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Soil and Tillage Effects on the Characteristic Size and Shape of Aggregates

Department of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091

*Corresponding author (eperfect{at}ca.uky.edu).

ABSTRACT

Measures of aggregate size and shape are needed in models to predict soil erosion, seed bed density, tensile strength, and solute diffusion. We hypothesized that size (i.e., equivalent cubic length L_c = [l₁(l₂l2)(l₃l3)]^1/3, where l₁, l₂, and l₃ are the shortest, intermediate, and longest axial lengths, respectively) and shape (i.e., aspect ratios, l₁/l₃ and l₂/l₃) vary with soil type and tillage treatment. Caliper measurements of l₁, l₂, and l₃ were made on 1800 aggregates from three sieved fractions (4–8, 8–16, and 16–31.5 mm) of a plowed, plowed plus disked, and no-till Maury silt loam (fine, mixed, mesic Typic Paleudalf) and a no-till Karnak silty clay (fine, montmorillonitic, nonacid, mesic Vertic Haplaquept). Overall means for l₁/l₃ (0.57) and l₂/l₃ (0.78) were close to those for a cube (0.58 and 0.82, respectively). The mean l₁/l₃ for Karnak was 18% lower than the corresponding value for no-till Maury. The L_c was lognormally distributed. The mean log₁₀(L_c) increased from 0.67 in the smallest fraction to 1.19 in the largest fraction. Soil effects on log₁₀(L_c) were most evident in the 8- to 16-mm fraction (1.01 for Karnak compared with 0.93 for no-till Maury). Tillage effects on log₁₀(L_c) were most evident in the 4- to 8-mm fraction (0.68 for no-till Maury compared with 0.66 and 0.65 for plowed and plowed plus disked Maury, respectively). The equation L_c = x_l(m/m_m)^1/3, where x_l is lower sieve aperture, m is aggregate mass, and m_m is modal mass, explained 96% of the variation in L_c. This equation can be used to predict aggregate size from sieve aperture size and measurements of aggregate mass.

Kentucky Agric. Exp. Stn. Contribution no. 96-06-113.

Received for publication July 17, 1996.

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