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National Soybean Research Laboratory
Soybean Production Basics

When is the best time to plant soybeans?
Typically, beans planted during early May have the best yield potential. However, yield depends on several other factors, too. Growing conditions at planting time will influence the success of seed germination and seedling vigor. Just because the calendar says it's time to plant doesn't guarantee that it's the optimum time to plant soybeans. Soybeans need a minimum soil temperature of 55 to 60o Fahrenheit to germinate. Germination rates increase at warmer temperatures. A seed that's in the soil but cannot rapidly germinate and emerge above the soil surface will have a higher chance of exposure to diseases and damping off.

What is the ideal soil for growing soybeans?
Soybean is a hardy plant and well adapted to a variety of soils and soil conditions. Producing the best quality crop and maximum yields will require top quality soil. Thus, soil is one of the first things to consider when planting a crop. A healthy, fertile, workable soil will actually provide seedlings and growing plants with protection from adverse weather including cold, frost, drought, excess water, and protection from pests and diseases.

Ideal soil for optimum soybean production is a loose, well-drained loam. Many fields have tight, high clay soil that becomes waterlogged when it rains. When the soil dries out, a hard crust surface may form which is a barrier to emerging seedlings. These high clay soils are low in humus and may have imbalance in mineral nutrients. Also, these soils may have few beneficial soil organisms (bacteria, fungi, algae, protozoa, earthworms and others). High clay soils may be amended with peat moss, sphagnum, organic mulch to increase the humus content. Sand may be added to loosen and aerate the soil and allow better drainage.

The advantages of loose, well-aerated soil include (1) movement of air to roots and nitrogen-fixing root nodules, (2) increased water-holding capacity with adequate drainage, (3) reduced erosion, (4) reduced weed populations, (5) maintenance of steady and balanced nutrients to roots and balance pH, and (6) increased potential to protect roots from harmful nematodes, insects pests, and pathogens.

How should the seedbed be prepared?
The ideal seedbed for soybeans should provide moisture and the appropriate temperature warmth for rapid germination and seedling emergence. Soil should remain friable without crusting over when dry. Germination of weed seeds should be delayed or prevented.
Soil moisture (about 50% of the soybean weight) must be sufficient to allow uniform and steady germination rates. If soybeans germinate and grow rapidly, weeds can be shaded out. To reduce or discourage weeds, allow the space between the planted rows to remain covered by residue. Another approach is to prepare the seedbed early, let the weeds germinate, then work the seedbed to kill sprouted weeds before planting.

If soybeans have not been grown in a particular location for three to five years, it is best to inoculate the seed with the proper strain of nitrogen-fixing bacteria (Rhizobium). Some strains are more effective nitrogen fixers than others. Both seed and soil inoculum are available.

What happens during germination and emergence?
Germination is time after the seed is planted in the soil and before the young seedling emerges above the soil surface. After being planted in the soil, the seed absorbs moisture (called imbibition), changing from less than 13% moisture to about 50% in several hours. After one or two days the first root (called the radical) emerges through the seed coat and begins growing downward to establish the root system.

About five to ten days after planting, the new seedling arches through the soil surface (this is called emergence). The hypocotyl ‘hook’ (the emerging portion just below the cotyledons) begins to lengthen pulling the remainder of the seed upward, and the oval seed leaves (called cotyledons) open up. The cotyledons provide the seedling with a temporary source of food (plant useable nutrients originally stored as the seed was formed). The cotyledons quickly turn green and begin making additional food by photosynthesis. Shortly after the first set of true photosynthetic leaves is formed, the cotyledons drop off.

Seed germination and emergence is a critical period in the life of a soybean because poor emergence due to low temperatures, a soil crust, or seed planted too deeply allows seedling pests or diseases to drastically reduce yield.

