Subsoiling

Subsoiling is defined as non-inversion tillage below a depth of 14 inches [1]. Figure 6.1 shows an example of an agricultural implement that has been used for uniform disturbance of a soil profile to depths of 14–20 inches. Soils compacted from traffic, animals or natural processes benefit from subsoiling because the compacted zone is disrupted. Subsoiling creates larger pores that increase rooting and infiltration. The benefits of subsoiling depend upon many factors including soil type, soil management and vehicle management. Much research has been conducted that provides evidence of the benefits of subsoiling. However, some research has shown no overall benefits to crop productivity. Reasons for the discrepancies include differences in equipment, climate, annual variations in weather, cropping systems, management practices and soil types.

The effect of subsoiling to a 15-inch depth was studied in sandy loams of South Carolina [12]. In this study, researchers found that subsoiling adequately disrupted the hardpan, reduced soil strength (see the sidebar, Determining the Depth of a Compacted Soil Layer), increased infiltration and increased rooting depth. Several other studies reported increased crop yields and reduced soil strength due to subsoiling [25]. However, most of these studies provided little crop management information, and it is assumed that conventional tillage practices were employed.

A four-year study on a sandy loam in Georgia evaluated the long-term effects of reducing soil strength by subsoiling to a depth of 14.2–15.0 inches [30]. It concluded that soil strength was reduced but that reductions were not detected after the second year. The use of a controlled-traffic system was recommended to increase the longevity of reduced soil strength. Another study showed that subsoiling down to 14.2 inches in a sandy loam in Georgia, along with irrigation, significantly increased grain yields [7].