Mechanical impedance (MI) is an important property of root-restricting, tillage-induced pans common to Ultisols in the Coastal Plain of North Carolina. The relationship between MI and other soil properties is not well understood. The purposes of this study were to quantify selected physical, chemical, and mineralogical properties of tillage pans and to correlate MI to these measurements.
Mechanical impedance of tillage-induced pans at 50 sites in the Coastal Plain of North Carolina was measured with a constant rate cone penetrometer on triplicate, undisturbed soil cores. Sites were selected to give a broad range of measured MI values. Impedance at a soil water pressure (SWP) of -0.10 bar ranged from 3.8 to 39.4 kg/cm2; for -1.00 bar, 19 to 98 kg/cm2. Other measured soil properties included particle size, the particle-size coefficient of uniformity (CU), bulk density (Db), saturated hydraulic conductivity, dense soil angle of repose (DSAR), soil water characteristic, cation exchange capacity (CEC), external and total surface area, free iron oxides, and the amounts of organic matter, Ca, Mg, P, K, Mn, gibbsite, kaolinite, and amorphous silica and alumina.
A stepwise regression procedure (maximum R-square improvement technique) was used to select the best models relating MI to the measured soil properties. For this grouping of soils, no one physical, chemical, or mineralogical parameter correlated well with MI. The best single variable model was found for Pw as the independent variable but this relationship had an R2 value of only 0.15. The best three-variable model included Pw, DSAR, and Db and had an R2 = 0.67. When MI data was subjected to a log10 transformation, the R2 for the three-variable model increased to 0.74.