The amount of organic matter in soil depends on cropping history, current production methods, soil type, and variations in climate and microclimate. The Soil Conditioning Index (SCI) is used by the USDA Natural Resources Conservation Service to predict changes in soil organic matter as affected by cropping system, tillage management and soil texture . When an SCI score is negative, organic matter is predicted to decline. When an SCI score is positive, organic matter is predicted to increase. The SCI is a useful way to look at the likelihood of a change in soil organic matter, but it does not predict an amount of change. In the following sections, the SCI was used to assess various cropping systems in these major land resource areas: Southern Piedmont, Southern Coastal Plain and the Southern Appalachian Ridges and Valleys .
Soils in the Southern Piedmont have moderately high clay content, are mostly well drained, and are at least moderately permeable . Historical tillage practices caused the loss of soil organic matter in these soils, resulting in organic-matter contents that are often less than 2 percent. The SCI predicts that continuous cotton production with no-till would increase soil organic matter marginally, but including a winter cover crop or grain in the rotation would do even better (Table 3.3). Increasing crop-rotation complexity with corn and short-term sod that could be used for livestock grazing would be the best way to increase soil organic matter. Surface-soil organic-matter contents of 4 percent are about as high as could be expected based on data from forest and pastures.
Cotton was the dominant crop for more than 150 years in the Southern Piedmont, causing great erosion scars in this sloping region . Despite adequate rainfall, limited infiltration due to crusting resulted in high water runoff and low soil-water storage under conventional tillage. Maintaining good residue cover is particularly important for reducing surface sealing, water runoff, soil loss and agro-chemical runoff [11, 39]. Conservation tillage systems lead to more soil organic matter, improved soil quality and greater cotton yield [16, 47]. When converting to conservation tillage in this region, use deep tillage without inversion such as subsoiling to initially overcome a lack of soil structure resulting from decades of intensive tillage.
Southern Coastal Plain
Agricultural soils in the Southern Coastal Plain are located on floodplains, river terraces and gently sloping uplands . These soils tend to have sandy textures and are moderately well drained to well drained. Conventional tillage in the Coastal Plain region causes loss of soil organic matter, as seen by the negative SCI in Table 3.4. Most agricultural soils in the region have organic matter contents less than 1 percent.
Management strategies to increase organic matter include reduced tillage or no-till, diversifying rotations with high-residue crops such as corn and cereal cover crops, applying animal manure, and including sod in rotations. For example, a Norfolk soil with continuous cotton using conventional tillage has an SCI score of -0.41. Changing management to no-till will increase the score to 0.44. Adding a cereal-rye winter cover crop and rotating cotton with corn increases the SCI score to 0.60. Because these sandy soils do not retain organic matter as well as the clayey soils of the Southern Piedmont, organic-matter contents of 2–3 percent are probably as high as can be expected.
In Coastal Plain soils, non-inversion subsoiling is needed in the spring to alleviate compaction due to traffic and natural reconsolidation, which can constrain root growth. Paratilling disturbs the soil and results in a loss of organic matter. In Bama soils, paratilling continuous cotton would likely decrease soil organic matter, but paratilling cotton rotated with corn using cover crops would increase organic matter (Table 3.4). An even more intensive rotation, corn>sunn hemp summer cover crop>wheat>cotton> white lupin/crimson clover mixture, further increased organic matter. Increased plant growth, particularly in the root zone, adds organic matter. Several types of non-inversion subsoiling tools are used, including subsoil shanks and paratills. Methods that cause the least surface-soil disturbance are best.
Southern Appalachian Ridges and Valleys
Soils in the Southern Appalachian Ridges and Valleys region are fine-textured silts and clays derived from limestone, sandstone, siltstone, shale and dolomite . Agricultural soils in this area typically have organic-matter contents of 2 percent under conventional tillage. The use of crop rotation and cover crops is particularly important. Continuous cotton production in the Tennessee Valley of northern Alabama is predicted to lose soil organic matter under both chisel plow and no-till (Table 3.5). By including a cover crop in a cotton>corn rotation, organic matter would likely increase, especially when applying poultry litter as a nutrient source. Even with soil disturbance with a paratill prior to cotton planting, including a cover crop in the rotation promotes increased organic matter. A soil organic matter content of 4 percent is probably as high as can be expected.
Soils in this region have a platy structure that leads to soil compaction and resistance to root growth, especially under no-till. In the early 1990s, the common practice was to plant without tillage directly into cotton stubble with no winter cover crop. Cotton yield reductions were common and jeopardized the adoption of no-till. It was later demonstrated that yields would increase with autumn non-inversion tillage under the row, coupled with an annual-rye cover crop to reduce compaction and to provide moisture-conserving surface residue [42, 43, 48].