Oct. 20, 1923 
Soil Reaction in Relation to Calcium Adsorption 
9i 
of boiled or unboiled extracts was 0.2 to 0.3 cc. of the standard solution. 
These figures are within the limits of the error of any data used as a 
basis for conclusions in the present investigation. The portion used 
for titration was also used for the determination of calcium. The ex¬ 
tracts were made acid with HC 1 and the calcium was precipitated as 
oxalate in the presence of NH 4 C 1 and sodium acetate. The oxalate 
was filtered on asbestos, dissolved in H 2 S 0 4 and titrated with 0.05N 
KMn 0 4 . 
MAKING THE ELECTROMETRIC MEASUREMENTS 
Before using, the electrodes were tested by making a blank deter¬ 
mination on a standard acetate solution (4, 35). This blank determina¬ 
tion was made every day. Experience showed that if the electrodes 
were carefully washed after each determination they could be used 
three or four times before they needed to be replatinized. The hydrogen 
was bubbled through the suspensions or extracts contained in the elec¬ 
trode vessels, which were shaken continuously during the measurements. 
Readings were taken at such intervals as were found necessary, and 
continued till they remained constant, within 1 millivolt, for five minutes 
or more. Most soil suspensions or extracts did not come to an apparent 
equilibrium in much less than an hour, and many required a longer 
time. Extracts or suspensions, the reaction of which is near P H 7 come 
to equilibrium much more slowly than those of a higher or lower P H . 
This was particularly true if CaC 0 3 was present. In some such cases 
the material required as much as five or six hours to come to equilibrium. 
The tables of Schmidt and Hoagland (40) were used for calculating the 
P H values corresponding to the voltmeter reading, the necessary cor¬ 
rection for the saturated calomel cell being made. 
EXPERIMENTAL RESULTS 
GROUP A.—INFLUENCE OR CALCIUM HYDROXID, CALCIUM CARBONATE, 
POTASSIUM CHLORID, HYDROCHLORIC ACID, AND OXALIC ACID, SINGLY 
OR IN COMBINATION, UPON P H OF SUSPENSIONS AND EXTRACTS OF 
NATURAL SOIL; ALSO UPON THE TITRATION AND CALCIUM CONTENT 
OF THE EXTRACTS 
EXPERIMENT I.—S 0 IL+WATER 
It has been shown by several investigators (23, 57, 41) that the 
reaction obtained on a soil suspension in water is different from that 
obtained on a filtered extract. This difference is due to the relative 
insolubility of the acid-forming substance. In Table I are found the 
P H values obtained on four soils, together with the titration values on 
100 cc. extract and the amount of water-soluble calcium. The extracts 
of these four soils came to equilibrium very slowly. These extracts 
were not boiled before titrating, and so contain the maximum amount of 
C0 2 likely to be present in such extracts. The amount of calcium ob¬ 
tained is small. This was to be expected, since the total calcium content 
of these soils is low. 
