Landscape effects on desiccation cracking in an aqualf

The rapid transport of water and solute through desiccation soil cracks can lead to crop water and nutrient stress as well as ground and surface water contamination. This study hypothesized that desiccation cracking varies with landscape positions. Cracking was quantified at three landscape positions in a Mexico silt loam (fine, montmorillonitic, mesic Mollic Endoaqualf). The elevation changes of seven disks placed at 15-cm depth increments in the profile were measured to determine changes in soil layer thickness. Crack volume in the upper 90-cm profile was determined from thickness changes of the six soil layers, Disk elevation and soil profile water content measurements were made weekly during the crop season (April-November) in 1992 and 1994 on summit, backslope, and footslope positions. During both years, the backslope position profile developed the largest amount of crack volume (4.3 cm(3) cm(-2) in 1992, 4.8 cm(3) cm(-2) in 1994) and had the least amount of water depletion (10.0 cm(3) cm(-2) in 1992, 9.7 cm(3) cm(-2) in 1994). At the backslope position, a thin, high-clay-content Ap horizon and a shallow, clay-rich Bt horizon (both high in smectite clay) lead to these developments. Differences in shrinkage characteristics of aggregates from Ap, Bt, and C horizons across landscape positions were related to textural and mineralogical differences, Bt horizon clay contents as high as 64.4%, comprised of up to 75% smectite, produced the greatest volume reductions (41%) of aggregates from the Bt horizon compared with volume reductions of aggregates from the Ap and C horizons. Soil texture and mineralogy influenced the magnitude of desiccation soil cracking and water depletion.