In general, conservation tillage practices in these regions are similar to conservation tillage practices in other regions. On Southern Coastal Plain and Atlantic Coast Flatwoods soils, keys to successful crop production with conservation tillage include:
- adequate residue cover and residue management
- good seed/transplant placement and crop establishment
- timely and aggressive weed control strategies
- management of soil compaction
Adequate Residue Cover
Adequate residue cover is needed in conservation tillage systems not only to reduce erosion but also to restore soil biology and enhance crop productivity. In addition to reducing erosion, a good residue cover:
- decreases raindrop impact and the potential for surface-soil crusting
- increases rainfall and irrigation infiltration into the soil
- decreases soil-water evaporation
- decreases soil temperature
A very high-residue cover can also aid in weed management. There are two proven methods for producing adequate surface residues. The first is rotating cash crops such as corn and small grains that leave a large amount of stalks that slowly decompose on the soil surface. Examples of common rotations that include high-residue crops are a one-year wheat>soybean rotation and a two-year corn>wheat>soybean rotation.
The second proven way to produce residues is to use cover crops. Common species of winter cover crops include small grains such as rye, wheat and oats; and legumes such as clovers, peas and vetches. Summer cover crops are also grown, primarily in rotations with vegetables or other short-duration rotations. Cowpeas, millet and sorghum-sudan grass are used for summer cover crops.
For a winter cover crop, rye is often used because it is more cold tolerant than the other small grains. Because of this, it can produce more biomass in the spring before it is killed. Small-grain cover crops may need a small amount of nitrogen fertilizer to produce adequate residue cover. Nitrogen fertilizer is not applied when cover crops follow legumes, such as soybeans or peanuts. Similarly, nitrogen is not applied to a cover crop that follows a non-legume cash crop that had low yield due to drought. The cover crop will use the nitrogen left in the soil after the cash-crop harvest.
Legume cover crops are planted to produce nitrogen for the subsequent crop. For best results when choosing a winter legume, match the cover crop species to the field’s plant hardiness zone. Growers can expect legumes to provide 50–100 pounds of nitrogen per acre to the subsequent crop under most growing conditions.
Crop Establishment
Higher crop yields and easier crop maintenance generally result when stands are uniform and seedlings grow vigorously. Successful conservation tillage planting or transplanting really begins at the harvest of the previous crop. Harvesting when the soil is too wet is avoided because it can result in ruts. Evenly distributed residues yield the best results. Planters have difficulty accurately placing seed into the soil when residues are not evenly distributed and there are residue mats.
Careful management of vegetation between crops is also important because the coarse-textured soils in these MLRAS do not store much water, only about 1 inch of water in the top 12 inches. The vegetation between crops is managed so the seedbed soil water is not depleted before the next crop is planted. Most recommendations call for cover crops or weeds to be killed two to four weeks prior to planting the cash crop. If conditions are dry and long-range weather forecasts indicate only a small chance of rain, then vegetation is killed earlier to reduce the depletion of soil water. This is especially critical in fields that are not irrigated. On the other hand, if precipitation is abundant and soil conditions are wet, letting the vegetation grow longer will help dry out the fields and will allow planting equipment to get in the fields sooner.
Planting or transplanting is done with equipment designed for conservation tillage systems. Settings are adjustable to match soil-water conditions and the composition and thickness of residue in the field. Drills and planters must:
- cut through the crop residue and soil
- place seed at a uniform spacing and depth
- completely cover or close the seed slot
- firm the soil around the seed
For planters, row cleaners can be used to move residues out of the row. This facilitates planting and allows for the soil around the seed to warm faster. If row cleaners are not used or when planting crops with drills, it is best to wait until the residue is dry to plant seeds. When residue is wet, coulters can push the residue into the seed slot rather than cutting through the residue as they are designed to do. This is called hairpinning. Hairpinning keeps seed from reaching the proper depth, inhibits closing of the slot and prevents good seed-soil contact. This is a common cause of poor stands in conservation tillage systems.
Crops established as seedlings are common in the Southern Coastal Plain and Atlantic Coast Flatwoods MLRAs. Researchers at Virginia Tech have developed a transplanter that can be used with high-residue, allowing for conservation tillage to be used for tobacco and other transplanted crops [8]. Named the Subsurface Tiller Transplanter (SST-T), the implement allows for more efficient and effective planting than previous transplant systems, provides higher capacity to set plants in heavy residues, and reduces the disturbance of surface residues [8].
Weed Management
Weed management in the Southern Coastal Plain and Atlantic Coast Flatwoods is similar to other MLRAs. However, all interviewed producers identified herbicide-resistant weeds as the most important challenge in the future. After years of successful no-till production, one of the interviewed producers, Kirk Brock, stated that herbicide-resistant weeds could be the reason to return to conventional tillage.
