The variability of in situ mechanical impedance in field soils hinders evaluation of impedance differences stemming from various tillage and soil management practices. Use of the penetrometer cone index (CI) is one method to measure mechanical impedance and many CI data have been treated statistically. This study was conducted to evaluate the mechanical impedance as measured by CI for Norfolk sandy loam for the following tillage treatments: (i) moldboard plowing to 22 to 25 cm followed by disking, (ii) chisel plowing to 27 cm, and (iii) subsoiling at 91-cm intervals to a depth of 45 cm followed by bedding. Mechanical impedance was continuously recorded to a depth of 41 cm with a tractor mounted, hydraulically driven, constant rate descent penetrometer at seven positions spaced 15 cm apart on a transect normal to the soybean row. Measurements were taken four times during the year, representing a wide range in soil water content; replicated gravimetric water content measurements were taken as a function of depth at two of the seven positions at the time penetrometer readings were taken.
Analysis of variance showed significant differences in CI for tillage treatment, soil depth, position, and all two-way interactions. Chisel plowing reduced impedance at all seven positions to values less than those measured for the moldboard plowing. Of the three treatments, the subsoiled plots generally had the lowest impedance only in or near the subsoil slit, but had the highest impedance in the trafficked interrow.
We recommend that future characterizations of mechanical impedance (and other soil physical properties which are induced or modified by tillage practices which introduce nonhomogeneity of these properties, i.e., mechanical impedance, water movement, etc.) include sufficient measurements to isolate not only tillage effects, but also position and depth effects.