SOIL AND TILLAGE EFFECTS ON THE CHARACTERISTIC SIZE AND SHAPE OF AGGREGATES

Measures of aggregate size and shape are needed in models to predict s oil erosion, seed bed density, tensile strength, and solute diffusion. We hypothesized that size (i.e., equivalent cubic length L-c = [l(1)( l(2)/root 2)(l(3)/root 3)](1/3), where l(1), l(2), and l(3) are the sh ortest, intermediate, and longest axial lengths, respectively) and sha pe (i.e., aspect ratios, l(1)/l(3) and l(2)/l(3)) vary with soil type and tillage treatment. Caliper measurements of l(1), l(2), and l(3) we re made on 1800 aggregates from three sieved fractions (4-8, 8-16, and 16-31.5 mm) of a plowed, plowed plus dished, and no-till Maury silt l oam (fine, mixed, mesic Typic Paleudalf) and a no-till Karnak silty cl ay (fine, montmorillonitic, nonacid, mesic Vertic Haplaquept). Overall means for l(1)/l(3) (0.57) and l(2)/l(3) (0.78) were close to those f or a cube (0.58 and 0.82, respectively). The mean l(1)/l(3) for Karnak was 18% lower than the corresponding value for no-till Maury. The L-c was lognormally distributed. The mean log(10)(L-c) increased from 0.6 7 in the smallest fraction to 1.19 in the largest fraction. Soil effec ts on log(10)(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 log(10)(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 dis hed Maury, respectively). The equation L-c = x(1)(m/m(m))(1/3), where x(1) is lower sieve aperture, m is aggregate mass, and m(m) is modal m ass, 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.