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Growing Great Soybeans

by Dr. Harold Willis

from Acres USA March 1989, pp. 1, 6-8

TABLE

ANATOMY OF GROWTH
LIGHT AND PODS
MATURITY GROUP
GETTING STARTED
NUTRIENTS
CROPPING

The soybean is a truly amazing and versatile crop plant. It is one of the oldest food plants, domesticated by 1 100 BC in northeastern China. Its ancestor is a wild vine-like plant which produces tiny, hard seeds that are useless for food unless properly prepared.

Over the next several hundred years the domesticated soybean (called Glycine max by botanists) spread throughout much of eastern Asia. It grew upright and yielded larger, more digestible seeds. A variety of foods was developed from the soybean, ranging from soybean sprouts to steamed raw beans to roasted seeds to soy milk to soy sauce to fermented soybean paste and cake to soy flour to the commonly eaten curd called tofu (or doufu).

Soybeans reached the western world by the early 1700s and were first grown in North America by 1804. Benjamin Franklin appears to have been involved in introducing soybeans from France to Philadelphia at that time. A number of varieties were grown and evaluated in the United States during the 1800s. The primary use for the crop was for forage, hay and green manure.

In the 1880s, French scientists discovered that the soybean contains practically no starch, so its use in diabetic diets began. Later its high protein content was recognized.

Modern uses. In the early 1900s the first processing of seeds t`or oil and meal was done in England. For the most part, soybeans were a neglected crop until World War II. Germany developed a soy oil lard substitute and a meat substitute. In the U.S. increasing from soybean sprouts to steamed raw beans to roasted seeds to soy milk to soy sauce to fermented soybean paste and cake to soy flour to the commonly eaten curd called tofu (or doufu).

Soybeans reached the western world by the early 1700s and were first grown in North America by 1804. Benjamin Franklin appears to have been involved in introducing soybeans from France to Philadelphia at that time. A number of varieties were grown and evaluated in the United States during the 1800s. The primary use for the crop was for forage, hay and green manure.

In the 1880s, French scientists discovered that the soybean contains practically no starch so its use in diabetic diets began. Later its high protein content was recognized.

Modern uses. In the early 1900s the first processing of seeds for oil and meal was done in England. For the most part soybeans were a neglected crop until World War II. Germany developed a soy oil lard substitute and a meat substitute. In the U.S. increasing amounts of soybean meal were used as livestock and poultry feed, especially after 1945 when consumption of meat increased dramatically. More recently, an increasing proportion of American soybean production has been used by the food processing industry--in such foods as margarine, shortening, ice cream, salad dressings and mayonnaise. Industry uses lesser amounts in products including paint, ink, putty, caulking, wallpaper, rubber substitutes, adhesives, fire extinguisher foam, electrical insulation and gasoline. The versatile soybean is a part of everyone's life in developed countries.

At present, most soybeans (over three-fourths of the world supply) are grown in the United States (especially in the cornbelt and Mississippi Valley), in Brazil and Argentina. China produces most of the soybeans grown in the Orient, while only a few are grown in Europe. In the U.S., the soybean is third in production (corn and wheat are first and second) and second in value (corn is first) of crops grown.

Growth and development. In order to best manage soybean production, one needs an understanding of how the plant grows and develops,

Germination. After being planted in the soil, the seed absorbs moisture, changing from less than 13% moisture to about 50% in several hours. After one or two days the first root (called the radicle) emerges through the seed coat and begins growing downward to establish the root system.

The upper part of the young plant (the hypocotyl) begins to lengthen, pulling the remainder of the seed upward. About five to fifteen days after planting, the new plant arches through the soil and the oval seed leaves (cotyledons) open up. The cotyledons provide the seedling with food (that u as stored in them) for about a week, plus they soon turn green and begin making a little additional food by photosynthesis. Later they drop off.

ANATOMY OF GROWTH

Seed germination and emergence is a critical period in the life of a soybean because poor emergence due to a soil crust cold temperatures or seedling pests or diseases can drastically cut yield.

