26 BULLETIN 61, U, S. DEPAETMENT OF AGEICULTUEE. 



extent, but only very locally, and the seepage water drains speedily away. WTiere 

 impervious layers occur, ground water may be retained and by evaporation leave its 

 burden of salts within the soil at varying depths. ^^Tiere soil surfaces are sloped, 

 underground drainage is facilitated, and, if the soils are not too coarse textured, or 

 capillarity' not an important factor, the soluble salts are drained away and deposited 

 in the level portions of the vallej'S. 



With moderate rainfall salines are distributed in sandy soils with the least propor- 

 tion at the surface and gi-eater amounts or accumulations at intermediate points. 

 Under arid conditions these accumulations would be nearer the surface, and in regions 

 of extreme aridity would be very close to and even at the surface. With heavy soils 

 (slow movement of ground water) the accumulations of salines would be nearer the sur- 

 face for moderate ramfalls and much closer to the surface for small rainfalls than for the 

 porous soils. 



Means ^ discusses the conditions under which alkali salts move within the soil and 

 his conclusions are pertinent here. He states that — 



(1) "Movement of alkali salts is caused by diffusion of the salt mixture; 



(2) "By the force of gravity in moving the salt mixture downward; 



(3) "By surface tension or capillary action which moves the salt mixture in 



any direction." 

 Means considers that the effects of the diffusion are practicably negligible. The 

 second and third causes may best be placed in his words: 



(2) '"Force of gravity. — T\Tien water is applied to the siirface of the soil the force of 

 gra\dty, assisted by surface tension, pulls the water down into the capillary spaces. 

 Soils will hold a certain percentage of water by capillary forces alone, and any excess 

 over this percentage will drain away. This excess is called gravity water. "\ATien the 

 surface of the ground is flooded, both surface tension and gravity act in pulling the 

 water downward. Since the rate of flow of water through capillary spaces depends 

 upon the size of the space, the flow through the large capillary spaces, root holes, worm 

 borings, and animal burrows is very much greater than that through the true capillary 

 spaces. When water is applied, the downward movement by gravity is almost 

 entirely through the larger noncapillary spaces, while the true capillary spaces are 

 filled by surface tension from the noncapillary spaces. In this way the salt which is 

 dissolved by the descending water is probably to some extent drawn back into the 

 capillary spaces, where there is very little downward movement, and remains there, 

 only escaping out into the channels of downward movement by diffusion. The 

 amount of salt which is washed downward by a heavy flooding is therefore not so great 

 as would be expected. 



(3) "The greatest movement of alkali salts is due to capillarity which operates 

 through surface tension. When water moves by surface tension, the films around the 

 soil grains move. As soon as the gravity water has drained away, the movements 

 become entirely by surface tension. A loss of water due to evaporation changes the 

 curvature of the water films and starts a capillary movement toward the point where 

 the evaporation takes place. But when water moves by capillary action it is the 

 water in the smaller spaces that moves, and not the water which is in the larger non- 

 capillary spaces. Therefore the water which was drawn back into the capillary spaces 

 and which carried some of the alkali salts as it flowed down into the soil starts upward 

 and carries with it the salts in solution. The evaporation of an inch of water on the 

 surface of the soil accumulates on the surface all alkali salts which were contained in 

 that inch of water, while, on the other hand, the same volume of water leached down 

 through the soil would probably not leach out an equal amount of the salts. From 

 this it will be seen that the tendency of the alkali salts under irrigation is to move 

 upward perhaps more rapidly than downward. 



"From the above discussion it appears that surface tension or capillary attraction, 

 as it is commonly called, is the most important agent in the movement of alkali salts 

 toward the surface of the soil. Therefore, a soil which Avould permit the most rapid 

 movements would be the most likely to accumulate alkali salts upon the surface. If 

 two soils with different capillary powers were placed side by side, with the level of 

 standing water the same, the soil which raised the water to the surface the more 

 rapidly would the sooner accumulate an alkali crust." 



THE POSITION OF MAXIMUM SOLUBLE SALT CONTAINED IN THE SOIL. 



The following table summarizes certain observations which have been made con- 

 cerning the depth at which soluble salts form accumulations. 



> Bui. No. 35, Bureau of Soils, p. 13. 



