This year we will see a dramatic increase in soybean acreage in Harrison County. There is no set of planting practices suited for all situations. Each location, year and set of growing conditions will alter planting recommendations. Planting date, planting depth inoculation, seeding rate and row width must be adjusted for specific conditions and taken into consideration for other production practices.
Planting on time is necessary to obtain enough plant growth and development for good yields. If planting is delayed beyond the optimum date, yields are reduced.Likewise, planting too early can reduce yields because of poor stands caused by cool soil temperatures or because day lengths are too short, causing plants to flower early and have reduced vegetative growth. Generally, however, planting soybeans too early will not be a problem in Kentucky because low soil temperatures will prevent early plantings.
The main factor governing initial planting dates in Kentucky is soil temperature. Although soybean germination can occur at 55 degrees F, it will be slow and emergence will require almost three weeks, too long for the seed and seedling to remain in the soil. It is best to plant soybeans after the soil temperature has reached a minimum of 65 degrees F or above, which will permit rapid germination and emergence within seven days. Soil temperatures of 65 degrees F at planting depth are normally reached by May 1 in the western half of the state and by May 10-15 as you proceed to the eastern and more northern portions of the state. The optimum planting period in Kentucky occurs from early May to mid-June (June 10-15).
Planting depth can greatly influence soybeans’ ability to emerge and establish a uniform stand. Ideal planting depth depends on soil texture and moisture. Plant the seed deep enough to be in moist soil but no deeper than necessary. Planting soybeans too deep is a frequent cause of poor stands. The ideal planting depth for best emergence is one to one and one half inches under most conditions. Avoid planting deeper than two inches if possible because at deeper levels emergence is delayed, seedling vigor is reduced, and it is harder for a soybean seedling to break through a crust that may have formed.
Varieties also differ in their ability to emerge from greater depths. As a result, although all soybeans emerge slowly when planted deep, the ability to emerge successfully differs among varieties. Soil type will also influence soybeans’ ability to emerge from deep plantings. Heavy rains can cause hard crusts to form on fine-textured, heavy soils. When this happens, it is harder for deep planted soybeans to break the crust. In such cases, the seedlings may completely deplete the food stored in their cotyledons before they emerge and eventually die. The most severe damage resulting from a soil crust is the breaking of the hypocotyl arch during emergence, resulting in a reduced stand. When crusts develop, rotary hoeing may help emergence. Various conditions can dictate slightly different planting depth recommendations.
The soybean plant is a legume and capable of supplying nitrogen for its growth through a symbiotic nitrogen fixation process with certain bacteria (rhizobia) in the soil. These bacteria form nodules on soybean roots and extract nitrogen from the air for the plant to use. If soybean plants are well-nodulated with effective nitrogen-fixing bacteria, this condition will supply the soybean’s nitrogen requirement and no additional nitrogen as a fertilizer will be needed. Thus, it is very important that the soybean plant have effective nodulation for good nitrogen fixation. The safest way to ensure good nodulation is to inoculate the soybean seed with the proper bacteria. Only Brady rhizobium japonicum has been shown to effectively nodulate and fix nitrogen with soybeans.
Many different kinds and brands of inoculant are available to the grower. The oldest type and most commonly used is the peat-based material which is applied to the seed before planting. In recent years, oil-based inoculants have become commercially available as well as granular-type inoculants, which are soil applied in the row at planting. Choice of inoculant should be based on cost, ease of application and planting equipment.
There are several methods of applying the inoculant to the seed. Thorough mixing of the rhizobia inoculant with the seeds in the manner recommended by the manufacturer is important. Methods include slightly wetting the seeds, applying the inoculant in a dry form on the seeds, or use of a sticking agent. Remember that inoculants contain live organisms and should be handled carefully to retain their viability. Keep them in a cool, dry place until use. Over-exposure to sunlight, excessive heat or drying will impair or destroy the bacteria’s effectiveness, so inoculate the seed just before planting to retain viability. Rhizobia’s viability on unplanted seeds is only a few hours. Further, since the bacteria do not survive indefinitely in the inoculant package, use fresh supplies. Most companies put an expiration date on the label.
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