Seed yield of monocrop soybean [Glycine max (L.) Merr.] grown on nonirrigated clay soils in the Mississippi River alluvial flood plain has been consistently low and marginally profitable. Deep tillage (subsoiling) of certain clayey soils in the fall when the soil profile is dry significantly increases yields and net returns from this production system. Yields and net returns were evaluated for seven deep-till treatments and a conventional-till treatment that was disked annually (C). The deep-till treatments included subsoiling annually and once every second, third, fourth, and fifth year (DT1, DT2, DT3, DT4, and DT5, respectively), Paratill annually (PT1), and profile modification every fifth year (MP). Over the 5-yr study, DT1 produced the greatest yield (3452 kg ha−1), which was 1016 kg ha−1 (42%) above the average yield from C (2436 kg ha−1). Yields from DT2, DT3, and PT1 were virtually identical and averaged only 198 kg ha−1 (5–6%) less than the average yield from DT1, whereas yields from DT4 and DT5 averaged 291 kg ha−1 (9%) and 613 kg ha−1 (18%) less. Net returns from DT1 ($436 ha−1) averaged the highest, whereas returns from C ($220 ha−1) were the lowest. Net returns from DT2 and DT3 were within 5% of the net returns from DT1, whereas net returns from the DT4, DT5, PT1, and MP treatments respectively averaged 9, 26, 9, and 31% less than returns from DT1. Thus, fall deep tillage should be performed at least once every 3 yr to maximize and sustain higher yields and net returns from soybean grown on nonirrigated Tunica clay (clayey over loamy, smectitic, nonacid, thermic Vertic Haplaquept).