Mar. i, 1924 
Movement of Water in Irrigated Soils 
667 
physical conditions through the exchange reactions described above. 
For this purpose such salts as calcium sulphate (gypsum) or aluminum 
sulphate are among the most available. Of these two salts gypsum is 
the cheaper and more generally accessible. It has the disadvantage of 
being less soluble than aluminum sulphate and consequently of reacting 
more slowly. 
The beneficial effects that follow the use of gypsum on soils that do 
not take water readily have long been known, and this salt is extensively 
used on irrigated land. The assumption that the physical condition of 
the soil that is associated with slow permeability was due to the existence 
of sodium carbonate in the soil solution made it natural for earlier 
investigators to explain the effect of gypsum as a reaction with sodium 
carbonate. When solutions of sodium carbonate and of calcium sulphate 
are brought together a reaction takes place by which calcium carbonate 
is precipitated from the solution. It was assumed that a similar re¬ 
action takes place when calcium sulphate is applied to a soil. It is not 
essential in the present connection to decide whether the calcium applied 
as gypsum reacts with substances in the soil solution or takes part in 
exchange reactions with the soil. The obvious fact is that the applica¬ 
tion of gypsum to a soil that is slowly permeable to water generally 
results in its improvement. 
It has been noted above that gypsum is not very soluble and that its 
effectiveness is limited on that account. It requires about 500 parts of 
water to dissolve 1 part of gypsum under favorable conditions. In 
many situations this low solubility is not a serious disadvantage because 
the reactions in the soil may go on slowly and be long continued. Where 
the conditions appear to require more effective action than may be 
obtained from gypsum it is possible to use calcium in a more soluble 
form as calcium chlorid or calcium nitrate or to use aluminum sulphate. 
This latter salt which occurs in nature in limited quantities is also manu¬ 
factured for use in industry and for clearing turbid waters for domestic 
and industrial use. It has been found that aluminum, like calcium, 
reacts through the soil solution with the soil to modify its physical 
condition (9, 15). 
In the application of these salts to the soil in the field it would seem 
advisable to limit their use to those spots where the soil is impermeable 
rather than to apply them to the whole field. The end sought for is to 
obtain more uniform conditions of water penetration, and this end may 
be attained more economically by confining the treatment to the spots 
that show the need of it. After a field has been irrigated an exploration 
of it, using a sharp pointed steel rod or a soil augur, makes it possible 
to locate the spots where the water has not penetrated to the depth 
desired. These spots may then be given appropriate treatment. 
An example of the effect of an application of aluminum sulphate to 
increase the permeability of the soil is shown in Plate 1. The field in 
which this experiment was made is at Sacaton, Ariz. A number of 
borders in this field included spots on which there was no crop growth, 
though other places in the same borders produced good crops. In the 
experiment illustrated in the figure two small basins were made side by 
side in one of the bare spots. To one of these* the one shown on the 
right, aluminum sulphate was applied at the rate of 5 tons per acre; 
the other basin was left untreated. The two basins were then irrigated 
with equal quantities of water about 3 inches in depth. The water 
