636 
Journal of Agricultural Research 
Vol, XXVII, No. 9 
In ordinary investigational work where the salt content of the soil 
solution is the subject, it is customary to treat a sample of the soil with a 
large quantity of water and then filter off a part of the water for examina¬ 
tion. Some investigators use 5 parts by weight of water to 1 part of soil. 
^Others use 10 parts of water and still others use 20 parts of water to 1 part 
of soil. There are also some differences of methods as to the length of 
time the water is left in contact with the soil samples and as to the extent 
to which the mixture of soil and water is agitated. 
It is assumed that the quantity of water used is adequate to dissolve all 
of the readily soluble material in the soil. An examination of an aliquot 
part of the water used in treating the soil sample is then made, and the 
result of this examination multiplied by the appropriate factor gives the 
quantities of the dissolved materials in terms of the dry weight of the soil. 
Thus the salt content of irrigated soils is usually given in terms of the 
percentage of the dry soil rather than as a percentage of the soil solution. 
It is obviously not practicable to attempt to state the salt content in 
terms'of the soil solution, for two reasons: (1) Because the quantity of 
the soil solution and consequently its concentration changes from day to 
day as water is lost by evaporation or added by rainfall or irrigation, and 
(2) because there is reason for believing that the quantity of material in 
solution is affected by the quantity of water present. It is largely because 
of this second reason that investigators hesitate to express the results of 
their soil analyses in terms of concentration of the soil solution, even 
when the water content of the soil has been determined. 
A single example will serve to show the range in concentration of the 
soil solution if it is assumed that the salt content of the soil is all dissolved 
in the soil solution. Where the salt content of the soil is 0.5 per cent 
and the moisture content is 25 per cent it might be assumed that the salt 
content of the soil solution would be 2 per cent. On the other hand, if 
the salt content is 0.5 per cent and the moisture content is 10 per cent 
then the salt content of the soil solution would be 5 per cent. 
It is the consensus of opinion among investigators that the roots of 
ordinary crop plants can not tolerate concentrations of mixed salts in 
solution much above 1.5 per cent. Notwithstanding this, many instances 
are reported of crop plants making fair growth in soils which show by 
analysis as much as 0.5 per cent of salt to the dry weight of the soil and 
where the moisture content frequently falls as low as 15 per cent. 
Such observations have led to the conclusion that when the soil solution 
becomes very concentrated through the loss of water by evaporation, 
some of the salt may be reabsorbed in some way by the soil. 
EXTRACTION OF THE SOIL SOLUTION 
While it is difficult to obtain a sample of the soil solution when the 
water content of the soil is at or near the wilting point, it is possible by 
the use of a centrifuge to obtain a sample when the water content is up 
to or just above the optimum for plant growth. The centrifuge used 
in the experiment here described was one designed by Briggs and McLane 
for use in determining the moisture equivalent of soils ( 2 ). The machine 
used differed from the one described in the publication cited in that it 
was equipped with an outer casing which serves to collect the water 
that is thrown out of the soil during the run. 6 
6 The writer is indebted to J. W. McLane for his cooperation in making these solution extractions. 