What are the optimum planting depth, row width, and plant density for soybeans?
Planting depth. Seeds should be planted deep enough to meet the moisture and temperature requirements for germination. Planting depth may be determined by variety, and some varieties can emerge from greater depths than others (usually the larger seeded varieties). Typical planting depths are 1-1.5 inches, but if soil is low in moisture or sandy, plant 2 inches deep. In cool, moist soil, seed can be planted 1 inch deep. Seldom should soybeans be planted deeper than 2.5 inches.

Plant density/population. Plant population varies depending on row spacing and environmental factors. A final plant population may range from 70,000 to 180,000. Typically, 150,000 is a good target for wide rows and 175,000 for narrow rows. Planting an excessive population may result in increased lodging, but an inadequate or uninformed stand may lead to higher weed populations. At lower populations, plants branch more and lodge less, while at high populations the opposite is true. Pods form higher on the plant in high populations. Weeds are more of a problem in low populations. Populations should be adjusted to reduce lodging and keep pods high on the plant. Populations can be increased when growing determinate, semi-dwarf and non-branching varieties. Additionally, the local soil type, environment, and seed quality can influence plant density.

Row width. An important goal is stand uniformity. In general, if weeds are controlled, soybeans will yield more in narrow rows than in 30 inch rows. Benefits from narrowing the row width will depend on location, soil conditions, weather conditions, planting date, and variety. In northern and central regions of the U.S., soybeans grown in narrow rows yield more than those grown in corn-width rows. In southern areas, there is a similar trend toward narrower rows and higher yield if good weed control is achieved. The ‘rule of thumb’ is that the soybean canopy should completely close (cover and shade the space between rows) by flowering time. The faster the soybean canopy closes, the fewer the number of weeds will grow. In narrow rows, weeds can not be cultivated easily.

Should fungicide treated seed be used?
Seed can be treated with fungicide, but this is not necessary. If the soil temperatures are warm and if the germination rate is over 85%, there is little advantage in using fungicide-treated seed. Lower germination seed may have a 5 to 10% increase in emergence if treated.

What is the growth rate of soybeans?
Soybeans are slower growing than most garden beans, requiring warmer weather and about 3 to 5 months for maturity. In cooler growing regions, the rate of development will be slower. Only the early maturing varieties should be grown in the northern growing regions, and the medium or late maturing varieties grown in the south regions. Planting soybeans can be done about the same time as tomatoes and other long-season, warm-weather crops are put in the garden.

What is the vegetative growth stage?
After the seedling has emerged from the soil the young stem and first leaves begin to rapidly grow upward. The seedling is very tough and frost resistant, but if the terminal bud (growing tip) of the stem is killed, side branches will begin to grow. The first six to eight weeds after emergence are called the vegetative period.

The first two leaves that develop are called unifoliolates, one simple leaf or blade supported by a petiole. The remaining leaves are compound leaves composed of three leaflets and are called trifoliolates. The cotyledons, unifoliolates, and trifoliolates are attached to the main stem at regions called nodes. Later, flowers will develop at the nodes between the petiole and stem, and branches also grow out from node regions. Newly formed upper leaves will shade older, lower leaves which may turn yellow and fall off.

How does the root system develop?
While the stem and leaves grow upward, the root system continues to grow deeper into the soil. Initially, the plant produces a main taproot, but soon after emergence numerous lateral roots branch off to produce a fibrous root system. The deepest roots may reach down five feet or more in loose well drained soil but most of the roots are found in the upper one foot of soil.

The young roots will develop root nodules within a week after emergence. The nitrogen-fixing bacteria, called Rhizobium, enter the nodules and after ten to fourteen days are able to supply most of the plant's nitrogen needs. In favorable soil conditions, about two dozen nodules will develop on the upper roots of a plant. Healthy nodules are pink or reddish inside.

What happens during the flowering stage?
Typically after six to ten trifoliate leaves have been produced, the soybean plant begins the reproductive period. The flowers are self-pollinated; that is, the flower fertilizes itself, and insects are not required to carry pollen from one flower to another. From 3 to 15 flower buds develop at each node of the stem.

