You can spot High sodium areas in your field right now.

In Major Land Resource Areas 113 and 114B (the Southern Illinois Claypan Region)  soils often have areas where the subsoil has high sodium.  The pattern is very intricate so soils are mapped in complexes such as Cowden-Piasa complex and Oconee-Darmstadt complex.  The pattern is often over looked, but is apparent when soils get dry.  Water is less available in high sodium areas, so when weather turns dry, drought symptoms appear first in the high sodium areas.  In the photo below you can see the corn in the foreground is fired up to the ears and yield potential is greatly reduced.  Corn in the background on non-sodium soils is still nice and green.  Soil test levels for major nutrients tend to be high in the high sodium areas because lower yields remove fewer nutrients. 

High Sodium vs. Non-Sodium Soils

Southern Illinois Claypan Regions