been repeatedly 
4 Weology, 1: 160. 1920. Especially pages 167-170. 
5 It is now generally believed that the source of the electric charge on colloids lies 
in adsorbed ions, especially H*+ and OH-. This matter is fully treated in modern 
books on colloid chemistry, and need not be discussed further here. 
