Pawel Wiatrak and Jason Norsworthy
Double-cropping soybeans after wheat is a popular practice in South Carolina, with an estimated two-thirds of the soybean crop planted in June after wheat harvest. Even though double-cropping has been a profitable system for many farmers, high costs, time constraints, and labor have caused more interest in adopting conservation tillage for both crops.
Previously, research has shown that no-till wheat and soybeans are not feasible because of problems with soil compaction and stands. Now, better drills, varieties, and deep tillage tools designed for breaking hardpans, while leaving the majority of surface residues in place, have enhanced the chances for success of conservation tillage.
The sandy soils typical of the southeast Coastal Plains are inherently low in fertility and water holding capacity. They are also subject to leaching and often exhibit significant runoff during the growing season when most rainfall comes as thunderstorms. The organic matter content of the topsoil (A horizon) for these soils is low (usually less than 1 percent), and with excessive tillage this figure may be closer to 0.5 percent. This situation results in poor soil tilth and reductions in rainfall infiltration potential.
Some agronomists attribute the lowering of soil productivity and crop yield potential to these factors. Also, excessively tilled low organic Coastal Plain soils possess less buffering potential against the effects of drought stress.
Another important characteristic of most Coastal Plain soils is a hardpan that restricts root growth and increases the potential for serious yield losses during drought. The hardpan, or E horizon, is usually 1 to 4 inches thick and about 7 to 15 inches deep and is lighter in texture than the topsoil or plow layer (A horizon). Subsoiling is the most common method farmers use to break the hardpan to encourage root growth into the clay or B horizon, where additional moisture and nutrients are available. Yield increases for wheat and soybeans can be from 10 to 15 percent, or more, with good deep tillage practices for alleviating the effects of soil compaction. Chisel plow tines are spring loaded and thus are relatively ineffective in disrupting the E horizon.
Deep tillage tools that feature bent shanks or "wings" are available for use in conservation tillage systems. These plows (Tye or Bingham Bros. Paratill, Worksaver's Terra-Max, and DMI's Ecolo-Till), equipped with coulters for cutting through surface residues, actually lift the soil and then drop it as they are pulled through the field. This action shatters hardpans similar to dropping concrete. Such practice effectively loosens the soil above the shanks or wings. Thus, there is almost a broadcast type (about 70 percent of the soil is affected) of deep tillage vs. the furrow type of tillage effect with conventional shank subsoilers. The idea is that this type of hardpan shattering will last longer than the furrow-type done by conventional shanks. Experience has shown that reconsolidation of compacted zones occurs earlier with the conventional shank type of deep tillage. Also, crop roots can more effectively "search" the profile for water and nutrients after use of these winged plows.
Conventional wheat planting systems in the Coastal Plains typically involve a minimum of two or three diskings to bury previous crop residue, followed by subsoiling, smoothing, and then planting. For soybeans, the small grain residue is either burned or disked, and then there is a one-pass subsoil-planting operation for soybeans. Energy consumption and the investment in equipment, time and labor is high. Also, little consideration is given to the potential for runoff and/or erosion with these conventional tillage systems. A reduced tillage, one-pass wheat/soybean system has the potential to save energy, reduce production costs and erosion, provide food for quail and other wildlife, and expand the planting interval available to farmers.
The bottom line in these discussions of reduced-tillage systems for profitable wheat/soybean double-cropping systems for South Carolina is as follows:
Better drills, Roundup Ready varieties, and controlled traffic systems will help enhance the chances for success.