Compaction can be a problem in some no-till (NT) soils, but accumulation of soil organic C (SOC) with time may reduce the soil's susceptibility to compaction. Relationships between SOC and soil maximum bulk density (BD max), equivalent to maximum soil compactibility, have not been well documented, particularly in NT systems. We assessed near-surface BD max using the Proctor test under long-term (>19 yr) moldboard plow (MP), conventional tillage (CT), reduced tillage (RT), and NT conditions in the central Great Plains and determined its relationships with SOC, particle size distribution, and Atterberg consistency limits. The experiments were located on silt loam soils at Hays and Tribune, KS, and loam soils at Akron, CO, and Sidney, NE. The near-surface BD max of the MP soil was higher than that of the NT soil by 13% at Sidney, while the near-surface BD max of the CT was higher than that of the NT soil by about 6% at Akron, Hays, and Tribune. Critical water content (CWC) for BD max in the NT soil was higher than in the CT and MP soils except at Tribune. The BD max decreased with increase in CWC ( r=-0.91). The soil liquid limit was higher for NT than for MP by 82% at Sidney, and it was higher than for CT by 14, 9, and 31% at Akron, Hays, and Tribune, respectively. The SOC concentration in NT soil was higher than in MP by 60% at Akron and 76% at Sidney, and it was higher than in CT soil by 82% at Hays. The BD max decreased ( r=-0.64) and the CWC increased ( r=0.60) with an increase in SOC concentration. Across all soils, SOC concentration was a sensitive predictor of BD max and CWC. This regional study showed that NT management-induced increase in SOC improves the soil's ability to resist compaction.