Vegetative growth. 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. If the terminal bud (growing tip) of the stem is killed side buds will take over.

After emergence for the first six to eight weeks the soybean grows its stem (and possibly branches) and leaves. This is called the vegetative period.

The first two leaves that develop are called unifoliates meaning that the leaf has a single flat surface the blade similar to the leaves of elm or maple trees. The remaining leaves are three-bladed, or trifoliates. Here the total leaf has three divisions all attached to a single "leaf stalk," or petiole. The place u here a leaf petiole attaches to the stem is called a node. Later flowers will develop at the nodes between the petiole and stem, and branches also grow out from here.

After the first fen leaves develop. Overall growth of the plant increases rapidly. If plants are spaced far apart more side branches will grow outward to capture as much light as possible, producing a bushy-looking plant. Plants in dense stands tend to grow upward, with few or no branches. Some soybean varieties tend to branch more than others.

As new upper leaves begin to shade older, lower leaves the lower leaves may turn yellow and tall off. This is nothing to be concerned about since the plant is just getting rid of unproductive leaves.

Roots. While the stem and leaves are growing upward the root system is growing deeper into the soil. At first the plant grows a main taproot but soon side roots branch off and still others grow off from them. 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 start to develop root nodules within a week after emergence if the proper nitrogen-fixing bacteria are present in the soil. The nitrogen fixing nodule bacteria, technically called Rhizobium enter the nodules and after ten to fourteen days are able to supply most of the plant's nitrogen needs, if the nodules are healthy. In favorable soil conditions a couple dozen or so pea-sized nodules will develop on the upper roots of a plant. Healthy nodules will be pink or reddish inside.

Flowering. In typical soybean plants after six to ten trifoliate leaves have grown, the next main stage in the plant's life begins the reproductive period. From 3 to 15 flower buds develop at each node of the stem.

There are two main types of soybean depending on how flowering occurs. Varieties called indeterminate 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 best in short growing seasons.

A few varieties are called determinate and complete their growth in height first. then all flowers bloom at about the same time. They are usually one-half to two-thirds as tall as indeterminate varieties and so are often called "semi-dwarfs." There are also some intermediate varieties, called semi-determinate which grow taller during the first part of their flowering period.

The flowers of soybean are tiny (1/4 inch) and white, pink or purple. They resemble the flowers of pea or clover, since the soybean is in the same plant family, the legume family. Many more flowers are produced than eventually produce seed pods. The extras drop off, anywhere t`rom 50 to 80% of the total.

The flowers are self-pollinated; that is, the flower fertilizes itself, and insects are not required to carry pollen from one flower to another.

LIGHT AND PODS

The light factor. The beginning of the flowering period is hastened by higher temperatures and a greater amount of vegetative growth, but a major factor that controls flowering is photoperiod--the length of the day. Flowering of a certain variety begins sooner when the days are shorter and later when the days are longer (if the plants are grown where there is artificial light during the night, they may never flower).

Each variety is adapted to flower and complete its life cycle at a certain geographic latitude (distance from the equator). Normally, if planted in the spring, the plants will begin flowering in mid-summer, after the days begin to get longer (in the northern hemisphere, the longest day, the summer solstice, is about June 21). But the days are longer the closer one gets to the pole (the sun never sets above the arctic circle during the summer). This means that if you try to grow a variety adapted to a certain latitude, say around St. Louis, Missouri, at more northerly locations, say Minneapolis, Minnesota, the days will be longer and the plants will not begin to flower until later, and they may not mature before frost. If grown to the south, they will mature too soon and yield will be reduced.

Therefore, 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.) Be certain to plant a variety adapted to your area.

Pod development. One or two weeks after the first flowers, the first seed pods appear, with most pods being set within the next three weeks. Inside the pod, three (or sometimes four) tiny seeds begin to grow and develop.

For the next 30 to 40 days, the seeds rapidly fill with food produced in the leaves. The seed-filling period is the most critical in the life of the soybean plant with regard to yield. If weather conditions are adverse, such as drought stress or leaf loss from hail, yields will be cut severely. At this time, the plant takes 30 to 40% of its total mineral needs from the soil, so soil fertility should be at a peak.

