Surface broadcasting is still the predominant mode of potassium (K) fertilizer application for soybean [Glycine max (L.) Merr.] in conservation tillage systems even though K is relatively immobile and vertical soil K stratification is often evident in conservation-till fields in North America. Soybean yield in conservation tillage systems may be improved by banding K fertilizer below seeding depth. Certain soybean seed quality parameters may also be affected by K fertilizer management systems since K is essential in various plant enzymes. The objectives of this research were to investigate K nutrition, yield, and quality responses of soybean resulting from alternate K fertilizer placements (band placement versus surface broadcasting, and applied directly before soybean versus residual effects associated with application to previous corn) in various tillage (such as zone-tillage, fall disk, and no-tillage) and row-width (from 18 to 76 cm) systems imposed after long-term no-tillage. All 17 experimental site-years had a minimum 5-year no-till history prior to treatment initiation. Soybean responses to K placement were generally unaffected by tillage and row width on medium- to high-testing K soils. Trifoliate leaf K concentrations were increased by K application even on medium to high K soils. Yield responded positively to direct K fertilization most frequently on low K soils. Despite vertical soil K stratification, deep-banded K increased yield, relative to broadcast K, only on low K soils and only when most soybean rows were in close proximity to fertilizer bands. On long-term no-till fields with medium soil-testing K levels, leaf K concentrations and yield of no-till soybean were affected more by residual K fertilizer rate than by K placement and tillage systems applied to previous corn. On low-testing soils, soil exchangeable K concentrations (0-20 cm depth) in previous corn rows were significantly higher than those between corn rows regardless of K fertilizer management applied to previous corn. Trifoliate leaf K concentrations of no-till soybean in preceding corn rows were 2.0-5.3 g kg^-1 higher than those from corresponding plants between corn rows. Yield of no-till soybean in previous corn rows increased 10-44% compared to those between previous corn rows. Previous corn row effects on K nutrition and yield of no-till soybean occurred even when K fertilizer was not applied in the previous corn season. Corn row effects were generally not affected by tillage system or corn hybrid employed in the previous corn crop. Therefore, the most efficient K management strategies for no-till narrow-row soybean should take this horizontal stratification of soil K caused by the preceding corn rows into account. Adjustments to the current soil sampling protocols for soil K may be warranted when no-till narrow-row soybean follow corn on soils with low to medium K levels. Potassium application frequently increased daidzein, genistein, and total isoflavone concentrations in soybean seed; however, glycitein concentrations rarely responded to K fertilization. Placement method of K fertilizer significantly affected isoflavone response to K application on low-testing K soils. Daidzein, genistein, glycitein, and total isoflavone concentrations were positively correlated with yield, and seed K and oil concentrations on low K soils. Our research suggests that K fertilization to increase seed yield when soil-testing K was limiting significantly improved seed quality from an isoflavone-based, functional-food perspective. For maximum grain yield, the critical leaf K concentration at the initial flowering stage (R₁) of development was 24.3 g kg^-1; this concentration is substantially higher than the current critical values that are used in many US Corn Belt states. Critical leaf K values for the maximum concentrations of oil and total isoflavone in seed were 24.1 and 23.5 g kg^-1, respectively. Our results suggest that critical leaf K concentrations for maximum yield and seed quality components were higher in the soils with greater vertical soil K stratification than those with lower K stratification. The extent of vertical soil K stratification seems to be one of the factors contributing to apparently higher critical leaf K concentration for conservation-till soybean.