Mar. i, 1924 
Movement of Water in Irrigated Soils 
657 
It should be definitely understood that there fe a real distinction implied 
by the term potential impermeability as applied to irrigated soils. The 
same soil may be actually permeable and potentially impermeable. A soil 
rarely shows symptoms of impermeablity while the soil solution contains 
a large proportion of dissolved material. The existence in the solution 
of appreciable quantities of such strong electrolytes as chlorin, sulphate, 
or nitrate appears to inhibit the soil reaction that makes for impermea¬ 
bility to water. This is true whether the solution is rich or poor in calcium 
and magnesium. Thus an impermeable soil may be made temporarily 
permeable by treating it with sodium chlorid or sodium sulphate or by 
leaching it with a solution of these salts. On the other hand, such treat¬ 
ment would almost certainly make a soil potentially impermeable in that 
a subsequent dilution of the soil solution, as by leaching the treated soil 
with distilled water, would induce impermeability. 
In view of what has just been said it will be clear that experiments in 
the dilution of the soil solution by leaching, either in the laboratory or in 
the field, may be expected to be successful only when the solution con¬ 
tains a substantial proportion of calcium and magnesium. Unless this is 
the case the experiment is doomed to end in disappointment. Even when 
this condition is fulfilled it is often found that the rate declines as the 
leaching goes on. But the decline is not likely to continue unless in the 
readjustments that take place the preponderance of sodium becomes 
large. 
In the leaching experiments now to be described it will be observed that 
both the soils used showed large proportions of soluble calcium and 
magnesium. With one of the soils these proportions continued high to 
the end of the experiment. With the other it declined as the experi¬ 
ments proceeded. 
The soil used for the first of these two leaching experiments was taken 
from an irrigated field in Arizona. When thoroughly dry it was pulver¬ 
ized and placed in a glass tube of such a diameter that 1,000 gm. of the 
dry soil occupied about 16 inches of the tube. The bottom of the tube 
was stoppered with a perforated cork. The column of soil was then 
leached with distilled water and the percolate was collected in successive 
fractions of 50 cc. each. 
These successive percolates were then analyzed with results as shown 
in Table XX. 12 The total solids are reported as percentages of the percolate, 
and the constituents that were identified are reported as percentages of the 
total solids. As the leaching progressed the solutions obtained became pro¬ 
gressively more dilute and the character of the solution was also changed. 
At first the chief constituents were calcium, 13 chlorin, and nitrate. After 
the fifth percolate, when 250 cc. of water had passed through the soil, 
the nitrate practically disappeared. The sulphate content of the first 
leachings was very low. As the solution became more dilute the ac¬ 
tual quantity of sulphate increased somewhat, while the proportion of 
sulphate to the total salts increased very greatly. 14 
l * This experiment and the analyses were made by J. F. Breazeale. 
1 * The ratio of calcium to ma gtiesi um continued throughout the experiment to be approximately as 8 to i. 
h Breazeale has observed in another experiment that while calcium sulphate is soluble in distilled 
water to the extent of approximately 2.000 parts per million, its solubility is reduced in the presence of 
calcium chlorid. With 26 per cent calcium chlorid he found less than 500 parts per million of dissolved 
calcium sulphate, while with a saturated solution of calcium chlorid he found calcium sulphate practi¬ 
cally insoluble. 
