The Early Soybean Production System (ESPS) with its use of indeterminate, non-branching, early-maturing varieties is by necessity a narrow-row system of production. Just how narrow rows should be depends on several factors. An important one is soil texture.
With adequate rainfall and/or irrigation to replenish depleted soil water, loamy soils (silty clay loam, silt loam, and fine sandy loam) promote rapid soybean growth and development because of relatively easy soil water availability. Conversely, clayey soils (generally greater than 50 percent clay particles) with adequate rainfall and/or irrigation to replenish depleted soil water will promote relatively slower growth because of their slow release of water.
Thus, a variety planted on a particular early date on most loamy soils will produce a larger plant than will that same variety planted on the same date on most clayey soils. Therefore, ESPS plantings grown in 30-inch-wide rows on loamy soils will probably form a complete canopy, while the smaller-statured plants that will be produced on clayey soils will require a narrower row, say 20 inches or less, to achieve complete canopy closure. (For soil texture classification, see ftp://ftp-fc.sc.egov.usda.gov/NSSC/Field_Book/FieldBookVer2.pdf p. 68).
Regardless of soil texture, risks associated with early planting of soybeans in the Mid-South will be decreased when beds are used. A bed planting system will reduce potential stresses associated with early-season surface water accumulation. Beds will also facilitate surface irrigation during the growing season. These same attributes will also benefit corn and grain sorghum grown in rotation with soybeans.
Experience and observation have shown that beds less than 30 inches wide are not practical. They do not keep their integrity during the months between construction and planting, they do not provide enough furrow and bed height to facilitate drainage and irrigation, and they are not easily planted.
Conversely, beds that are 36 to 40 inches wide can be constructed effectively. They will most likely maintain themselves during the off-season, especially if rolled flat following formation to minimize bed degradation during high-rainfall winter months.
A twin-row planting system is being used with increasing frequency. This system allows planting on beds but at row spacings that are less than the bed width. For example, twin rows spaced 8 inches apart on a 38-inch-wide bed give the same row spacing between rows on adjacent beds as a single row planted on a 30-inch-wide bed. This row spacing is preferred for corn and grain sorghum, and is adequate for soybeans planted on loamy soils.
A 30-inch row spacing may be too wide for ESPS plantings made on clayey soils because of the growth differences between loamy and clayey soils described above. The problem, then, is how to construct and maintain a bedded planting system on clayey soils that require narrow row spacings to ensure canopy closure.
A wide-bed planting system may be the answer. These beds can be constructed in the fall soon after harvest using an implement that combines sweeps to dig furrows with wings to distribute the resulting loose soil over a wide area, say 80 inches. The resulting wide bed will accommodate various row spacings, and will be suitable for different crops that may have varying row spacing requirements. (See http://msucares.com/pubs/bulletins/b1072.htm for description of prototype bedder and view of beds).
This is probably the most manageable bed system for clayey soils. This system will also facilitate furrow and flood irrigation on clayey soils that crack and allow lateral movement of irrigation water between furrows.
Soybeans grown in ESPS plantings will be harvested from late July through early September. Since this is normally the driest time of year in the Mid-South, there should be no ruts left by the combine. Thus, a bed system can remain in place from year to year. Beds can be refurbished without destructive primary tillage in the fall when necessary. Consequently, a production system using beds for planting should result in minimum tillage, less soil loss from erosion, and a lower cost of production.
Larry G. Heatherly is a retired USDA-ARS Research Agronomist and current crop consultant. e-mail firstname.lastname@example.org