ALKALI SPOTS OF THE YOUNGER DRIFT SHEETS 261 



salts are slow to find their way out of the soil through the drains, 

 capillary flow during dry weather readily brings them to the sur- 

 face, where evaporation raises the percentage of magnesium in the 

 soil moisture in the zone of root-growth beyond the point of tolera- 

 tion by the plants; continued evaporation of the surface moisture 

 produces the efflorescence. 

 If the floor of the slough was 

 originally somewhat peaty, 

 and therefore porous, the 

 efflorescence is fairly evenly 

 distributed over the area 

 formerly occupied by the 

 water; but if, as is more 

 usually the case, the slough 

 has been silted up with fine 

 material, the efflorescence 

 appears in a zone corre- 

 sponding to its former mar- 

 gin. It is the occurrence of 

 this annular area of barren 

 ground in an otherwise pro- 

 ductive field which mysti- 

 fies the farmer and the agri- 

 cultural writers. 



The occurence of this annular zone of poisonous soil is accounted 

 for on the following considerations: The surface water which finds 

 its way into the slough carries with it both fine and coarse material ; the 

 particles of the grade of sand are deposited first, while the silt is dis- 

 tributed farther from the border. The soil and subsoil of the margin 

 are consequently more open-textured than the soil of the interior; 

 hence there is a predominant flow of capillary moisture toward the 

 porous margin, where evaporation renders conspicuous both the 

 efflorescence, and the poisonous condition of which it is the sign. 



The essential features of typical "alkali spots" are shown in the 

 accompanying figures. 



Adsorption is undoubtedly a considerable factor in geological chemistry, and should 

 receive greater attention from soil investigators. 



Alkali Zone 



Fig. 1. — Group of small annular "alkali 

 spots" on the Story County Poor Farm, Iowa. 

 (Horizontal scale: i inch = 200 feet.) 



