Mar. i, 1924 
Movement of Water in Irrigated Soils 
681 
connected with the water of the river and may be regarded as a part of 
its underflow. The volume of this underflow is large, but its movement 
appears to be slow. The field wells from which the samples were ob¬ 
tained that are reported in Table XXV are located on the Bard Experi¬ 
ment Farm. The nine wells are so distributed over the farm of 160 acres 
as to give samples of the underground water for the whole tract. This 
underground water probably represents a composite of river underflow 
and of leachings from the irrigated fields. The figures reported are the 
means of six sets of analyses from nine wells. These results show a 
condition where the underground water differs very little from the 
irrigation water, either in total solids or in the composition of these solids. 
The irrigation water that is applied to the land is dissipated in two 
or three ways. One part of it is absorbed by plants, another part is 
necessarily lost by evaporation from the soil, and where the quantity 
applied is more than the sum of these two losses the remainder percolates 
into the subsoil. The water that is absorbed by the plants is not ordi¬ 
narily of the same composition as the soil solution; that is to say, the 
plant roots do not absorb the soil solution with which they are in contact. 
They absorb the water from this solution and with the water they take up 
only such dissolved constituents as are needed. This process is known 
as selective absorption. 
It is obvious that crop plants do not absorb the soil solution as it 
exists in the soil. It is not uncommon to find that the soil solution in 
irrigated land contains as much as 1 per cent of dissolved mineral matter. 
It has been found (3) that the ordinary crop plants transpire as much 
as 500 pounds of water for each pound of dry matter produced. Of 
this dry matter only 10 per cent or less is mineral matter. From this 
it may be seen that the plant may transpire as much as 5,000 pounds of 
water and take into its system only 1 pound of mineral matter. This 
means that the water that is absorbed and transpired by plants leaves 
its dissolved mineral matter in the soil almost to the same extent as the 
water that is lost by evaporation from the soil. 
When irrigation is so conducted that the water applied is all dissipated 
by plant absorption and by evaporation, the salts contained in the irriga¬ 
tion water are then deposited in the soil. Even where the salt content 
of the irrigation water is low the accumulation goes on and it is merely a 
matter of time until it reaches critical proportions. It is not uncommon 
to find irrigation waters in use that contain 1,000 parts per million of 
dissolved minerals. This is equivalent to 1.35 tons per acre-foot of 
water. As a basis of comparison it may be said that an acre-foot of soil 
which contains 15 per cent of moisture holds 300 tons of water. If this 
water holds in solution 1 per cent of mineral matter there would be 3 
tons of such material in each acre-foot. From this it is clear that in the 
practice of irrigation it is essential that some part of the water applied 
pass downward through the soil, for it is only .in this way that the exces¬ 
sive accumulation of salts may be avoided. 
While many crop plants may obtain water from a soil solution which 
contains more than 1 per cent of dissolved mineral matter, it would 
appear to be advisable generally to keep the solution below that degree of 
concentration. In order to do this it follows that in applying irrigation 
water it should be the aim to use enough so that the proportion lost by 
percolation is large enough to offset the concentration due to evaporation 
and transpiration. Thus if the irrigation water contains 1,000 parts 
81990—24 - 5 
