Abstract Coarse‐textured soils are typically not highly aggregated due to low surface reactivity of sand‐sized grains. However, soil erosion and surface sealing can still be a concern from frequently tilled soil in relatively flat landscapes of the Coastal Plain and Flatwoods of the southeastern United States. Soil at 0‐to‐10‐cm depth was sampled from 120 randomly chosen cotton ( Gossypium hirsutum L.) fields in North Carolina, which could be separated into three categories of tillage system: frequent disk tillage, rotated conservation and disk tillage, and continuous conservation tillage. This study assessed soil stability index derived using (a) mean‐weight diameter (MWD) following water immersion and oscillation for 10 min relative to that of (b) MWD following dry sieving of soil. This whole‐soil analysis had significant variation in soil stability index (interquartile range of 0.58 to 0.95 mm mm –1 ) along a sand concentration gradient, but that was differentiated by tillage system. The strongest association of MWD from dry‐stable crumbs ( r = .71, p < .001) and of MWD from water‐stable crumbs ( r = .65, p < .001) occurred with soil‐test biological activity, suggesting that organic C resources near the soil surface with continuous conservation tillage promoted both biological activity and soil stability. Physical manipulation of soil by biota (i.e., fauna, microbes, and roots) into water‐stable crumbs was a consequence of continuously undisturbed organic C resources near the soil surface, which also led to greater soil‐test biological activity and net N mineralization.