Agronomics

Soybean Cultural Practices

Overview
Soybeans are one of the most flexible crops grown. They are easily adapted to a number of different production systems and cultural practices. The type of tillage, plant population, row spacing, and planting date are four major management decisions that soybean farmers must consider. Other considerations include planting depth, double-cropping, and use of inoculants and seed treatments.

Because U.S. soybeans are grown over such a wide geographical area with a wide variety of local climates and soil types, how individual producers choose to handle each management application will depend on their own farming circumstances. State Extension personnel will be able to guide you to local research and specific recommendations for your area.

Tillage
Modern tillage practices fall on a continuum.

Clean till

  • Plowing, disking or use of other implements to completely bury all previous crop residues.
  • This method is best suited to fields that are not at risk from water or wind erosion.

Minimum / reduced-till

  • Some tillage occurs to place residue in better contact with the soil surface (but not buried).
  • Better residue-soil contact enhances microbial breakdown of the residue and reduces erosion risks.
  • Various types of implements may be used.

No-till

  • Other than possibly mowing the residue of the previous crop, the new crop is planted directly into a stale seed bed with no tillage prior to planting.
  • Planters with the ability to cut through old crop residues and firmly reseal the seed furrow are necessary for good stands.
  • No-till seed beds are generally cooler and wetter, which may slow seed germination. Seed treatments typically are a good investment when planting no-till.
  • No-till provides maximum protection against both water and wind erosion.

The practice of no-till is currently on the rise among farmers in many soybean growing regions as producers look to reduce tillage input costs and protect the soil from erosion. Uniform residue distribution, effective weed control, proper seed placement, correct planter adjustment, soil testing, and fertilizer management are important for success in conservation-tillage systems.

Crop rotation

  • Continuous cropping of soybeans over a several year period is highly discouraged because of the threat of soybean cyst nematode and other soybean diseases including Phytophthora root rot, sudden death syndrome and frogeye leaf spot. Some insect problems such as soybean gall midge can be worse in continuous cropping as well.
  • Crops in a rotation with soybeans usually receive a benefit compared with continuous monocrop systems. Additionally, yields are greater when soybeans are grown in a rotation vs. continuous monocrop (5 to 10 bushels per acre) in a no-till system.
  • Crop rotations also promote lower pest pressure. Where soybeans are rotated with a grass crop including corn, wheat or sorghum, use of different herbicide modes of action can delay the development of herbicide resistant weeds. Residue-borne foliar diseases (e.g. frogeye leaf spot and Septoria brown spot) are also reduced where rotation is practiced.

Planting Practices

  • Planting date
    Soybeans can be planted over a wide range of dates under good soil moisture availability. Data from many universities shows that earlier planted soybeans tend to have higher yields than soybeans planted later in the growing season. Producers seeding early need to check that the seedbed is in the right conditions (temperature, moisture, nutrients) since planting into a sub-optimal seedbed will lose yield instead of gaining it. Other special considerations include soil pathogens, insect pressure, and the possibility of frost. Fungicide, insecticide, and nematicide seed or in-furrow treatments are options used to help reduce the risk of soil pathogens and insect pressure. Knowing the chance of frost, especially in northern growing regions can help determine how early seed can safely be sown. The distribution and amount of rainfall during pod formation and grain-filling have a large influence in defining yield potential. Under high risk of drought and heat stresses, diversifying planting dates may be a good approach.
  • Row width
    The row width that producers may choose is highly variable. Producers using grain drills may plant rows with a spacing of no more than 7 inches. Other common row spacings include 15 inch, 30 inch and in the south, 40 inch row spacings may still be used. Research has shown that where yield potentials are high, narrow rows result in equal or greater yields compared to 30-inch rows (regardless of planting date, seeding rate, or maturity).

Benefits of narrow rows include early canopy cover, light capture, weed control, and reduced erosion. Poor stands can occur more frequently with narrow row spacing than with wider row spacing when a similar seeding rate is used. Proper planting speed, planter unit down pressure, and adequate press wheel pressure may help reduce or eliminate narrower row stand issues.

Photo Credit: United Soybean Board
  • Seeding rate / plant population
    Plant population is the number of seeds planted in a given area. Population is usually expressed as plants or seeds per acre. Plant population along with row width are the two major factors that determine canopy closure (when the plants cover the space in between the rows) and other yield components. A higher seed population will close the canopy faster and reduce soil moisture loss. A high plant population does not necessarily equal a high yield. For many areas of the soybean belt, the recommended seeding rate is 125,000 to 140,000 seeds per acre, but it is good idea to check with state Extension agronomists for local recommendations. The goal is to achieve a final stand of 100,000 plants per acre. Planting extra seed gives the producer added insurance that each acre will attain a final stand of 100,000 plants, but it also can increase seed costs.

    The number of seeds or plants per linear foot of row decreases as the row spacing becomes narrower and increases with the plant density. Late planting dates may require increasing plant density to compensate for the reduction in the length of the growing season by compensating for fewer nodes per individual plant. Low yielding environments might require an increase of optimum plant densities. For higher yielding environments, plant density changes might not be required.
  • Planting depth
    Optimum planting depth under good seed bed conditions is from 1 to 1 ½   inches. Late planting under dry conditions requires deeper seed placement to place the seed in contact with moisture. For early planting, place seed slightly shallower to shorten the time until emergence (related to soil temperature). Soil crusting causes poor stands. It is exacerbated by deeper seed placement (deeper than 2 inches) and may result in greater disease pressure from soilborne pathogens.

Seed treatment
A good fungicide seed treatment package is recommended when planting into cold (cooler than 58°F), damp seedbeds. Seed treatment research at various universities has shown inconsistent benefits from their use. However, stand and yield benefits are more consistent when used on early planted soybeans when soil conditions may not be optimal, or in no-till production systems where cooler, wetter soils may slow germination. Consult Fungicide Efficacy for Control of Soybean Seedling Diseases for the latest product evaluations.

  • Inoculation
    Planting soybeans without inoculation in fields out of soybeans for more than two to three years or in Conservation Reserve Program (CRP) land can result in poor nodulation and nitrogen deficiency. From the total nitrogen demand of the crop, the nitrogen-fixation process provides on average 50 to 60% of this amount. For further details check: Biological Nitrogen Fixation and Soybean Productivity in the Midwest.

Double cropping
In southern production regions, double-cropping soybeans, usually after wheat harvest, can achieve high yields with a similar maturing variety relative to a full-season crop. Double cropping often benefits from narrow rows and increasing seeding rates under adequate moisture.

Calculation of plant density
Soybean seed size varies among and within the same variety grown under diverse climatic conditions. Thus, planting rates need to be calculated as the total number of seeds per acre.

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Source: Information on soybean cultural practices is provided by Douglas J. Jardine, Professor Emeritus, Kansas State University 7/2020
Images provided by Crop Protection Network, IPM images and USB Photo library and Joseph L. Murphy, Iowa Soybean Association