Organic Carbon Influences on Soil Particle Density and Rheological Properties

Soil particle density (ρ s ) is not routinely measured and is assumed to range between 2.60 and 2.70 Mg m −3 or to be a constant (2.65 Mg m −3 ) when estimating essential properties such as porosity, and volumetric water and air relations. Values of ρ s for the same soil may, however, differ significantly from the standard range due to management‐induced changes in soil organic carbon (SOC) concentrations. We quantified the ρ s and Atterberg limits of a Rayne silt loam for five long‐term (>22 yr) moldboard‐plowed continuous corn ( Zea mays L.; MP), no‐till continuous corn (NT), no‐till continuous corn with beef cattle manure (NTm), pasture, and forest systems. We also assessed the relationships of SOC concentration with ρ s and the Atterberg limits and the impact of ρ s on soil porosity. Mean ρ s across NT, NTm, and pasture (2.35 Mg m −3 ) was ∼7% lower than that for MP in the 0‐ to 10‐cm soil depth (2.52 Mg m −3 , P < 0.01). Forest had the lowest ρ s of all soils (1.79 Mg m −3 ). The NTm caused a greater reduction in ρ s and a greater increase in SOC concentration, liquid limit (LL), plastic limit (PL), and plasticity index (PI) than NT. Surface soils under MP had the highest ρ s and ρ b and the lowest SOC concentration, LL, PL, and PI. The SOC concentration was correlated negatively with ρ s ( r 2 = 0.75) and positively with Atterberg limits ( r 2 > 0.64) at >20‐cm depth. Estimates of soil porosity for NT, NTm, and pasture using the constant ρ s overestimated the “true” porosity by 12% relative to that using the measured ρ s