After most seeds have filled, the growth activities of the plant rather suddenly slow down (called senescence). The leaves slow down their photosynthesis and begin to turn yellow, eventually dropping off. Root nodules cease producing nitrogen.

Maturity. 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 (if it is a yellow-seeded variety).

In warm, dry weather, seed moisture will continue to drop to about 13 to 14%, when the crop can be harvested. In some varieties especially, the dying plants tend to lodge, making harvesting difficult, and in some varieties, pods tend to split open (shatter), dropping the seed and reducing harvestable yield.

As soybean seeds lose moisture they change from large, kidney bean shaped to smaller and nearly round. When dry, the seed contains about 40% protein, 21% oil, 34% carbohydrates and 5% ash.

Varieties. There is an amazing number of soybean varieties. Just about every valley in China, Japan and Korea grows its own variety, adapted to local conditions. A collection of over 10,000 strains of soybean seeds is maintained by the USDA. A glance of an assortment of these seeds reveals seeds of every color and description--some red, some green, some black, some brown, some speckled or streaked, some large and some tiny.

The great majority of soybean varieties grown commercially today is for animal feed and oil production (for food processing and industrial uses). Most are yellow-seeded field varieties. Other varieties can be obtained for special uses: forage and hay (with an abundance of stems and leaves; small seeded black and brown late varieties) and human food (large-seeded, various colored varieties). For the most part, we will stick to commercial field varieties in this book, except for the last chapter.

Hybrids. Commercial hybrid soybean seed is very difficult to produce. This is because of the way the soybean reproduces: it is self-pollinating. Hybrids are made by soybean seed breeders, but it is a laborious, expensive process. From various ancestral and hybrid varieties, the commercial varieties are developed, both by agricultural experiment stations and private seed companies.

Seed quality. Varieties are developed to produce high yields of good quality seed, to mature properly for the geographic area, to be resistant to lodging and shattering, to be cold and drought tolerant, and to resist diseases and pests. Factors of seed quality mag include low numbers of defective or shriveled seeds, high germination rate, high oil and/or protein content and human food value.

Soybean seeds sold by reliable seed dealers should come with certain important information: the variety, the Maturity Group number, percent inert matter, percent weed seed, percent other crop seed, germination rate and resistance to diseases and/or pests. The U.S. plant variety protection act of 1970 and the earlier federal seed act, as well as state seed laws, provide standards and protection to dealers, but some private growers may not adhere to these standards. Anyone can save some of his seed to grow the next year, but this is no assurance of quality.

Selecting a variety. In selecting which variety you wish to plant assuming you are growing field soybeans. sou need to consider several things. First, buy the best quality seed you can find. Certified tested seed is usually worth the cost. You can test for germination rate by counting out 25 whole seeds and roll them up in a damp cloth. Keep in a warm (70 to 80 degrees F.) place Sprinkle with water if necessary to keep the cloth moist. After five or six days, unroll the cloth and count the seeds that have germinated out of 25. Multiply by 4 and divide by 100 to get the percentage germination.

MATURITY GROUP

Be sure to get seed of a Maturity Group adapted to your area. You may want to vary slightly the maturity group depending on soil type (an early variety for cool, wet, fine-textured soils and a later variety on coarse, well-drained soils). Avoid early varieties in fields where tall broadleaf weeds may get out of hand. If you want to follow the soybeans with fall-seeded small grains, use an early-maturing soybean.

One way to allow for uncertain weather conditions is to plant more than one maturity, either in different fields or as a seed blend, a mixture of varieties. That way at least one variety should give a reasonably good yield. If you save your own seed to replant, sou will not get the same proportion as what was in the blend.

Select a variety that is shatter and lodging resistant, especially if you intend to plant high populations, since the plants will grow taller, more slender stems.

Disease and insect resistance may be important if these have been a problem in your area; however, by improving your soil's fertility and structure, most such problems should disappear.

