Mar. i, 1924 
Movement of Water in Irrigated Soils 
637 
In making these solution extractions the dry soil was placed in the 
centrifuge cups and then carefully moistened to a point just below 
saturation. It was left in this condition overnight, protected from 
evaporation, and then centrifuged. The solution extracted represented 
that part of the soil solution that is above the limit of the moisture 
equivalent and below the limit of saturation. The moist soil was weighed 
both before and after centrifuging, and the dry weight was obtained 
later. From these figures the percentage of moisture in the soil was 
determined before and after the run, and it was assumed that the sample 
of the solution extracted represented a moisture condition midway 
between the two. The salt content of the extracted solution was deter¬ 
mined by means of the electrolytic bridge. Table IX shows the salt 
content of the solution extracted from four samples of soil. 
Table IX .—Salt content of the soil solutions extracted by the centrifuge process from 
four samples of soil 
Sample No., 
Percentage 
of solution 
to soil, a 
Salt content (per cent). 
Of solution. 
Of soil 
(indicated). 
384.. 
33 
36 
33 
30 
i 
1. 020 
. 050 
. 062 
2. 200 
o -337 
. 018 
. 020 
.660 
?8e. 
286. 
?8t;a b . 
O u J“ • * *. 
a This is the midway figure between the percentage of moisture in the sample before the run and the 
percentage after the nm. 
b Sample 385a was a duplicate of 385 to which approximately 0.6 per cent NaCl was added before the 
solution extract was made. 
Another method of extracting a sample of the soil solution has been 
used in the laboratory by the writer. It has the advantage of not re¬ 
quiring the use of the centrifuge. By this method a portion of the 
soil solution is obtained by displacement. A leaching pot, such as is 
shown in figure.4, is used for this purpose. After placing a small filter 
paper in the pot, a sample of 300 gm. of dry soil is poured in a little at 
a time and enough distilled water is added with each portion of the soil 
sample to moisten just below the saturation point. The pot is then 
covered to prevent loss of water by evaporation and allowed to stand 
for 24 hours to reach a condition of approximate equilibrium. It is 
then weighed to determine the moisture content. 
The quantity of soil used will usually hold from 90 cc. to 150 cc. of 
water. It has been found that by adding from 25 cc. to 50 cc. to the 
surface of the soil in the pot this added water will displace approximately 
an equal quantity which leaches out from below. A test of successive 
fractions of the percolate has shown that its salt content remains fairly 
constant until the larger part of the original soil solution has been dis¬ 
placed. 
The salt content of the soil solution obtained by this method of dis¬ 
placement from samples of the same soils used in the centrifuge test is 
shown in Table X, together with the computed salt content of the soil. 
The moisture percentage shown in the table is the result obtained by 
weighing the moist soil just before leaching. 
