THE DUST MULCH IN MINNESOTA. 467 



with which it is mingled than that of cohesion among the molecules 

 of the liquid itself. In other words, when the liquid will wet the 

 substance with which it is brought into contact capillary attraction 

 will take place. Quicksilver instead of rising in glass tubes above 

 the general level will always be depressed below it. 



From what has been said it will now be plain that, since water 

 is slow to penetrate a dry and open-grained material, the movement 

 of that contained in the unsaturated soil a few inches below the sur- 

 face up through the light .powdery dust blanket spread out above 

 it will be so slight as to do comparatively little injury. Every one 

 who has tried to absorb with a blotting pad a drop of ink acci- 

 dentally let fall upon his writing paper by inserting a corner of the 

 pad into it has discovered that the blotter must be moistened before 

 the water will feel the influence of the attraction. Moisten the 

 blotter a little and the ink at once rises from the writing paper and 

 distributes itself through the pad. 



Dry, powdery soil is a much greater obstruction to the upward 

 movement of water by capillarity than the unmoistened blotting pad, 

 and, to make it a still greater bar, it does not come in contact with 

 water or even with soil that is near the point of saturation ; the 

 texture lacks firmness, the pores are disconnected .and too large. 

 More than all else, the established connection between the soil be- 

 low and the upper portion or dust mulch is thoroughly disturbed, 

 so that the passage ways of the ground water to the surface are 

 closed and surface evaporation is brought to a stand still. This ac- 

 counts for the fact that in a comparatively dry time moist earth is 

 found in a naked fallow but a few inches from the top of the ground 

 which has been kept well pulverized, while in an adjoining field of 

 heavy oats you may dig a foot or more without striking moisture. 

 In the first case, the water which was in the ground at the beginning 

 has been locked up there and safely kept ; in the second, though the 

 direct evaporation has been trifling the indirect has been enormous. 

 Not less than six hundred tons having been absorbed by the air 

 irom every acre. 



We may say, then, that in order to carry a crop through a 

 drought as successfully as possible, two things are essential : ( i ) 

 Capillary attraction must go on constantly, otherwise the little water 

 near the surface will soon be taken up by the plant roots, and food 

 being thus withheld the crop suffers, and if rain does not soon come 

 to its relief will perish. (2) The amount of water that finds its 

 way to the air above must be reduced practically to zero, so that the 

 water which is brought up by capillarity from the depths below will 



