Oxygen Flux, Air-filled Porosity, and Bulk Density as Indices of Vertisol Structure

The structure of irrigated Vertisols in Australia is frequently damaged by cultivating wet soil. This may reduce crop growth and yield, but there is a lack of suitable indices that characterize the physical damage to the soil. Oxygen flux density (F), air-filled porosity (ε), and bulk density (ρb) were compared as indices of soil structure in undisturbed soil cores of a cracking gray clay (Vertisol, Typic Pellustert) at Narrabri, New South Wales, Australia. All three parameters were measured on the same cores. The soil had received two cultivation treatments: cultivating when wet and cultivating when dry. Samples were taken from each treatment on six occasions over a range of soil-water content at five depth increments from 0 to 0.075 m to 0.4 to 0.475 m. Wet cultivation decreased F and ε relative to dry cultivation at most sampling times, but effects on ρb were smaller and inconsistent. All three parameters increased as gravimetric water content (θg) decreased. The variation of each parameter with respect to θg was obtained by linear regression, and the ratio of the slopes of these regressions between the two cultivation treatments was called the sensitivity ratio. Sensitivity ratios for F, ε, and ρb were 22.7, 6.9, and 1.2, respectively, for the 0.2- to 0.275-m depth, which was most affected by cultivation treatments. Low sensitivity of ρb was related to low ε of the wet soil, which precluded compaction during wet cultivation. The sensitivity of ε was attributed to the greater structural shrinkage (conservation of pore volume on drying) observed in soil cultivated when dry compared to soil cultivated when wet. The high sensitivity of F is related to the dependence of F on ε and on the tortuosity and continuity of air-filled pores, all of which appeared to be adversely affected by wet cultivation. Oxygen flux density is therefore recommended as the most sensitive of the three indices for detecting structural changes.