Herbicides are the main tool for managing weeds in most conservation tillage systems. Repeated use of the same herbicides has resulted in herbicide-resistant weeds such as Palmer amaranth. Management of herbicide-resistant weeds in conservation tillage fields is difficult. Multiple applications of herbicides with different modes of action are needed, including residual pre-emergent chemicals. High-biomass cover crops such as wheat or rye cover the soil surface and aid in weed suppression. During the growing season, fields are routinely scouted and if small pockets of potentially herbicide-resistant weeds are found they are removed. For example, if Palmer amaranth stands are found that are close to mature and suspected of being glyphosate resistant, the plants are removed from the field to eliminate the chance of reseeding.
Glyphosate-resistant Palmer amaranth is not the only herbicide-resistant weed species found in the Southern Coastal Plain and Atlantic Coast Flatwoods. Other weeds in the region that have been found to be herbicide resistant include goosegrass, common cocklebur, Italian ryegrass, prickly sida, smooth pigweed, lambsquarters and horseweed. These weeds and associated herbicide modes of action are identified in Table 11.2. Information on herbicide-resistant weeds and herbicide resistance can also be found at weedscience.org, a website documenting herbicide resistance internationally [4].
Use the following management guidelines [7] to help delay herbicide resistance in weeds:
- Rotate classes of herbicides used to control the same weeds.
- Tank mix a combination of herbicides that have different modes of action.
- Use Integrated Pest Management (IPM), including scouting, crop rotations and other cultural or biological control practices.
- Monitor fields for weeds that have not been successfully controlled by prior herbicide applications, and control them before they set seed.
- Clean harvesting and other equipment to prevent moving resistant weed seeds between fields.
For more information on weed management, see Chapter 11.
Soil Compaction Management
Many of the soils in the Southern Coastal Plain and Atlantic Coast Flatwoods are coarse textured, highly weathered and inherently low in fertility. These soil properties, combined with hot summers and years of tillage, have resulted in soils that are low in organic matter. This makes the surface soils prone to compaction from machinery, trucks and rainfall. Also, the soils often have a subsurface hardpan, the E horizon, that limits root growth to the upper 12 inches. Surface traffic, especially when soils are wet, can increase subsoil compaction.
To alleviate subsurface compaction, subsoiling implements are often used. One such implement is an in-row subsoiler that consists of a shank that penetrates 10–16 inches into the soil. Since 1970, Extension specialists throughout the Southeast have recommended subsoiling for row crops grown on soils with a hardpan layer [11]. Because of compaction, conservation tillage equipment developed for other parts of the nation did not work well in the Southern Coastal Plain and the Atlantic Coast Flatwoods. Some researchers [14] have attributed the slower adoption of conservation tillage in these two MLRAs to problems associated with root penetration through the eluviated hard pan. Conservation tillage equipment capable of planting in these soils finally became available in the late 1970s. That is when the “No-Till Plus” implement was introduced by Harden et al [3]. This implement combined in-row subsoiling with no-till planting for a one-pass subsoiling-planting operation. The implement consisted of a no-till coulter, a subsoiler shank, a wheel to close the slit behind the shank, a double disc opener that places seed directly over the slit and a packing wheel to firm the soil around the seed [3].
Since the introduction of the No-Till Plus planter, many variations of planting and subsoiling equipment have been used. There are many planting and compaction management strategies:
- No-till planting is a viable option for soils without compaction problems. This is usually accomplished with just a coulter in front of the planter to open the seed furrow.
- In-row strip tillage is similar to the No-Till Plus system and is often called a version of strip tillage. In this system, a coulter is followed by a narrow shank called a ripper. The ripper tills to a depth of 10–16 inches and leaves a slit in the soil that is closed by packing wheels or other equipment. Often, planters are attached for a one-pass planting operation.
- Strip-tillage is widely used for row-crop planting. Strip-tillage rigs consist of coulters, rolling baskets, spider gangs, firming wheels and other devices. The tilled zone is 6–12 inches wide and 6–8 inches deep.
- Fall or winter ripping, followed by no-till planting in the spring, is also widely used. In this practice, compacted layers are loosened with straight-shank subsoilers or bent-legged subsoilers, such as the paratill.
- For drilled crops like winter small grains, bent-legged subsoilers can be used before the crop is planted to disrupt more of the soil profile than straight-shank implements. If used prior to planting the preceding summer crop, running the implement again may not be necessary in the fall unless the small grain is being managed for high yield.
Subsoiling is expensive, especially since tractor horsepower requirements are generally over 25 horsepower per shank, depending on the implement. Thus, it is critical that their use be optimized. Subsoilers are run so that the E horizon is disrupted, but no deeper. Generally, this is just at the top of the B horizon. Deeper tillage requires more energy and probably will not result in increased crop yields. If the tillage is too shallow, root growth will be limited. Checking soil compaction levels before tillage with a penetrometer can determine the depth of tillage necessary. Penetrometers can also tell whether subsoiling is needed. For example, if employing controlled traffic and planting near the row subsoiled the previous season, the penetrometer can determine if subsoiling is needed in the row again.
