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DIVISION S-1—SOIL PHYSICS

Fractal Analysis of Soil Water Desorption Data Collected on Disturbed Samples with Water Activity Meters

^a Dep. of Earth and Planetary Sci., Univ. of Tennessee, Knoxville, TN 37996-1410, USA
^b Dep. of Plant Production, Univ. of Buenos Aires, 1417 Buenos Aires, and Nitragin Argentina S.A., Calle 10 y 11, Parque Industrial Pilar, 1629-Pilar, Buenos Aires, Argentina
^c Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091, USA

^* Corresponding author (eperfect{at}utk.edu).

We combined water activity meters and fractal modeling to facilitate rapid, physically based characterization of the soil water retention curve. Desorption data (6 points per sample) were collected in the tension (h) range 2.0 x 10² to 1.5 x 10⁵ kPa using the gravimetric method to measure water content (w) and water activity meters to measure h. Thirty-two disturbed samples from a long-term nitrogen fertilization and tillage comparison study on a silt loam soil were analyzed. A new form of an established fractal equation was derived: w = ah^D–3 – _w/_s, where a is a compound parameter including the bulk density (_b) and air entry tension (h_a), D is the mass fractal dimension, _w is the density of water, and _s is the particle density. This model was fitted to the measured water retention curves by nonlinear regression analysis. The a and D parameters were estimated, while _w and _s were fixed at 1.00 and 2.65 Mg m^–3, respectively. Convergence was always achieved and the equation fitted the data extremely well; residual sums of squares ranged from 1.2 x 10^–6 to 6.7 x 10^–5, with a median value of 2.2 x 10^–5. Estimates of a (0.62–0.74) and D (2.948–2.963) were physically reasonable, and sensitive to soil management practices. The a parameter increased, r = 0.80 (P < 0.01), (signifying decreasing _b and/or h_a) whereas D decreased, r = –0.75 (P < 0.01), (signifying more rapid capillary drainage) with increasing soil carbon content. Additional research is needed to test this approach on other soil types, and to assess the influence of soil disturbance and variations in _s on the model's performance.

Abbreviations: NT, no-till • PD, plowed and disked treatment

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