Regarding flower development, soybean plants can be grouped by two main types: determinate and indeterminate. Indeterminate plants continue growing upward at the tip of the stem for several weeks after flowering begins lower on the stem. Upper nodes will not flower until later. Most commercial varieties are indeterminate. They typically grow taller and do well in short growing seasons. Determinate plants complete their growth in height and then produce all the flowers at about the same time. They are usually one-half to two-thirds as tall as indeterminate varieties.

The flowers of soybean are very small (1/4 inch) and are white, pink, or purple. They resemble the flowers of pea or clover, since the soybean is also in the legume plant family. Only about 50 to 80% of the total flowers actually produce pods.

What happens during pod development?
One or two weeks after the first flowers are produced, the first seed pods appear. Most of the pods are set within the following three weeks. Three to four seeds are produced per pod. For the next 30 to 40 days, the plant will store ‘food’ produced by the leaves in the seeds. The seed-filling period is very critical to yield. If environmental conditions are adverse (drought, hail, or disease), seed-fill will be restricted, and yields will be cut severely.


What happens to the seeds as they mature?
The newly formed seeds contain about 90% moisture. As the seeds fill with food, moisture content decreases to about 60 to 65%. When seeds are mature (filled), the moisture content is 45 to 55% and the pods and stems of the plant are yellow or brown. The mature seed itself will also be completely yellow when mature. Some soybean varieties vary in color and may include black, purple, brown, tan, or mottled coloration. As soybean seeds lose moisture they change from large, kidney bean shaped to a smaller and nearly round shape. When dry, the seed contains about 38% protein, 18% oil, 15% soluble carbohydrates, 15% insoluble carbohydrates, and 14% moisture/ash/other.

When should soybean be harvested?
For use as a green vegetable (called edamame), soybean pods should be harvested when the seeds are fully grown but before the pods turn yellow. Most varieties produce beans in usable condition over a period of a week to 10 days. The green beans are difficult to remove from the pods unless the pods are boiled or steamed 4 to 5 minutes, after which they are easily shelled.

What soybean varieties are available?
Many soybean varieties are available. A collection of over 10,000 accessions of soybean seeds is maintained by the USDA. This USDA collection represents the diversity of soybean germplasm including seeds of every color and description including red, green, black, brown, speckled, streaked, large, and small.

Most of soybean varieties grown commercially today are yellow-seeded field varieties used for animal feed and oil production (for food processing and industrial uses). Other varieties can be obtained for special uses: forage and hay (with an abundance of stems and leaves) and human food (large-seeded, various colored varieties).

Are soybean hybrids available?
Because soybean is self-pollinating, commercial hybrid soybean seed is very difficult to produce. Hybrids are produced by soybean seed breeders, but it is a labor intensive and expensive endeavor.

How important is the soybean ‘MATURITY GROUP’?
Plan to plant seed that is appropriate for the length of your growing season. Soybean varieties are grouped into 13 maturity groups, depending on the climate and latitude for which they are adapted. These maturity groups are given numbers, with numbers 000, 00, 0 and 1 being adapted to Canada and the northern United States, and numbers VII, VIII and IX being grown in the southern U.S. (Group X is tropical.)


maturity group map
The 10 maturity groups correspond to horizontal bands across the United States. The soybean varieties that are best adapted to Illinois conditions are from groups II through V. Credit: StratSoy


What nutrients do soybeans need?
Healthy plants need various amounts of nutrients from the soil. Some nutrients are required in large amounts (macronutrients) and some in small amounts (micronutrients). Most soils either have deficiencies or imbalances in the amounts of nutrients available to the plants. Here is a brief summary of the soil nutrients:

Nitrogen (abbreviated N) is a macronutrient and needed by the plant for certain enzyme functions, to make proteins, and as a necessary part of chlorophyll, nucleic acids, vitamins and several other substances. Soybeans can obtain all the nitrogen they need from root nodule nitrogen-fixing bacteria. Soybean is a legume and which normally provides itself with adequate nitrogen through a symbiotic relationship with N-fixing bacteria of the species Bradyrhizobium japonicum. In this symbiotic relationship, carbohydrates and minerals are supplied to the bacteria by the plant, and the bacteria transform nitrogen gas from the atmosphere into ammonium-N for use by the plant. In fact, in tests where fertilizer nitrogen was added to soil, no yield increase occurred, plus the root nodules fixed less nitrogen.

