Planting considerations
Pawel Wiatrak, Jim Dunphy, and Jason Norsworthy
Producing soybeans offers the farmer much flexibility in management and risk control options. For example, soybeans fit into a number of rotation systems with benefits to all involved crops. Also, soybeans can be planted from late spring, as a full-season crop, through mid-summer, or double-cropped after harvest of a winter annual crop such as wheat, rye, barley, oats, triticale, or canola. The following are suggestions for successfully planting soybeans to achieve stands for top yields.
Field selection
Soybeans can be produced successfully on a wide range of soils. However, soils that are inherently fertile, well-drained, and possess good water-holding capacity will produce the highest yields. Based on the current cost of producing soybeans (see chapter one Production and Management Risks), fields should be selected with soils that have a productive potential (under dryland conditions) of at least 40 bushels per acre. Information to assist with field selection is presented in the County Soil Survey, available from local USDA-NRCS offices. This publication contains aerial maps which show the soil types in each field in the county. Reference can then be made to detailed descriptions of the soils, including their productive potential for various adapted crops. 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 of the subsoil within fields.
Rotation
Rotating soybeans with other crops like corn, wheat, cotton, or tobacco is an important practice for managing pests, using soil-applied nutrients efficiently, and sustaining soil productivity. Rotation can result in a reduction in production costs due to less chemical inputs for controlling pests like weeds, diseases, or nematodes. Soybeans can also effectively utilize residual nutrients left in the soil. In addition to these positive benefits, rotations usually improve yields due in part to the improvement of various soil biotic and physical characteristics.
Tillage
The sandy light-textured soils of South Carolina's Coastal Plain are low in fertility and waterholding capacity, and tend to form natural compacted zones or hardpans that are restrictive to root growth and development. Two types of hardpans are evident in most of these soils: a tillage pan 6- to 8-inches deep, caused by excessive tillage; and a lighter-colored sandy layer about 8- to 15-inches deep and 1- to 4-inches thick (the E horizon). The E horizon is more compacted than either the topsoil or subsoil (B horizon) and is difficult, if not impossible, for roots to penetrate. On Coastal Plain soils, deep tillage implements (in-row or broadcast subsoilers) are often needed to break compaction zones enabling roots to grow into the subsoil to access residual soil moisture and leached nutrients (including nitrogen, sulfur, and potassium).
South Carolina research has shown that in-row subsoiling in wide rows (30 to 38 inches) ahead of planting can increase soybean yields in the Coastal Plain by 10 to 15 percent with even greater response during dry years. It is critical that these tillage practices are conducted during periods in which soils are not excessively moist. Deep tilling “wet” soils will fail to destroy the restrictive layer and will create compacted areas along the sidewalls of the tillage shanks (see Figure 1). NOTE: For more information on tillage, please see the Doublecropping Wheat and Soybeans with Conservation Tillage chapter.
Figure 1. Subsurface view of tillage under “wet” and optimum moisture conditions. Note the side wall compaction that occurs when soil is too wet. 
Tillage depth
For achieving maximum economic returns from deep tillage, it is important to know the average depth to the subsoil and the depth and thickness of the restrictive layer. Research in the 1980’s at the Clemson University Edisto Research and Education Center on three different soils showed that row subsoiling should be done 1 to 2 inches into the subsoil, so that roots can optimally access moisture and nutrients (see Table 1). Note: There is likely no benefit of tillage at depths greater than 2 inches below the upper surface of the subsoil.
Table 1. Soybean yields as influenced by depth to subsoil and depth of in-row subsoiling at the Edisto REC in Blackville, SC, 1984.*
In-row subsoiling depth (inches) |
Depth to subsoil (inches) |
||
8 |
12 |
16 |
|
|
Bu/acre |
||
No subsoiling (disk only) |
29 |
28 |
21 |
12 |
33 |
39 |
33 |
14 |
35 |
41 |
39 |
18 |
36 |
42 |
42 |
*Research by Tom Garner and Harold Musen.
Seedbed preparation
Conventional seedbed preparation increases the costs of producing soybean and has several associated risks. While seedbed preparation using a disk harrow is practiced by many farmers, it is a practice that has many costs. Disking soil prior to planting stimulates weed emergence, compacts the soil at the 6- to 8-inch depth, causes rapid moisture loss from the soil surface, and increases wind and water erosion of the upper most layer of soil.
Seedbeds are most often prepared using conventional tillage when:
- Lime and/or fertilizer is to be incorporated into the plow layer
- Residual herbicide(s) must be incorporated
- Dense layer of weedy and/or crop residue exists on the soil surface
- A well aerated seedbed is desired for optimum seed germination and emergence.
Note: All but residual herbicides can be overcome.
Seedbed preparation for strip-tilled soybean involves a coulter and a narrow furrow opener (e.g., double disk). The coulter cuts through the surface residue (e.g., small grain residue if double-cropping) and the furrow opener prepares a narrow seedbed about an inch wide. Care must be taken to ensure the seed has good soil contact so that optimum germination/emergence can occur. Stand problems with conservation tillage can seriously cripple chances for good yields.
Soil moisture
Having enough soil moisture present at planting is extremely important for good soybean stands. The soybean seed must imbibe 50 percent of its weight in water before germination can begin. This is why soybean seed swell within an hour or so after planting if soil moisture is present. However, there can be enough moisture for "swelling" and not enough to complete the germination process, especially under extremely hot soil conditions. Seed in this situation are often attacked by bacteria or fungi, and rot in the soil. Seed planted in soils too hot or dry can present other problems. Farmers who "dust in" their soybean crop and wait for a rain often have to replant because the seed baked, much like being in an oven. If soil moisture is limited, or if soil temperatures 1 to 1½ inches deep are over 100 degrees, the farmer should not plant until soil conditions are optimum.
No-till planting
No-till planting is widely practiced for soybeans following wheat. Planting directly into wheat residue reduces soil temperatures, conserves soil moisture, and reduces soil erosion. However, getting a good stand can be challenging. During soybean planting, the seed furrow must be kept clean from the wheat straw. Otherwise, we may expect a reduced stand due to poor seed-soil contact. Wheat should be cut high to allow better herbicide penetration. The research conducted at Clemson University has shown that fertilization and deep tillage prior to planting wheat in the fall is usually sufficient for soybean planting in the summer.
Some other recommendations regarding planting soybeans in the no-till include increase of seeding rate by 10 to 15 percent and use planters with cutting coulters, double disk openers, and packer wheels to increase seed-soil connection and therefore soybean stand.