Subsoiling for Sunflower Production in the Southeast Coastal Plains

Crops grown on the Paleudult soils of the South Atlantic Coastal Plain often benefit from disruption of root‐restrictive subsoil layers. In this physiographic area, the response of sunflower ( Helianthus annuus L.) to subsoiling was unknown. We hypothesized that in‐row subsoiling would benefit sunflower performance, and that plant performance could be related to profile penetration‐resistance patterns. Sunflower was grown on Norfolk loamy sand in Florence, SC, and on Orangeburg loamy sand (both fine‐loamy, siliceous, thermic Typic Paleudults) in Blackville, SC, in 1985 and in Blackville only in 1987. Plots were either subsoiled using 0.45‐m shanks or not subsoiled. Distribution (and, in most instances, magnitude) of cone indices were significantly different for subsoiled and nonsubsoiled profiles. Accumulation frequency of low cone indices was greater for Florence and Blackville in 1985 but not for Blackville in 1987. For subsoiled treatments, lower cone indices below planted rows persisted to late summer in 1985 at both locations, which favored plant growth. Reduction of soil profile strength produced increased seed yield, oil concentration, oil yield, and seed size in these cases. In 1987, accumulation frequency of soil strength was similar for non‐subsoiled and subsoiled plots shortly after tillage. In this case, plant parameters were not statistically improved with subsoiling. No tillage × N‐rate, hybrid × tillage, or hybrid × N‐rate interactions were observed. If low‐cone‐index isopleths persist throughout the season, then a positive response to subsoiling can be expected.