34 



SCIENCE. 



[N. S. Vol. XV. No. 266. 



face of the soils of humid regions have been 

 reported, but very little has been written about 

 them. Cameron has, in Bulletin No. 17, Divi- 

 sion of Soils, described a number of occur- 

 rences of crusts in humid regions, and has 

 called my attention to several others which 

 were not known to him at the time his paper 

 was published. All of these cases were after 

 a short season of dry weather, but it must be 

 admitted that their occurrence seems rather an 

 anomaly when the heavy rainfall is considered. 

 For what is the reason that this salt remains 

 near the surface of the ground when the water 

 from the rains passes down through the soil? 

 If the salt which is soluble in water is dis- 

 solved by the downward percolating rains, 

 why is it not continually washed deeper into 

 the subsoil? Why is it that, in spite of the 

 fact that more water passes downward than re- 

 turns to the surface by evaporation and capil- 

 lary movements upward, analyses of soils in 

 the humid regions invariably show more 

 soluble matter in the surface soil than in the 

 subsoil ? 



There are several reasons which may account 

 for this seemingly anomalous condition of 

 affairs. First, in the soils of the humid region 

 the great bulk of the decomposition of the soil 

 minerals and the consequent liberation of solu- 

 ble matter takes place within the soil proper 

 in which the greatest aeration takes place, 

 where the bacteria are most numerous and 

 where tillage and sunlight and changes of 

 temperature have a maximum influence. 



A second reason which might be given is 

 that of absorption. Very little definitely is 

 known about the phenomenon called absorption, 

 beyond the fact that it is a property of soil 

 grains or of any surface by virtue of which 

 matters in solution are held so that is diificult 

 to wash them off, so that salts which are liber- 

 ated during the processes of weathering are 

 held near the surface by the absorption. 



There is a third factor which seems to assist 

 in accounting for the salts at the surface, and 

 that is that there is a difference between the 

 rates of downward and upward movements of 

 salts within the soil. 



When water falls oil the soil both gravity 

 and capillary attraction act in the downward 



movement. Capillary attraction is more ef- 

 fective in the smaller spaces between the soil 

 grains, while gravity is more effective in the 

 larger openings. When water leaches through 

 a soil in a field, by far the larger part of it 

 passes through the larger openings — those pro- 

 duced by insects, worm burrows, root holes, 

 cracks, large interstitial spaces formed by 

 coarse grains, etc. That such is the case is 

 very easily proven if the rate of percolation is 

 measured through a block of soil in field con- 

 dition, and the same block is broken up dry, 

 so as to prevent puddling and the rate of per- 

 colation is measured again. A simple exami- 

 nation of any soil in the field will reveal the 

 presence of these larger openings, and as the 

 resistance to flow varies as the fourth power 

 of the diameter of the tube, a much larger 

 amount of water passes downward throiTgh the 

 large openings, than passes through the 

 smaller true capillary spaces. These larger 

 openings might well be called the gravitational 

 spaces, and the smaller spaces in the soil 

 grains the capillary spaces. 



When water moves upward through a soil to 

 replace that lost by evaporation or removed by 

 plants, the movement is entirely capillary and 

 the entire film around the soil grains moves. 



Now let us consider the action which takes 

 place when rain falls upon a soil covered with 

 a thin soluble crust. First of all the soluble 

 matter is dissolved and carried down into the 

 soil. The downward-moving wave penetrates 

 most rapidly along the gravitational spaces, 

 since here the resistance is least and the front 

 of the wave is drawn laterally into the true 

 capillary spaces by surface tension. These 

 capillary spaces, therefore, largely fill with 

 water from the front of the wave, and since 

 the front of the wave contains the greater part 

 of the salt dissolved, this salt is thus retained 

 in the capillary spaces. As soon as the capil- 

 lary spaces are filled, practically all movement 

 in them ceases, except the slow downward per- 

 colation caused by gravity, and in a soil of 

 average texture this movement is practically 

 nothing. The "movement in the gravitational 

 spaces continues. The salt in the water which 

 was drawn back from the front of the pene- 

 trating wave remains stationary or only 



