Mar. x, 1924 Movement of Water in Irrigated Soils 687 
Such evidence as is available appears to indicate that the improvement 
of the physical condition of irrigated soil to the end of making it more 
readily permeable to water depends upon disposing of the sodium, whether 
this exists in the soil solution or in replaceable combination with the soil. 
It seems clear that it is the sodium and not the carbonate of black alkali 
that causes deflocculation and impermeability in irrigated soils. 
THE REMOVAL OF SODIUM FROM THE SOIL 
The sodium which causes trouble in irrigated soil must be thought of as 
existing partly as in readily soluble salts and partly as in combination 
with the soil material. The sodium salts that are readily soluble, such as 
sodium sulphate and sodium chlorid, may be removed from the soil by 
leaching, which is in effect merely replacing the solution in the soil by 
another solution applied at the surface. This is a very simple matter if 
the soil is readily permeable to the downward movement of water. The 
sodium that is combined with the soil material and which in such combi¬ 
nation is only slightly soluble is the part that is difficult to remove from 
the soil. It is this combined sodium that causes deflocculation of the soil 
and consequent impermeability. 
The removal of this combined sodium from the soil by leaching is pos¬ 
sible only by replacing it with another base. It is possible to replace 
sodium from its combination with the soil with any other soluble base. 
There would be no point, however, in attempting to do so with potassium, 
for example, because such an exchange would not mend matters. Long 
experience has shown that calcium is the safest, most effective, and most 
easily available base to use in replacing sodium from the soil. The com¬ 
bination of calcium with the soil induces flocculation and makes for im¬ 
proved physical condition and permeability. Certain other bases, such as 
magnesium, iron, and aluminum, appear to produce the same effect. 
A great variety of methods and of materials have been proposed for 
correcting the injurious effects resulting from the combination of sodium 
with the soil. Calcium sulphate, commonly known as gypsum, has long 
been recommended and extensively used. As a preventive of bad con¬ 
ditions it appears to be altogether satisfactory. As a remedy to be used 
when bad conditions have developed it is sometimes inadequate because 
it is only slightly soluble. Calcium chlorid and calcium nitrate are both 
very soluble, but the first is so deliquescent that it is difficult to handle 
except in solution and the second is relatively expensive. Iron sulphate 
is a by-product of certain manufacturing industries and is often to be had 
at prices that might justify its use in land reclamation. Aluminum sul¬ 
phate is also a readily soluble salt and aluminum in solution actively 
displaces sodium from its combination with the soil and improves the 
physical conditions. With both iron and aluminum there is a possibility 
that when used in excess colloidal hydroxids may be formed which might 
cause temporary impermeability. 
The assumption that the deflocculated condition of the soil is caused 
by the existence of alkaline carbonates in the soil solution has lead to 
some attempts to remedy this condition by the use of strong acids. Some 
experiments have been made with sulphuric acid, which is often very 
cheap, and elemental sulphur has also been used to some extent. The 
implication is that elemental sulphur when applied to the soil is oxidized 
through bacterial action and converted into sulphuric acid. This 
oxidation process takes place slowly, so that the product seldom reaches 