Indeterminate varieties should be used in the North, and determinate varieties do not do well in soils that crust. For wide rows, bushy varieties are best, to fill in the space quickly.

If you use a grain drill for planting, avoid seed lots with many large seeds, which do not flow well through the drill. Use seed lots with 2,400 seeds per pound or less. Small-seeded varieties have some advantages: the seedlings emerge better through crusted soil, fewer pounds of seed are needed to establish a certain plant population, and it is often easier to produce high quality grain (because smaller seeds suffer less damage during harvesting and handling).

You can often get valuable advice on selecting varieties from your agricultural research and extension personnel or from seed dealers. They may have performance test results which can be a rough guide of what to expect from a variety.

GETTING STARTED

The first thing we need to think about before doing any field work is the soil and its fertility, for without good soil it is impossible to grow a good crop. And a good soil will actually give the plants protection from adverse weather--cold, frost, drought, excess water--as well as protection from pests and diseases.

Fortunately, the soybean is a hardy, not-too-particular plant and can do reasonably well in a variety of soils and soil conditions, but to produce high yields of top quality soybeans, you need to get your soil into really good condition.

The ideal soil. Ideal soil for peak soybean production is a loose, well-drained loam. All too many fields these days have tight, crusty soil that becomes waterlogged when it rains. More than likely, such soil is low in humus and has an imbalance in mineral nutrients. Probably there are few beneficial soil organisms (certain bacteria, fungi, algae, protozoa, earthworms and others). In short, the soil is "dead."

The advantages of loose, well-aerated soil with adequate humus and abundant living organisms include the following: (1) Loose, aerated soil allows air to get to roots and nitrogen-fixingroot nodules, plus it soaks up rain and lessens erosion, and it discourages many of the worst weeds. (2) Humus and soil organisms provide steady, balanced nutrition to roots, soak up and hold moisture (provide "droughtproofing"), and protect roots from harmful nematodes, insects and disease pathogens. (3) Organic matter also tends to buffer soil from extremes in pH (acidity and alkalinity).

Modern agriculture. Yet many of today's agricultural practices tend to degrade soil and produce the tight, crusty, lifeless conditions mentioned earlier. The overuse of synthetic salt fertilizers and anhydrous ammonia tends to reduce soil life and humus, leading to hard soil. Some of the herbicides and pesticides also do the same thing. Too much field traffic and heavy machinery compact soil. Even using the wrong kind of lime may in some cases lead to soil degradation.

NUTRIENTS

Nutrient needs. Plants need various amounts of nutrient elements from the soil as they grow and produce seeds. Other than nitrogen, they should be present in adequate amounts in ideal soils, but 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 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. In fact, in tests where fertilizer nitrogen was added to soil, no yield increase occurred, plus the root nodules fixed less nitrogen.

You should need to add no nitrogen fertilizer when growing soybeans unless root nodules do not form well, which can happen the first time soybeans are grown in a field or when soil conditions are toxic to the nodule bacteria. From 60 to 80 pounds per acre of supplemental nitrogen can then be applied between one month after emergence and first flowering.

Phosphorus (P). Soybeans need a lot of phosphorus, which is used 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 available phosphorus.

The soil has plenty of phosphorus, but most of it is tied up in insoluble soil minerals and in soil organic matter. The best way to get phosphorus to crop plants is to have soil with adequate levels of humus and beneficial soil organisms, which decompose organic matter and break down soil minerals to release nutrients to the plants.

Adding soluble phosphorus fertilizers (superphosphate, triple superphosphate, etc.) does little good because these soluble forms quickly change back to insoluble mineral phosphate. Good sources of soil-building natural phosphate fertilizers are soft rock (colloidal) phosphate and basic slag. These contain a small proportion of available phosphorus, plus some calcium and trace elements.

Potassium (K). 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.

As with phosphorus, most soils (except sand) contain large amounts of potassium, but mostly tied up in the minerals of the soil. If soil organisms are healthy and active, the crop plants should receive enough potassium, since soil microbes break it down from minerals.