Phosphorus (abbreviated P) is a macronutrient and is needed for general growth and metabolism and for photosynthesis. It carries energy from one part of a cell to another and helps transport food from one part of the plant to another. It also makes up part of cell membranes, nucleic acids and other components. It is necessary for growing really high quality crops. Young seedlings especially need phosphorus. The most efficient and economical way to get phosphorus to crop plants is to maintain soil with adequate levels of humus/organic material and beneficial soil microbes which decompose organic matter to release phosphorus and nutrients to plants.

Potassium (abbreviated K) is a macronutrient and is needed for the plant's enzyme functions, food transport, protein and chlorophyll production, and in regulating water balance, potassium is needed by soybeans in fairly large amounts. Most soils contain large amounts of potassium which are tied up and not available to plants. Soil microbes function to release potassium and other nutrients to plants.

If the soil is very low in potassium, a suggestion for an overall fertilizer source is potassium sulfate (0-0-50). Avoid using fertilizer formulations with chloride because the chloride ion can injure soil microbes as well as soybeans themselves if present in high amounts. Potassium sulfate is more expensive than potassium chloride, but only about one-half as much is needed, and the extra sulfur is usually beneficial.

Calcium (abbreviated Ca) is a macronutrient and is very important for growing high quality soybeans. Calcium is critically important for cell division, root hair growth, enzyme functions, and production of normal cell walls. Calcium improves plant's resistance to disease and gives higher quality, more nutritious crops.

In the soil, calcium and magnesium "compete" for plant absorption. Too much magnesium disrupts the plant's uptake of calcium and potassium, causing low quality crops. Additionally, excess magnesium causes soil to develop hard, crusty conditions. Most soils should have adequate magnesium. In general, soils in the western two-thirds of the U.S. have adequate calcium, while those in the eastern one-third may be deficient.

The best source of calcium is high-calcium lime (calcium carbonate) which has low magnesium and dissolves quickly in water. In alkaline soil, gypsum (calcium sulfate) is the best source of calcium.

Sulfur (abbreviated S) is a macronutrient and is needed to build proteins and assist enzyme functions. Many soils have adequate sulfur because of air pollution from burning high-sulfur coal, but some soils are deficient.

If sulfur is needed for healthy soil, the most readily available source is sulfate-containing fertilizers (calcium sulfate, potassium sulfate). Elemental sulfur (flowers of sulfur) is slow to release and become available.

Micronutrients are required by plants in small amounts and include iron (Fe), zinc (Zn), copper (Cu), boron (B), manganese (Mn), molybdenum (Mo), cobalt (Co) and chlorine (Cl). Molybdenum is needed by nitrogen fixing bacteria. In soybeans, the most frequent micronutrient deficiencies are for iron, zinc, manganese and molybdenum. But such deficiencies usually occur in poor, weathered or sandy soils, or in soils that are very alkaline or excessively high in organic matter (mucks and peats). A loamy soil with adequate humus and soil life should not have micronutrient deficiencies. If a micronutrient is deficient in your soil, only that element should be added. Too much of some micronutrients will be toxic.

Nutrient Balance and pH. For healthy crops and high quality yields, nutrients must be available to the plants in the proper amounts and in the right balance. The soil pH (acidity or alkalinity) affects the availability of soil nutrients to plants. The pH scale is expressed as a numerical scale ranging from 0 (most acid) to 14 (most alkaline), with a 7 being neutral. Soybeans grow best in slightly acid soil but can tolerate a wide range of pH (pH 5.8 to 7.0). Soil pH also affects the types and ability of soil organisms to live, including nitrogen-fixing bacteria. Humus in soil will buffer extremes in pH, and lime can be added to amend soil and counteract acid soil.