Mar. i, 1924 
Movement of Water in Irrigated Soils 
661 
In the second of the two leaching experiments described on the pre¬ 
ceding pages it was shown that the addition of sodium chlorid to the soil 
solution was followed by a pronounced increase in the quantity of calcium 
in the solution. This phenomenon, which has been called the exchange 
of bases, is one that has been reported upon by several investigators. 16 
The exact nature of this phenomenon has not been determined. It 
has been observed that exchanges take place between the basic constitu¬ 
ents of the soil solution and the basic constituents of certain insoluble 
or very slightly soluble substances in the soil. These exchanges of basic 
constituents that take place between the solution and the soil are prob¬ 
ably more important in their effect on the physical properties of the soil 
than are those changes in the proportion of the acid constituents that 
have been discussed above. Until the nature of these reactions is better 
understood it is not possible to do more than speculate concerning their 
processes or their exact character. The results may be recognized with¬ 
out difficulty. 
When a soil is treated with a solution of a sodium or potassium salt 
that is more concentrated with respect to these elements than the 
solution already present, a part of the basic element is withdrawn 
from the solution and its molecular equivalent of other bases passes into 
solution from combination with the soil, until a new condition of equi¬ 
librium is established. Similar reactions take place when other basic 
elements are used, such as calcium, magnesium, iron, and aluminum. 
The consequences of these exchange reactions is to modify the character 
of the soil as well as of the solution. 
These exchange reactions proceed in the direction of establishing a 
condition of equilibrium between the soil and its solution. It has been 
found that a limit may be reached beyond which no further reaction 
takes place. With the soil as with the solution when the limit of satura¬ 
tion is reached reaction ceases. A soil is said to be saturated with 
respect to a certain basic element when it ceases to show an exchange 
reaction except when the concentration of that element in the solution 
is increased. 
The results of these exchange reactions on the physical properties of 
the soil are strikingly different with the different bases. With respect 
to their effect on the soil the bases that occur most commonly in the soil 
solution fall into two groups. These groups are sometimes designated 
as the alkaline and the earthy bases. The alkaline group includes sodium, 
potassium, and ammonia. The earthy group includes calcium, magne¬ 
sium, iron, and aluminum. While it may be possible to distinguish 
between the reaction effects produced in the physical condition of the 
soil by elements of the same group, these differences if they exist are 
very slight as compared with the differences produced by representative 
elements of each. In order to simplify the discussion of the effects pro¬ 
duced by the elements of each group on the physical condition of the 
soil, one element from each may be taken as typical. Thus the alkaline 
group may be represented by sodium and the earthy group by calcium. 
Before proceeding to discuss in detail the effects* on the physical prop¬ 
erties of the soil of the exchange reactions between sodium and calcium 
it may be well to establish more definitely the fact that such exchange 
reactions actually take place. Evidence on this point has been contrib¬ 
uted by a number of investigators, but its interpretation has been 
M See: Way, J. Thomas (ax, 22), and Gkdroiz, K. K. (q). A comprehensive bibliography of this sub¬ 
ject is given by C ummin s and Kelley (6). 
