Mar. i, 1924 
Movement of Water in Irrigated Soils 
651 
In combination with the alkali bases, sodium and potassium, it forms 
salts that are indefinitely soluble in water under certain conditions 
though soluble only with difficulty after being dried. In solution these 
salts of silica give a strong alkaline reaction like the carbonates. The 
combination of water with silica, which is known as silicic acid, H 2 Si 0 3 , 
is in some ways analogous to the combination of carbon dioxid and water. 
Both are known as weak acids, but in many respects they are altogether 
unlike. 
From what has just been said it may be inferred that in dealing with 
silica in its relation to the soil solution one finds much that is unknown 
and baffling. There appears to be some justification for the view that 
silica, considered as an acid ion, is chiefly involved in those reactions 
of replacement or exchange that are known to take place between the 
soil and the basic constituents of the soil solution. Until the facts are 
more definitely established it is not possible to do more than to conjecture 
the nature of these reactions and their relationships to the observed 
changes in the physical condition of the soil. The known facts are at 
best extremely fragmentary. We may observe that as a result of treat¬ 
ing the soil with a solution of a sodium salt certain reactions take place. 
Some of the sodium disappears from the solution and equivalent quanti¬ 
ties of other bases appear in the solution. If the solution is then replaced 
as by leaching the soil with pure water, the new solution formed from the 
contact of the water with the soil gives an alkaline reaction and the soil 
manifests symptoms which we designate as “colloidal.” 
These phenomena may be explained by saying that when the solution 
containing sodium was in cpntact with the soil some of the sodium of the 
solution entered into combination with the silica of the soil and replaced 
from combination other bases; also that the silicate compound resulting 
from this exchange reaction remains insoluble and inert as long as the 
solution with which it is in contact contains quantities of the strong 
acid ions—sulphate, chlorin, or nitrate. Finally, if this solution contain¬ 
ing one or more of the strong acid ions be replaced or diluted by pure water 
then the sodium silicate passes from its previously inert or flocculated 
condition into a dispersed condition. In this latter condition it exhibits 
the properties of a colloid in that it may form a hydrosol or a hydrogel. 
In the condition of a hydrogel it absorbs and holds water, thus making 
the soil impermeable to the movement of water through it, and when its 
absorbed water is lost by evaporation the gel cements the soil particles 
together into solid masses. 
It should be clearly understood that the above explanation of the 
phenomena that may be observed from the treatment of . a soil with a 
solution of a sodium salt is proposed merely as a working hypothesis 
and not as a statement of facts. The facts remain to be established by 
further investigation. 
In view of what is known it seems probable that a better understanding 
of the causal relationship between the character of the soil solution and 
the physical condition of the soil awaits further information concerning 
those silicates of the soil that lie along the border line of solubility. 
EFFECT OF DILUTING THE SOIL SOLUTION 
When the soil of an irrigated field contains so much soluble material 
that the soil solution is too concentrated for normal plant growth, the 
only practicable remedy is to apply an excess of irrigation water and carry 
