CHARACTERIZING THE STRUCTURE OF UNDISTURBED SOILS

Soil structure is often viewed as the spatial heterogeneity of compone nt soil properties. To explore a method for determining this heterogen eity, we examined the use of spectral analysis to discern the periodic spatial variance in depth series measurements of penetration resistan ce and oxygen diffusion rate (ODR) in undisturbed soil cores. Mean pow er spectra of penetration resistance residuals for 105 moldboard-plowe d (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, p ower across the frequency range of O to 1.0 cycles mm(-1). Harmonic an alysis related this to increased aggregate strength in the NT treatmen t. 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, power across the fre quency range of O to 0.125 cycles mm(-1). Geometric mean aggregate dia meters showed tillage-induced differences of 6.8 mm for the MP and 10. 8 mm for the NT treatments. The mean frequencies corresponding with ma ximum power for the individual spectra were not different between till age treatments or between the independent penetrometer and ODR methods , yielding aggregate diameters of 15.3 mm. The absence of tillage-indu ced aggregate diameter differences from analysis of penetration resist ance and ODR data, as contrasted with the observed aggregate diameter differences from sieving, suggests that sieving may not precisely refl ect the in situ aggregation of soils.