PHYSICAL CONDITION OP SOILS 369 



coming to tlie surface, at lower levels, in the form of springs. 

 The capacity of a soil to care for the water it receiyes from 

 rains is, perhaps, the most important of any one property. 

 It has been demonstrated that the soils of the semi-arid regions 

 of the West will produce abundant crops of wheat and corn, 

 though receiving but about half the amount of water from rain- 

 fall that would be requisite in the East. This is accounted 

 for wholly on physical grounds, and is explained as follows:^ 

 Water falling upon a perfectly dry soil descends very slowly, 

 and indeed, in extreme cases, may continue to fall for hours 

 without wetting the mass for more than a few inches belowl;he 

 surface, while it will be absorbed very rapidly by a soil already 

 wet but not saturated. This is due to the fact, as explained 

 by Whitney, that in a dry soil the tension or contracting power 

 of the surface of tjie water is greater than the attraction of the 

 soil grains. If, on the other hand, there is any appreciable 

 amount of moisture in the soil, the tension of the water sur- 

 face will cause it to contract and pull the water from above 

 into the sub-soil. It follows, then, that the water of rains fall- 

 ing in semi-arid regions will not penetrate into the dry sub- 

 soil, until the overlying portions have become successively so 

 far saturated that they can no longer hold the water back, 

 and it will pass downward, therefore, very gradually into the 

 lower depths, saturating, or nearly saturating, each successive 

 depth as it progresses. Unless, then, as rarely happens in this 

 region, the rainfall is so great and so continuous as to saturate 

 the soil to a considerable depth, the whole supply of moisture 

 absorbed will remain within a short distance of the surface, 

 either immediately within reach of plant roots, or where it can 

 be brought upwards once more by capillarity when evaporation 

 from the surface begins. With a continuously wet sub-soil, 

 however, as in the East, a very considerable portion of the 

 water passes at once to depths beyond the reach of roots or 

 capillary attraction, and is, so far as our present considerations 

 are concerned, completely lost until, in the course of nature's 

 endless cycle, it shall once more be returned as rain. Within 

 certain limits, a light rainfall, equitably distributed, is more 

 advantageous to agriculture than are the heavier precipita- 



1 Conditions in Soils of tlie Arid Region, by Milton Whitney, Yearbook 

 U. S. Dept. of Agriculture, 1894. 



