Mar. z, 1934 
653 
Movement of Water in Irrigated Soils 
change in its physical character when the solution is diluted. On the 
other hand, when the dissolved bases are chiefly sodium and potassium 
the dilution of the soil solution is accompanied by a pronounced 
reaction of the soil that is manifested in several ways, among which 
impermeability to the movement of water is one of the most striking. 
Before undertaking a detailed discussion of the reactions that take 
place and the changes that occur in the physical condition of the soil 
when the soil solution is diluted, it may be proper to cite some examples 
of changes in the balance of the constituents of the solution that go on 
during the process of dilution. There are at least three ways in which 
such dilutions may be made in the laboratory: (1) By digesting similar 
samples of soil with different quantities of water; (2) by repeated 
digestions of the same sample, withdrawing a part of the solution after 
each digestion and replacing it with an equal quantity of distilled water; 
(3) by leaching a sample of soil with distilled water, analyzing successive 
fractions of the percolate. While none of these methods exactly simulates 
conditions that exist in the field, each of them gives results that contribute 
to an understanding of the reactions that may be brought about by the 
application of irrigation water to a tract of salty land. 
Reference has been made in a preceding chapter to the fact that 
when samples of the same soil are digested with different quantities 
of water and a portion of the water subsequently withdrawn for analysis 
it is found that with some soils the apparent salt content is higher 
when a large quantity of water is used for digestion than when a smaller 
quantity is used. This phenomenon might be interpreted as indicating 
that some of the soluble material of the soil is very slightly soluble and 
that as more water is used for digestion more and more of this slightly 
soluble material comes into solution. 
This interpretation is not wholly satisfactory, because it can be shown 
that if the solution obtained by digesting a soil sample with a large 
volume of water is concentrated by evaporation, precipitation does not 
occur until the solution has become much more concentrated than the 
solutions obtained by using a smaller quantity of water for digestion. 
Furthermore, the known solubility under laboratory conditions of such 
salts as would be indicated from the constituents identified in the solution 
is much higher than is obtained from these digestion experiments. The 
implication is that some part of the soil, presumably the clay, reacts 
toward some of the soluble constituents in much the same way as water 
does. The result of this reaction appears to be to retard the passing of 
these constituents into the soil solution. 
A more detailed discussion of these relationships between the soil and 
its solution may be postponed for the present until more of the pertinent 
facts have been established. Some of these facts may be brought out 
by comparing the character of the solutions obtained by various dilu¬ 
tions. It can be shown that the increased dilution of the soil solution 
with certain types of soil not only increases the apparent salt content 
of the soil but modifies the character of the solution in so far as this is 
indicated by the relative proportions of its constituents. 
An example of these differences in the quantity and character of the 
material dissolved from a soil by different quantities of water is shown 
in Table XVIII. The soil used in this experiment was from an irrigated 
field in Arizona. The solution of this soil showed only traces of calcium 
