The addition of mature (high C:N ratio) plant residues, and two immature (low C:N ratio) residues of barley and sorghum to soil, signifi­cantly reduced Fusarium root rot of soybean. Disease rating was posi­tively correlated with total soil inorganic nitrogen and nitrate. Soil amendment with residues of mature or immature barley and mature oat appreciably increased stem length and fresh root weight of Clark soyb­ean. Mature and immature residues of barley or oat decreased Fusarium population in soybean roots or rhizosphere, especially after 80 days from addition of these residues; however, other residues had no effect. Addition of soybean residues to the soil increased the incidence of soyb­ean seed infected by Fusarium oxysporum, while soil amendment with barley or oat residues had a reverse effect. Decomposing mature residues were more inhibitory to chlamy­dospore germination in vitro and in soil than the corresponding immature residues. Failure of chlamydospores to germinate in vitro and in soil was related to nutrient deficiency in the soil as well as to the formation of an inhibitory material from the decomposing residues. Fungistasis due to nutrient deficiency, but not toxicant production, was overcome in soil in the presence of nutrients. To improve physical, chemical and other soil properities for any soil contaminated with Fusarium root rot the addition of mature crop residues of high C:N ratio is recommended.