90 
Journal of Agricultural Research 
Vol. 26, No. 3 
PREPARATION OF THE SOIL SUSPENSION 
Soil, water, Ca(OH) 2 solution, or other chemical, two or more as the 
particular experiment called for, were placed in 500 cc. wide-mouth 
bottles and closed with rubber stoppers. These were then placed on a 
shaking machine which had a 2-inch forward and back motion at the 
rate of about 90 vibrations per minute. From preliminary experiments 
it was found that it was best to leave the soil in contact with the 
Ca(OH) 2 for about 24 hours. Experiments in which contact existed for 
48, 72, and 96 hours indicated that slight chemical 
changes took place after 24 hours, but so small in fact 
as not to interfere with the main purpose of these 
experiments. The usual plan was to prepare the mix- 
/ |[| \ q tures in the afternoon, shake for an hour, let stand 
IJBJ over night and then shake for a while the next morn¬ 
ing. The suspensions were then transferred to 250 
cc. wide-mouth bottles used as electrode vessels, and 
the electrometric measurements made the same day. 
As six of these measurements were carried on simul¬ 
taneously, 12 or 18 determinations were a convenient 
day's work with those suspensions or extracts which 
jjj attained equilibrium in the usual time. 
PREPARATION OF SOIL EXTRACTS 
For the extracts 20 gm. of soil and 200 cc. of Ca(OH ) 2 
were generally used. These quantities were suffi¬ 
cient to produce 100 cc. of material for the titra¬ 
tions and calqium determinations and at least 50 cc. 
for the electrometric measurements. After shaking in 
the same manner as for making the suspensions the 
soil was allowed to settle. When the quantity of elec¬ 
trolytes present were sufficient to cause coagulation, 
a clear solution could be obtained by pipetting off 
the supernatant liquid. In other cases the contents 
of the bottles were transferred to 500 cc. bottles and 
centrifugalized for about five minutes. The centrifugal 
force attained was about 1,300 times gravity. A per¬ 
fectly clear solution was not always obtained by this 
method, as some colloidal clay frequently remained 
in suspension. The opalescence was approximately 
that obtained by long-time settling of clay in water. 
This did not interfere with the titrations, and usually 
did not interfere with the calcium determinations. 
Some calcium silicate may have been present in colloidal 
suspension and may account for a small part of the 
calcium obtained in some extracts. The colloidal 
matter seemed to affect the electrometric measure¬ 
ments, making them more uncertain and reducing 
the number of times an * electrode could be used before replatinizing. 
ui, , 
Fig. 4.—-Hydrogen elec¬ 
trode supported in 
rubber stopper which 
fits the bottle used as 
electrode vessel. C, 
capillary glass tube 
making connection 
with the saturated 
potassium chloride 
calomel cell. B. glass 
tube through which 
excess gas may es¬ 
cape. The hydrogen 
enters at D. 
TITRATION AND CALCIUM DETERMINATION 
The titrations were made by means of 0.05N HC 1 and 0.05N NaOH, 
using phenolphthalein as an indicator. The extracts were usually 
boiled before titrating. The maximum difference in titrating 100 cc. 