If your soil is very low in potassium, the best overall fertilizer source is potassium sulfate (0-0-50). Avoid using potassium chloride (0-0-60, muriate of potash), since it has a high salt index, and 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 (Ca). Adequate available calcium levels are very important in growing high quality soybeans. Calcium is vitally important for cell division, root hair growth, enzyme functions and normal cell walls. Calcium improves plant's resistance to disease and gives higher quality, more nutritious crops.

Calcium and magnesium (Mg) are connected in plant usage. Magnesium is needed as part of chlorophyll and in nucleic acids, cell membranes and protein-producing structures. In the soil, calcium and magnesium "compete" for plant absorption. Too much magnesium upsets the plant's use of calcium and potassium, giving rise to low quality crops, plus in some soils, excess magnesium leads to hard, crusty conditions. Most soils (except acid, sandy soils) should have plenty of magnesium, so none should be added. 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 way to add calcium to soils is to use high-calcium lime (calcium carbonate). It has little magnesium and dissolves fairly quickly (more so if finely ground). The use of dolomitic lime (calcium magnesium carbonate) is unnecessary if soil already has enough magnesium, plus dolomitic lime is hard and slow to dissolve. In alkaline soil, gypsum (calcium sulfate) is the best calcium source.

Sulfur (S). Soybeans use quite a lot of sulfur, which is needed to build proteins and in enzyme functions. Many soils have adequate sulfur because of air pollution from burning high-sulfur coal, but other soils are deficient.

If sulfur is needed, use sulfate-containing fertilizers (calcium sulfate, potassium sulfate), not elemental sulfur (flowers of sulfur), which is slow to become available.

Micronutrients. Other elements are needed by plants, but only in very small amounts. Thus they are called the micronutrients or trace elements. Important are iron (Fe), zinc (Zn), copper (Cu), boron (B), manganese (Mn), molybdenum (Mo), cobalt (Co) and chlorine (Cl). Eco-farming suggests that half a hundred in some way figure in the production sequence. Molybdenum is needed by nitrogen fixing bacteria. For a more detailed discussion on this topic, see An Acres U.S.A. Primer.

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, add only that element, not a "shotgun" trace element fertilizer, since too much of some micronutrients will be toxic.

Balance. For healthy crops and high quality yields, it is important that nutrient elements be available to the plants in the proper amounts and in the right balance. Too much or too little of some elements can cause deficiencies of others.

pH. The acidity or alkalinity of the soil is called pH. It is expressed on a numerical scale ranging from 0 (most acid) to 14 (most alkaline), with a 7 being neutral. Soybeans can tolerate a wide range of pH if they have adequate nutrients, but do best in slightly acid soil, from pH 5.8 to 7.0.

Soil pH affects the availability of nutrient elements and the types and ability of soil organisms to live, including nitrogen-fixing bacteria. Extremely acid (low pH) or alkaline (high pH) conditions are bad, but in normal fertile soil, pH can fluctuate over a growing season without harm. Adequate humus levels in soil will buffer extremes in pH and bring soil toward best pH levels.

Lime has been used to counteract soil acidity and raise pH, but its primary value is adding calcium. A healthy, humus-rich soil is the best insurance against extreme pH.

Soil tests. In order to get your soil into a proper balance of nutrients, you should have frequent soil tests made (at least once or twice a year, in spring and fall). The trouble with soil tests is that some are more reliable than others, and there are various ways of testing soil, some of which give accurate results but tell you little about what your crops really need. The type of soil test which gives the most useful information is a water-soluble test. This test tells how much nutrient is available to the plant at that time, rather than the total nutrients in the soil (but mostly unavailable). Most testing labs do not run water-soluble tests unless you request them.

Tests may vary slightly, but using one method (the LaMotte system), desirable water soluble levels for major nutrients are:

2000 pounds/acre calcium
400 pounds/acre phosphate (P2O5)
200 pounds/acre potassium
40 pounds/acre nitrogen

These figures do not translate to non water-soluble tests and may be higher or lower than most experts recommend, but they do produce high quality crops. Generally, one should not worry about trace elements until the major elements are at proper levels.

