Cotton (Gossypium hirsutum L) usually does not exhibit Fe deficiency chlorosis symptoms, as do certain monocot plants. Field observations showed plant mortalities from a severe cotton root rot disease (Phymatotrichopsis omnivorum) were considerably higher in areas where monocot plants (sorghum) showed Fe chlorosis. This possible association of iron (Fe) stress in cotton and incidence of cotton root rot (PRR) gave impetus to greenhouse and field studies of Fe requirements of cotton and the effects of Fe nutrition on severity of PRR on highly calcareous soils. Cotton responded to soil amendments, which reduced soil pH and also to supplemental Fe applied as various Fe carriers. Plant mortalities from PRR occurred later in the growth cycle when Fe was applied. Substantial reductions in plant losses were measured with 7,140 and 14,980 lb ac-1 applied S. This highly CaCO3 buffered soil showed large reductions in soil pH with the increased dry matter and reduction in plant losses to PRR.