Mar. i, 1924 
Movement of Water in Irrigated Soils 
635 
in turn largely influenced by its chemical composition, and its chemical 
composition is largely influenced by the character of the salts dissolved 
in the soil solution. 
Thus it is possible to modify very greatly the rate of water movement 
through the soil under a given set of conditions by changing the character 
of the salts dissolved in the soil solution. There is abundant evidence 
bearing on this point which is well known to all investigators of irrigated 
soils and to many practical irrigators (6, 16). 
The crux of the matter appears to be that clay in the presence of water 
containing dissolved salts participates in chemical reactions by which 
exchanges may take place between certain elements combined with the 
clay and other elements dissolved or dissociated in the water. As a 
result of these exchange reactions the chemical composition of the clay 
is altered and in consequence its physical properties also. It is to be 
noted that the changes in the physical properties of the clay appear to 
be disproportionately large as compared with the changes in chemical 
composition resulting from these exchange reactions. 
The manifestation of the changes that may take place in the physical 
properties of clay are not confined to its effect on the movement of water 
through it. It is shown also in the behavior of the soil on drying and iff 
that complex of properties known as plasticity. 
From the above observations it seems safe to conclude that the most 
important factor influencing the rate of percolation in soil is to be found 
in the clay portion and also that the percolation rate is determined by 
the physical condition of the clay rather than by its relative quantity in 
in the soil. 
THE SOIL SOLUTION 
It will be apparent from the foregoing statements that the movement 
of water into or through the soil of an irrigated field is largely influenced 
by the character of the material in solution in that water. This relation¬ 
ship between the character of the salts in the soil solution and the permea¬ 
bility of the soil is an aspect of the so-called alkali problem that has not 
been extensively investigated. By far the larger part of the investigations 
concerning alkali troubles have been concerned with the realtionships 
between the crop plants and the soil solution. 
It is fairly well recognized that when the quantity of dissolved material 
in the soil solution becomes excessive, crop plants are injured. There is 
no definite agreement as to what proportions of dissolved material in the 
soil solution are to be regarded as excessive. In fact, it has been so 
difficult as to be almost impossible to determine what may be called the 
critical concentrations of salts in the soil solution. Numerous investiga¬ 
tions have been made to determine the toxic limit of solutions with plants 
grown in water cultures, but the translation of these results into terms of 
the concentration of the soil solution has not been satisfactory. 
The direct determination of the quantity or of the character of the ma¬ 
terial dissolved in the soil solution is rendered difficult by the fact that 
it has not been practicable to obtain samples of this solution as it exists 
in the soil under such conditions as make it available to the plant roots. 
In other words, it is very difficult to extract water from the soil when the 
water content is no higher than what may be regarded as optimum for 
plant growth. 
