Pawel Wiatrak and Jim Camberato
Corn can be grown on a wide range of soils. Field selection should be based on a potential to produce optimum yields. Information to assist with field selection is presented in the County Soil Survey, available from local USDA-NRCS offices. In addition to soil survey maps, Clemson scientists are utilizing geo-referenced equipment (Veris 3100) that can map the spatial variability in soil type and depth to the subsoil within fields.
Corn rotation with other crops like soybeans, peanuts, tobacco, and cotton, can help with reducing the weed, insect, and disease pressure, and improve yields. Additionally, rotating crops helps to remove phytotoxic substances produced by corn and improve soil physical properties. Growing legumes prior to corn add residual nitrogen to the soil, which can be utilized by corn, and therefore the nitrogen rates can be reduced on corn by 20 - 30 lb/A.
Most of the soils in South Carolina are sandy and have low water and nutrient holding capacity. These soils tend to form natural compacted zones or hardpans that are restrictive to corn root growth and development. Hardpans are formed either by tillage operations (6 - 8 inches deep) or naturally (8 - 15 inches deep) right above, but not extending into, the clay subsoil. These compacted layers or hardpans range in thickness from 2 to 12 inches. When present, they prevent or delay root development into the subsoil, resulting in lower grain yields.
Corn is sensitive to the presence of hardpans and responds well to deep tillage, which breaks the hardpan layers to allow corn roots to reach for water and nutrients at lower soil depths and therefore better withstand a short drought periods during the growing season.
Research over the past 20 years has shown significant yield increases as a result of deep tillage when hardpans exist. Deep tillage, such as in-row subsoiling, promotes deeper root penetration into the soil, allowing roots to extract more subsoil moisture and nutrients. The advantages of subsoiling over shallow cultivation are usually more pronounced in drier years. Fields should be checked to confirm the presence of a hardpan each year. This can be done by pushing a sharpened metal rod through the topsoil when soil moisture conditions are good.
A hardpan can usually be detected by an increase in resistance to rod penetration between the 4 and 14 inch soil depth. The depth of the subsoil depends on the soil type. To properly adjust the operating depth of the deep tillage equipment it is necessary to know the depth to clay throughout the field. Chisel plows can operate to a depth of 11 to 12 inches but have a very high draft requirement when run at these depths. In-row subsoilers perform more efficiently at these depths or deeper, and offer the advantage of not inverting the topsoil or mixing the subsoil with the topsoil.
For subsoiling to be economically effective, the subsoiler must reach to the bottom of the hardpan layer but not into the clayey subsoil. Subsoiling deep into the clay will not increase yields but will increase the costs of operation. In deeper sandy soils where the subsoiler will not reach to the clay, subsoil to a depth of 16 to 18 inches if possible. Because of the curvature of the subsoiler shanks, the depth of penetration of the shanks is seldom equal to the length of the shank. Inadequate penetration into the soil is a common problem in South Carolina that does not increase grain yields, but increases time, energy, and labor costs.
In-row subsoiling must be combined with some additional method to prevent the seed from settling too deep in the loose soil slit made by the subsoiler shank. Several types of slit-closing devices are available such as spider-gangs or rolling cultivators, fluted coulters, concave disks, and press wheels. These closing devices require that their position, angle, and spring tension be adjusted based on subsoiler depth and the soil and residue conditions. The benefits of deep tillage may be reduced if subsoiling is conducted when soil conditions are too dry (creates a cloddier seed zone) or too wet (compacts soil adjacent to subsoiler slit). If needed, disking operations should be conducted before subsoiling to prevent recompaction. When in-row subsoiling is done correctly, it should substitute for deep plowing and part of the secondary tillage.
Conservation tillage refers to soil preparation that leaves a percentage of the soil surface covered by some form of plant residue after a crop is established. The minimum portion of surface coverage to qualify as conservation tillage is 30 percent after planting. This residue coverage on the soil surface reduces the risk of erosion and protects water quality from degradation caused by runoff carrying sediment and possibly traces of fertilizer, pesticides, or both. Tillage systems affect soil properties such as temperature, moisture, bulk density, aggregation, organic matter content, and plant properties such as root density.
Spring soil temperatures are usually lower in conservation tillage systems than other tillage systems. Conservation tillage systems increase the amount of water stored in the soil profile by reducing the evaporation and help the plants survive short periods of drought. Long-term conservation tillage practices also increase organic matter levels in the soil. Lower soil temperatures and increased soil moisture contribute to slower rates of organic matter oxidation. In conservation tillage systems, reduced tillage leads to nutrient distributions that differ from those in other tillage systems. Less mixing of the soil in reduced tillage systems leads to distribution of phosphorus and potassium near the soil surface. Also capturing more of the rainfall through a good use of conservation tillage can often help to improve corn yields and profitability.
Good soil preparation is important for uniform placement of seeds in the soil to achieve high corn yields and quality. It should protect the soil from water and wind erosion, provide a good seedbed for planting, break the hardpans or compacted layers, and allow an increase of organic matter.
Cultivate the soil so that the soil is just fine enough to give good contact between the seed and soil. Good seed to soil contact is necessary so the seed can absorb moisture from the soil particles. Overworking the soil does not help the corn seed to germinate or emerge, but it will increase production costs, the risk of soil crusting, and soil erosion during periods of heavy precipitation.
The seeding depth should be adjusted according to soil conditions. Avoid planting corn into cold (soil temperature below 50°F at the top two inches) and wet soils if at all possible. Corn should be generally planted between 1-1/2 to 2 inches deep. If planted early, the seed can be planted at 1-1/2 inches deep due to generally higher soil moisture and less evaporation. Later planting in warm soils requires deeper seed placement of up to 2 inches. It is important to establish a good seed-soil contact while planting by using seed-press wheels.
The risk associated with more shallow than 1-1/2 inch depth is the possibility of poor development of the secondary (permanent) root system. Check the seeding depth periodically to ensure proper depth across the field. Irregular depth of planting may result in uneven plant emergence and reduced yields. Germination time will depend on moisture and temperature. Temperature in the top 2 inches is influenced by air temperature. Cooler soil temperatures will extend the time from corn planting to emergence. Below 50°F little, if any, germination can be expected. With deeper planting, seeds are placed in cooler soil, which lead to slower emergence. Planting depths greater than 2 inches result in seedlings with less vigor, slower growth and development, and less yield.
Planting corn seeds too deep may result in the coleoptile growth terminating below the soil surface. As the shoot grows through the coleoptile tip, it will continue to grow unprotected towards the surface. In heavy soils, crusted or compacted soils, an unprotected shoot will be torn apart before it can emerge.
Regarding the organic soils where drainage is a problem, the recommended planting depth should be 1 inch during the early part of the planting season. Deeper planting on organic soils often results in poor germination and emergence.