Fractal Description of Soil Fragmentation for Various Tillage Methods and Crop Sequences

Soil structure has been difficult to quantify and, at best, has been studied semiquantitatively. Fractal representation of soil fragmentation can provide an indication of soil structure. The purpose of our study was to use fractal analysis to quantify soil fragmentation under various tillage and crop sequence treatments at different times during the growing season. We collected soil samples from four tillage treatments (established 10 yr earlier) of chisel, disk, no-till, and moldboard plow in factorial arrangement with two crop sequences of corn (Zea may L.)-soybean [Glycine max (L.) Merr.]-corn (C-S-C), and soybean-corn-soybean, (S-C-S) on a Sharpsburg (fine, montmorillonitic, mesic Typic Argiudoll) soil. Aggregate-size distribution was used to calculate fractal dimension (D) for each treatment. Higher D values indicate greater soil fragmentation and a soil dominated by smaller aggregates. The opposite is true for lower D values. Differences in soil fragmentation observed for tillage treatments after autumn tillage became even greater over winter. Soil fragmentation increased over autumn and winter, with D increasing in the order of plow > chisel > disk > no-till. Formation of larger soil aggregates increased during the growing season for all tillage systems. The D values for C-S-C were smaller than S-C-S in the no-till, indicating that the previous year's corn in C-S-C provided more large aggregates. Soybean appears to have negative effects on large-aggregate formation in no-till. Aggregate densities, averaged across tillage and crop sequence, increased from 1.25 to 1.77 Mg m⁻³ as the aggregate diameter decreased from 6.38 to 0.162 mm. Fractal analysis was found to be useful in determining soil fragmentation differences due to different tillage methods and crop sequences. Contribution of the USDA-ARS in cooperation with the Nebraska Exp. Stn., Lincoln, NE, as Paper no. 9909.