Submitted to: Ecological Modeling
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/10/2003
Publication Date: 1/6/2004
Citation: Pachepsky, Y.A., Guber, A. 2004. Fractal mass-size scaling of wet soil aggregrates. Ecological Modeling. pp. 317-322. Interpretive Summary: Soil aggregate analysis presents an important characterization of soil structural organization. Experiments with dry soil aggregates have shown that the mass of aggregates changes with their size so that the aggregates become looser as their size increases. Such dependence has been successfully explained by the fractal scaling theory, and predictions of this theory are in good agreement with experimental data. The theory furnishes two parameters to characterize mass-size relationships. One of them is the exponent that shows how pronounced is the loosening with the size increase. Another parameter shows the reference bulk density for aggregates of the unit size. We note that aggregates in field soil are not dry. The mass-size dependence for such aggregates differs from that in dry aggregates because soil shrinks as water content decreases. Our objective was to find out whether the fractal scaling theory is applicable to aggregates that are not air-dry. Aggregates from the plow layer of the Greyzem soil were brought to four different levels of water contents, and the aggregate mass was then measured using the kerosene method. It appeared that the wetter aggregates are the less prone to loosening as the water increases. The reference bulk density increased as the water content increased. Both parameters of the fractal theory were simple linear functions of the water content. Those findings introduce convenient, physics-based parameters to characterize changes in soil structure during wetting-drying cycles and corresponding changes in soil ability to retain and transmit water and solutes.
Technical Abstract: Structure is an important factor of soil functioning in ecosystems. Soil aggregate size distributions are commonly used to characterize soil structure. Relationships between density of dry soil aggregates and aggregate size present a different way to use aggregate-related information in soil structure characterization. Those relationships have been simulated assuming soils to be mass fractals. Aggregates in field soil are not air-dry. The relationships between mass and size differ between dry aggregates and wet aggregates because aggregates shrink as water content decreases. Our objective was to find out whether the mass fractal model can be applied to wet aggregates. Aggregates from the plow layer of Greyzem soil were brought to four different levels of water contents, and the kerosene method was used to measure mass of aggregates within diameter ranges of 3 to 5 mm, 5 to 7 mm, and 7 to 10 mm. It appeared that the wetter aggregates were less prone to loosening as the water increased. The mass fractal model was applicable to wet aggregates, and the assumption about linear dependence of the fractal dimension and unit size aggregate mass on gravimetric water content resulted in R2=0.9999 for the regression line simulated vs. measured aggregate mass. Fractal modeling of mass-size scaling in wet soil aggregates presents a set of aggregate-based parameters for soil structure that may reflect soil properties and can be explored as an index of soil ability to retain and transmit water and solutes.