Characterizing the Structure of Undisturbed Soils

Soil structure is often viewed as the spatial heterogeneity of component soil properties. To explore a method for determining this heterogeneity, we examined the use of spectral analysis to discern the periodic spatial variance in depth series measurements of penetration resistance and oxygen diffusion rate (ODR) in undisturbed soil cores. Mean power spectra of penetration resistance residuals for 105 moldboard‐plowed (MP) and 109 no‐tillage (NT) cores had peaks at a frequency of 0.031 cycles mm −1 . The NT treatment, however, averaged 0.71 higher log e power across the frequency range of 0 to 1.0 cycles mm −1 . Harmonic analysis related this to increased aggregate strength in the NT treatment. Mean power spectra of detrended ODR data from 96 MP and 95 NT cores had peaks across the frequency range of 0.017 to 0.025 cycles mm −1 . The NT treatment again averaged 0.78 higher log e power across the frequency range of 0 to 0.125 cycles mm −1 . Geometric mean aggregate diameters showed tillage‐induced differences of 6.8 mm for the MP and 10.8 mm for the NT treatments. The mean frequencies corresponding with maximum power for the individual spectra were not different between tillage treatments or between the independent penetrometer and ODR methods, yielding aggregate diameters of 15.3 mm. The absence of tillage‐induced aggregate diameter differences from analysis of penetration resistance and ODR data, as contrasted with the observed aggregate diameter differences from sieving, suggests that sieving may not precisely reflect the in situ aggregation of soils.