640 
Journal of Agricultural Research voi. xxvii, no. 9 
(3) The electrical conductivity of the solution is measured by the use of a Wheatstone 
bridge. The bridge reading may be converted in terms of percentage salt content 
by the use of a calibration table. 
(4) A fairly complete analysis may be made, determining quantitatively each of 
the constituents found to be present and the sum of these reported as the total salts. 
(5) A few of the more important acids may be determined volumetrically and these 
results calculated to their equivalents as salts of sodium, added together, and reported 
as total salts. To do so involves assumptions that may be misleading. All that is 
known concerning the composition may be given quite as clearly by reporting the 
quantities of the various elements or ions that have been identified. 
The various constituents of the soil solution are usually reported as 
parts per million. It is convenient to remember that parts per million 
may be converted into parts per hundred or percentage by moving the 
decimal point four places to the left, e. g., 12,000 parts per million equals 
1.2 per cent. Likewise, where there is occasion to convert parts per 
million into pounds per acre-foot of water or of soil, it may be remem¬ 
bered that an acre-foot of water weighs approximately 2.75 million 
pounds and an acre-foot of soil weighs about 4 million pounds. Thus, 
if a sample of water is found to carry 730 parts per million of salt that 
would be equivalent to 1 ton of salt per acre-foot of water. 
There are in addition a number of modifications of these methods, but 
these examples serve to show something of the diversity of methods that 
are in general use for estimating the salt content of the soil solution. 
The material dissolved in the soil solution is assumed to exist as salts 
in equilibrium as to acids and bases. The methods of chemical analysis 
do not permit the identification of these salts as such, but only of the 
acid or basic radical or ion. Thus, we may determine with a fair degree 
of accuracy the quantity of chlorin (Cl), of sulphate (S 0 4 ), or nitrate 
(N 0 3 ) in a solution and also the calcium (Ca), the magnesium (Mg), or 
even the sodium (Na), but we can not with certainty know how these 
exist in the solution with reference to each other. It is questionable 
whether it is desirable to attempt to state the composition of soil solution 
or of irrigation and drainage waters in terms of combined salts. 
. IMPORTANT CONSTITUENTS OF THE SOIL SOLUTION 
The term soil solution as here used is intended to include not only the 
solution as it exists in the soil of an irrigated field but also the accumu¬ 
lated underground water or drainage and the water used for irrigation. 
In other words, water that has been in contact with soil is here termed 
soil solution. It is not intended to include as a part of the soil solution 
any of the inert suspended matter that may be removed by careful 
filtering. 
The more important constituents of the soil solution as it exists in 
relation to irrigated soils may be enumerated as follows: The bases are 
calcium, magnesium, sodium, and potassium. The acids are sulphate 
(S 0 4 ), chlorin (Cl), bicarbonate (HC 0 3 ), carbonate (C 0 3 ), nitrate (N 0 3 ), 
phosphate (P 0 4 ), and silica (Si 0 2 ). Other bases, such as manganese, iron, 
and aluminum, are sometimes found in soil solutions, and some other 
acids, particularly organic acids, also occur, but these are not often 
separately identified. 
There is some diversity in the methods used for quantitative determina¬ 
tion of the constituents of the soil solution. It is not the purpose here 
to give a detailed description of these methods, but merely to give an 
account of them that may serve as a basis for a later discussion of soil 
reactions in which the character of the soil solution, as determined by 
these methods, plays an important part. 
