Mar. 
1924 
Movement of Water in Irrigated Soils 
629 
in permeability to water. Such differences are a matter of common 
•observation in the field in irrigated areas and constitute one of the most 
important distinctions in irrigated soil. The importance of this matter 
of permeability to water lies in the fact that to function properly in the 
support of crop plants the soil must act as a reservoir for the storage of 
water from one irrigation to the next. If the soil is not permeable to 
water and does not readily absorb water when irrigated, it does not ' 
function well in its capacity as a reservoir. 
There is another aspect of the case that is also important. Some 
irrigated soils contain soluble salts, commonly known as alkali, in such 
quantities that the soil solution is so concentrated as to be toxic to crop 
'plants. When conditions are such that the irrigation water penetrates 
the soil readily and passes away in the underground drainage, it is not 
difficult to leach out the excess of soluble salt and thus to reclaim the 
alkali land. But when the soil is not readily permeable to water, such 
reclamation becomes difficult or impossible. If conditions are such 
that the irrigation water soaks down only a foot or two into the soil, it 
does not carry the salts away. Instead of doing so it evaporates, leaving 
in the soil not only the salt originally there but also any additional salt 
which is carried in solution by the irrigation water. 
In view of these facts, it is clear that a soil to be used successfully for 
the production of crops under irrigation must be readily permeable to 
water, not only that it may serve as a suitable reservoir to hold water 
for the use of plants but also that any excess of soluble salts may be 
leached away. 
MEASURING THE RATE OF WATER PENETRATION 
It is not to be supposed that the rate of water penetration in dry 
pulverized soil in glass tubes in the laboratory is the same as the rate of 
penetration of irrigation water in the field. There is, however, reason 
for believing that the differences in the rate of penetration for different 
soils as shown in laboratory tests are to be found also in the field and 
that these differences are in the same direction and substantially of the 
same degree. Numerous comparisons have been made in the laboratory 
between soils from different fields and different irrigated regions, and 
these results appear to accord very well with field experience. This 
field experience shows clearly that the rate of water penetration varies 
greatly even in different parts of the same field. An example of this 
variation is shown in figure 2 and in Table IV. 
In making the laboratory tests of water penetration as described above 
there are certain factors that make for a lack of uniformity. Even when 
portions of the same soil sample are used in different tubes there are 
likely to be differences in the degree of fineness of the material used and 
differences in the way the material settles together in the tube. Table 
VII shows the differences observed in the rate of water penetration in 
5 tubes that were filled from the same sample of soil and given the same 
quantity of water. These results show that the differences in rate of 
penetration are relatively slight when compared with those shown by 
different soils even of closely similar textures, as shown in Table VI. 
