Corn root distribution and yield response to subsoiling for paleudults having different aggregates sizes

Chisel plowing loosens dense root-restricting layers within 0.25 m of the soil surface, while subsoiling can be used to routinely loosen dense layers down to a depth of approximately 0.45 m. Selecting one tillage method over the other to maximize yield is problematic because the lower depth of a root-restricting layer is often difficult to determine precisely if the layer contains structural aggregates. We hypothesized that the ability of roots to penetrate dense soil layers is related to the size of aggregates in the dense layers. Therefore, this study compared root distributions for chisel plowed and subsoiled tillage treatments and related them to soil properties and corn (Zea mays L.) grain yield for soils having dense layers that varied in their aggregate size. Yields and root distributions were determined for the two tillage treatments on eight Typic and Arenic Paleudults. Each site had a dense, high-strength soil layer below the depth of chiseling. Subsoiling significantly (P = 0.10) increased yields over chisel plowing at five sites. At four of these sites, the proportion of roots between depths of 0.3 and 1.0 m in the chiseled treatment was low (less than or equal to 0.20) due to the root-restricting layer extending below the depth of chiseling. Although all sites had pan-like layers, which were characterized on the basis of bulk-density and cone-index measurements, only layers whose ped meanweight diameter was less than or equal to 1.2 mm resulted in root proportions being less than or equal to 0.20 in the chiseled treatment. These virtually nonaggregated layers also tended to have low amounts of plant-available water (< 0.12 m3 m-3). To adequately characterize root-restricting layers that reduced yield in these soils, some measure of subsurface aggregation or a correlated property such as plant-available water or texture had to be evaluated along with bulk density or cone index.