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Sodium-affected soils (SAS) or sodic soils are soils that have been adversely affected by sodium salts and/or exchangeable sodium; they commonly occur in arid, semiarid, and subhumid climates. The occurrence of SAS in humid south-central Illinois, USA provides an opportunity to study and understand the genesis of SAS in this unique setting. Background historical information and studies, description of SAS chemical and physical properties, an update on current SAS research in Illinois, and suggestions for further study are presented. An understanding of the distribution and genesis of these unique soils will aid in discovering soil management methods that will address the high sodium content and poor physical makeup of SAS.

Samples of 255 noncalcareous A, Ap, and AB horizons from selected major land resource areas (MLRA) in the north central USA were used to develop equations for predicting organic C content, as determined with a Leco C analyzer, from weight loss‐on‐ignition (LOI). Organic C concentrations of the samples ranged from 1.09 to 114.6 g kg−1Within each MLRA, strong linear relationships were observed between LOI and organic C measured by the Leco instrument, with r2ranging from 0.94 to 0.98. Predictive equations developed by least‐squares regression were significantly different for individual MLRA's. Loss‐on‐ignition is a rapid, inexpensive, and accurate method for estimating organic C concentration in soils of the north central USA. We recommend that unique predictive equations be developed for individual soil‐geographic regions.