Mar. i, 1934 
Movement of Water in Irrigated Soils 
633 
This may be done by means of chemical analyses of the two waters. 
By this means the quantity and character of the substances dissolved in 
the water may be determined and the identity of the water may be es¬ 
tablished. 
While it is possible by the means indicated above to demonstrate that 
the percolation of irrigation water proceeds by displacement, it is not a 
simple matter to determine the rate of this displacement under field con¬ 
ditions. The rate of displacement differs greatly in different parts of the 
same field because of differences in the texture or structure of the soil, and 
there is also some mixture of the water and some diffusion of the dis¬ 
solved material from each water to the other. 
Something as to factors that influence the percolation of water through 
the soil may be learned in the laboratory by the use of the simple device 
illustrated in figure 4. This consists of a glass pot with a small hole 
drilled through the bottom to provide drainage. The pot shown in the 
figure is 3X inches across the top and inches high. It will hold con¬ 
veniently 300 gm. of soil, leaving space for as much as 100 cc. of water 
at the top. The loss of soil through the drainage hole may be prevented 
by placing a small filter paper in the bottom of the pot before putting in 
the soil. The leaching pot may be set into an ordinary drinking glass to 
collect the percolate and may be covered with a Petri dish to prevent 
evaporation from the moist soil. 
This apparatus is useful not only for measuring the percolation rate of 
soils but for investigating the changes that take place in the character 
of the soil solution during the process of leaching. It is possible also with 
this apparatus to determine the relative water-holding capacities of 
different soils, though the water-holding capacity shown in this way is 
usually much higher than that shown by the same soil under field 
conditions. 
The percolation rate in cubic centimeters per minute as determined 
in the laboratory by means of the apparatus described above using several 
different types of soil was as follows: Soil sample No. 326, 0.049; No. 338, 
0.100; No. 325, 0.273; No. 324, 0.353; No. 340, 0.506; No. K82, 0.560; 
No. 339, 0.701; No. K90, 2.109. 
The rates of percolation given represent the mean of several observa¬ 
tions. A quantitative expression of the percolation rate with two dif¬ 
ferent soils is shown in figure 5. 
With soil No. 324, which showed a percolation rate of 0.353 cc. per 
minute, the discharge for 12 hours would be 254 cc. With soil N0.326, 
having a percolation rate of 0.049 cc. per minute, the discharge in 12 
hours would be only 35.6 cc. A comparison of the percolation rates 
shown by these two soils with the rate of water penetration shown by the 
same soils in figure 3 would indicate that the factors that influence the 
movement of water into a dry soil tend also to influence in the same way 
the movement of water through a saturated soil. 
FACTORS INFLUENCING THE PERCOLATION RATE 
It can not be doubted that such factors as the texture of the soil and 
its structure 5 have a large part in determining the rate of percolation. 
In fact, if the meaning of the term “structure” is made broad enough it 
5 As here used the phrase “texture of the soil" refers to the composition of the soil as determined by 
mechanical analysis or to the sizes of the particles composing it. The word “structure" refers to the 
arrangement of the soil particles in relation to each other, as in the formation of granules or of larger 
aggregates and in the development of shrinkage cracks on drying. 
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