678 
Journal of Agricultural Research voi. xxvii. No. 9 
This lack of uniformity of the soil in many irrigated fields is very con¬ 
spicuous. It tends to become more pronounced as the conditions result¬ 
ing from the accumulation of soluble material become critical. One of 
the first signs of such trouble is the appearance of spots in the field where 
crop growth is abnormal. Spots of poor growth or bare spots appear 
and increase in size as the conditions become worse. These irregularities 
in the soil are manifested also in the underground water where conditions 
are such that it accumulates. 
In the reclamation of saline soils the same diversity of conditions is 
found. In some spots improvement is marked and immediate; in others 
it is much slower. It seems evident that the water used for leaching as 
well as the water used for ordinary irrigation does not soak down into the 
soil at a uniform rate in all parts of the field. It is also apparent that 
the lateral movement of underground water to drainage ways, whether 
natural or artificial, does not proceed uniformly. There are channels or 
strata of permeable material in the subsoil often interspersed or cut off 
by other strata or barriers of less permeable material. In reclaiming land 
or in irrigating it, one of the most important aims should be to equalize 
the movement of the water. It is not enough to level the land so that 
the water may be applied uniformly; it is necessary also to make pro¬ 
vision for holding the water for a longer time on those spots in the field 
where the soil is less permeable. 
In planning a system of drainage where the accumulation of subsoil 
water makes drainage necessary it is no less important to take into account 
the conditions of permeability in the subsoil. It is not sufficient in laying 
out a drainage system to consider merely the surface contours of the land. 
Where a drainage system is designed only to collect and dispose of sur¬ 
face waste waters its location is determined chiefly by surface contour 
conditions. But when its chief function is to relieve an accumulation of 
underground water, its location should be determined by underground 
conditions. Whenever it is possible to do so, such drainage should be 
designed to intercept water that is moving into a section where trouble 
exists or is anticipated, as well as to provide outlets from such sections. 
In making the surveys preliminary to constructing a drainage system to 
prevent or relieve the accumulation of underground water, it is believed 
that chemical studies of the water should be helpful in locating the chan¬ 
nels of free movement and consequently in placing drains. 
COMPARISON OF IRRIGATION AND UNDERGROUND WATERS 
It has been shown in the preceding pages that when a solution comes 
in contact with the soil, reactions may take place as a result of which 
constituents are exchanged. Both the solution and the soil are made 
different by these reactions. Furthermore, a large part of the irrigation 
water applied to the land is dissipated by evaporation or absorbed and 
transpired by crop plants, leaving the dissolved material in the soil. 
In consequence, the soil solution which accumulates as underground water 
in irrigated land is likely to be very different in concentration and com¬ 
position from the irrigation water. 
The comparison of the analyses of underground or drainage waters 
with those of the irrigation water affords a basis of understanding the 
reactions that are going on in the soil through which the water has 
passed. Such an understanding is much to be desired because it may make 
it possible to anticipate and to prevent some of the difficulties that follow 
