66 o 
Journal of Agricultural Research 
Vol. XXVII. No. * 
It would be natural to expect that succeeding percolates would show a 
large increase in the proportion of chlorin and consequent decreases in 
the proportion of carbonate and sulphate. The actual quantities of 
carbonate and sulphate in the solution were not at first materially 
changed. This was not true with the calcium. 15 While the first per¬ 
colate after the addition of the sodium chlorid showed no pronounced 
change in the proportion of calcium to the total salts, the actual quan- 
tity of calcium in the solution increased from less than 300 parts per 
million to more than 1,600 parts per million. This reaction by which 
sodium added to the soil solution tends to replace combined calcium will 
be discussed later. 
As these leachings continued the concentration of the percolates 
became gradually reduced, the proportion of calcium declined rapidly, the 
proportion of the chlorin declined also, but less rapidly, while the pro¬ 
portion of carbonate began to increase as the solution became more dilute. 
Not only did the proportion of carbonate (identified in the solution as 
bicarbonate) increase as the percolates became more dilute but the 
actual quantity increased also. The first percolates showed 120 parts 
per million of HC 0 3 , while the final one showed 450 parts per million. 
These various experiments in diluting the soil solution all show that 
such dilution, by whatever method it is accomplished, does not operate 
in the same way with the different constituents of the solution. The 
evidence available appears to justify the inference that there are con¬ 
tinuing reactions between the soil and its water, so that the character 
of the solution is essentially modified as its concentration changes. 
These reactions between the soil and its water appear to operate not 
only to influence the character of the solution but also to influence the 
physical character of the soil in its relation to the movement of water 
through it. It is well known also that these changes in the physical 
character of the soil are manifested when the soil is dried. The same 
physical condition that retards the movement of water through the soil 
causes the soil particles to become cemented together into a solid mass 
when the water is lost by drying. Soils that are readily permeable to 
water usually crumble on drying. 
EXCHANGE REACTIONS IN THE SOIL SOLUTION 
The leaching of a saline soil in the field must be done ordinarily with 
irrigation water. Such water always contains some dissolved material 
derived from its contact with the soil. When it is obtained from wells 
or from most streams the water is itself a soil solution. The character 
of this solution is a matter of large importance. Its use on the land may 
result in a complex of conditions wholly different from the one already 
existing. In this respect again leaching experiments in the field are 
likely to differ from those conducted in the laboratory. 
It has been noted above that certain changes in composition of the 
soil solution take place when the concentration is changed, as by the 
addition of pure water. When another solution is added to the one 
already present in the soil, the reactions that follow involve not only 
the two solutions but the soil also takes part in them. Certain constitu¬ 
ents in the blended solutions may be taken up by the soil and other 
constituents be released from combination with the soil and pass into 
solution. 
15 In these solutions the proportion of magnesium was so low as to be negligible. 
