May 1 6 , 1921 
Quality of Irrigation Water 
267 
THEORY OF THE REACTIONS 
The reactions that take place in a soil that is leached with a dilute 
salt solution were first investigated in detail by Way. 1 Later investi¬ 
gators have dealt with various aspects of this subject, but too often 
without distinguishing clearly between the chemical and physical reac¬ 
tions involved. It was shown by Way that when a solution of a salt 
was leached through a soil containing clay a chemical reaction took 
place as a result of which some of the basic element of the salt was re¬ 
tained in the soil and an equivalent quantity of other bases was released 
into the soil solution and appeared in the percolate. Thus, if a solu¬ 
tion of ammonium sulphate, sodium sulphate, or potassium sulphate was 
leached through a soil a portion of the base was retained by the soil and 
the other bases, chiefly calcium, passed into solution. It was definitely 
pointed out that these chemical reactions were essentially different from 
the physical property of soils known as adsorption which is common also 
to other finely divided substances such as charcoal and carbon black. 
The reactions reported in the present paper appear to belong to the 
same category as those discovered by Way. There is, however, an addi¬ 
tional fact to be noted. In the present investigation the question under 
consideration was not primarily the changes taking place in the soil 
solution but the changes in the physical character of a soil, both with 
respect to its permeability to water and to its characteristics upon 
drying. It has been found that leaching sodium salts through a soil 
causes it to become less permeable to subsequent leachings with water, 
and, furthermore, upon drying the soil becomes hard, a condition fa¬ 
miliarly described in agriculture as “baking.’’ Calcium salts, on the 
other hand, have been found to render the soil readily permeable for 
water and less likely to become hard or “baked” upon drying. 
It has been shown by experiments on small samples, not here reported 
in detail, that the soluble salts of aluminum, such as chlorids and sul¬ 
phates, are much more effective than the similar salts of calcium in 
improving the physical condition of impermeable soils. Soluble alumi¬ 
num salts, such as aluminum sulphate, if leached through a sample even 
of extremely refractory soil, increases its permeability for water and com¬ 
pletely removes the tendency to become hard on drying, the soil remain¬ 
ing very soft and powdery. Furthermore, these characteristics persist 
even after repeated leachings with distilled water. 
The theory advanced to explain these facts is that the different bases 
in the solutions used partially displace the bases in the compounds on 
the surface of the soil particles. For example, when a solution of a salt 
of sodium is leached through a soil, the sodium will displace a portion 
1 Way, J. Thomas, on the power of soils to absorb manure. In Jour. Roy. Agr. Soc. England, 
v. 11, p. 313-379- 1850; v. 13, p. 123-143. 1852. 
a -ON THE INFLUENCE OF LIME ON THE “ABSORPTIVE PROPERTIES” OF SOILS. In Jour. Roy. Agr. 
Soc. England, v. 15, p. 491-514. 1854. 
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