The occurrence of the cotton root rot fungus, Phymatotrichum omnivorum (Shear) Duggar, in sharply defined, localized areas in certain fields led to the suggestion that there may be a specific soil chemical factor which limits the spread of the fungus. The available sodium, potassium, calcium, and magnesium contents of infested and adjacent noninfested soils in 13 fields were determined and compared. At seven sites, there were no significant differences in cation contents of infested and adjacent noninfested areas (p = 0.05). Available sodium levels were higher in infested areas at three sites, and higher in noninfested areas at three sites. Potassium and calcium levels were higher in the infested area at one site, and higher in the noninfested areas at two sites. Magnesium levels were higher in the noninfested areas at three sites. Contrary to previous reports, the fungus was found to cause severe disease in soils with available sodium contents as high as 4.2 milliequivalents per 100 g soil. Applications of sodium chloride at rates ranging from 1800 to 4000 kg/ha did not reduce disease severity or increase yield significantly in ten field trials.