Another way to reduce subsoil compaction is to include deep-rooted annuals and perennials in the crop rotation. The roots provide channels for root development through the restrictive layer by subsequent crops.
Enterprise Budgets
Most universities with agricultural programs provide crop enterprise budgets to help determine which production systems and practices are most economical for a farm. Budgets take into consideration both fixed and variable costs and provide projected incomes at different yield levels and prices. For example, Clemson University provides crop enterprise budgets for the major row crops grown in South Carolina, with more options for crops that have the most acreage in the state. There are five budget options for corn through which farmers can calculate net economic returns at different yields, with or without irrigation, with conventional or conservation tillage, and with or without using new genetic technologies. There are nine budgets for cotton that differ in similar variables. In contrast, only one or two budget options are available for crops like tobacco, oats and barley. The comparison between conservation tillage and conventional tillage is available for all major crops grown in South Carolina except tobacco, oats and barley. In conservation tillage, the following variable costs are different than the costs in conventional tillage: land preparation, herbicides, machinery and labor. Fixed costs for tractors and machinery are also different in the two systems.
Other Considerations Specific to the Region
The adoption of conservation tillage practices by growers in the two MLRAs was slower than in other MLRAs. In addition to the compaction problems, there was little environmental incentive to adopt the technology because the soils are generally flat and water erosion is less than in other MLRAs. Rotations with crops that are dug are common in the two regions. Peanuts are widely grown throughout the two MLRAs, and sweet potatoes occupy a significant acreage, especially in North Carolina. There were two concerns about using conservation tillage for these crops. One was the ability of crops to properly grow in fields with high residues. For example, pegging in peanuts can be inhibited by surface residues. The second concern was digging- and harvesting-equipment operating properly.
Another reason for slower adoption of conservation tillage was that research found small differences in yield between conservation and conventional tillage systems. This is contrary to research results from other MLRAs, especially the Southern Piedmont. For example, North Carolina studies in the 1980s conducted in the Southern Coastal Plain found that corn grown with conservation tillage had higher yield than conventional tillage in only one of five years. There was no yield difference between conventional and conservation tillage in any of the five years for soybeans [17]. Another study in South Carolina on the Southern Coastal Plain [6] reported lower yield of wheat when conservation tillage was used than when moldboard plowing or chisel plowing was used. Researchers attributed part of the lower no-till yield to the inability to get uniform stands with the drill they used in that study. With this experience, agricultural advisors in the region were less aggressive in promoting conservation tillage than advisors in other regions.
Long-term experiments in the region are providing evidence that using conservation tillage slowly improves soil characteristics. In Florence, S.C., organic matter in the top 2 inches of soil in conservation tillage was 76 percent higher than soil in conventional tillage. This is based on a study of soil properties after 25 years of conventional and conservation tillage. The soil organic carbon under conventional tillage was 0.95 percent and was 1.67 percent under conservation tillage [1]. In this experiment, high-residue crops were grown in every year of the study. It included a two-year rotation of small grains double-cropped with soybeans followed by corn in the second year (small grains>soybeans>corn). Cotton replaced soybeans in five years of the study. Contrary to what has been found in other regions, analysis of the trend over 25 years showed that soil organic matter levels were continuing to increase in the top 2 inches of the soil in those plots [10]. This analysis indicates the need for continuous conservation tillage on these soils for maximum soil improvements.
Building soil organic matter in these soils improves the biological and chemical properties of the soil. Low soil organic matter makes the soil prone to compaction, and as compaction increases, bulk density increases. A long-term study (1996–2003) in Goldsboro, N.C., found that conservation tillage increased organic matter and decreased soil bulk density in the surface 2 inches of soil compared to conventional tillage. It also found a strong inverse correlation between soil organic matter and bulk density [9]. However, the soils managed with conservation tillage had lower soil organic matter at the 2- to 5-inch depth and higher bulk density in that layer if the soils had low silt content. In coarse-textured soils, 2.16 percent soil organic matter (1.25 percent organic carbon) is needed to keep bulk density at levels where root activity is not inhibited in the 2- to 5-inch depth range.
Sod-based rotations also increase soil quality in conservation tillage systems. The University of Florida has developed a new crop rotation scheme for irrigated production that has higher economic viability than conventional production. In this four-year rotation, two years of bahiagrass are followed by one year of cotton and one year of peanuts. Cover crops are grown during the winter following cotton and peanuts. The bahiagrass is either grazed or baled for hay. Although this is not a “permanent” conservation tillage rotation because the peanuts must be dug, the system does provide long-term soil improvements. In experiments, the soil organic matter in the top 6 inches of soil increased 0.1 percent per year [18]. See Chapter 8 for more information about sod-based rotations with grazing.