Plant tissue testing, as done by most labs, is not as informative as water soluble soil tests. Tissue tests only test the soluble contents of the cells. Some nutrients- are part of the cell structure and are not soluble. Sometimes the soil may have plenty of nutrients, but they are not getting into the plant because of poor root functions or toxic soil conditions.

Tillage. Tillage is done for three reasons: to prepare a seedbed or improve soil structure, to incorporate organic matter and fertilizers, to aerate the soil, and to control weeds. There are several commonly used tillage methods. The moldboard plow lifts and turns the soil, inverting the plow layer. This causes drastic disturbance in the soil ecosystem, but can be useful in heavy soils if done in the fall. Winter freezing and thawing may improve soil structure.

Chisel plows fracture the soil rather than turning it. Less energy is needed to pull the plow, and the soil is disturbed less. Some plant residue is left on the surface, which is helpful for reducing erosion.

Discs cut and loosen soil and incorporate much of the plant residue, but they compact the soil beneath the blades.

Field cultivators and springtooth harrows dig and lift the upper layers of soil and do not compact lower soil. Little residue is incorporated.

Rotary hoes break up clods and crusts and leave a fine-particle layer.

Subsoilers and deep chisels are used to fracture subsoil and break up hardpans, in an attempt to improve drainage and deep soil structure. Generally the effects are temporary, and without - increasing soil humus, hard soil conditions will return.

In general, tillage on humus-poor, heavy soils causes deleterious effects, especially if overdone. Soil structure is destroyed, organic matter disappears and erosion increases. Tillage operations should be kept to a minimum if soil is poor.

No-till. The above disadvantages of tillage in poor soils have led to the development and promotion of various reduced- and no-till systems. By using special planters that can operate in surface crop residue and by using high levels of herbicide for weed control, crops can be grown fairly successfully (except in northern climates on poorly drained clay soils).

While it is true that reduced-tillage systems do reduce erosion and save fuel, the requirements for high amounts of fertilizer and pesticides and the longterm tendency for deep soil to become depleted in oxygen and toxic are disadvantages. Soil-living pests and diseases often increase, and springtime soil temperatures may be cold.

All of these disadvantages of no-till could be eliminated and most of the advantages obtained if an adequate level of humus (up to 10 to 12%) is maintained in the soil and if the use of materials toxic to soil organisms is reduced or eliminated (pesticides, some herbicides, high-salt and chlorine-containing fertilizers, over-use of raw manure). Humus and soil life create loose, non-crusting soil structure and break up hard subsoil and hardpans, improving drainage. Erosion is greatly reduced because humus holds soil particles in small clumps (aggregates).

Ridge planting. A fairly new tillage method that works well in some cases for corn and soybeans is called ridge planting or ridge-till. Rows must be at least 30 inches apart to allow ridges and valleys to be built up (branching varieties of soybeans must be used). The crop is planted on top of the ridges, with crop residue left in the valleys. Earlier planting is possible because ridge tops warm up soon, and wind erosion is reduced. Ridges catch more snow in winter. Weeds can be cultivated out in the valleys. Ridges must be built up each year, and machinery must be compatible with the ridge widths.

Which? The tillage methods you use should depend on your climate, soil type, slope, crop rotation, machinery and costs.

CROPPING

Cropping systems. Most people grow soybeans in a crop rotation sequence, typically with a non-legume such as corn, small grains, sorghum or cotton. The yield of the non-legume is improved because of the left over nitrogen from the soybean root nodules. Also, disease, pest and weed problems are reduced in rotations compared to growing one crop continuously. These disadvantages can be overcome if soil is in peak fertility and condition.

Soybeans are also often grown in a double-cropping system, with two crops being grown in the same year. Winter wheat followed by soybeans is the most common; snapbeans or peas followed by soybeans is another. Timing is critical in more northerly areas.

Intercropping, in which two crops are planted in alternating rows or strips, or in which one crop is broadcast into the other, has been tried with mixed success. Sometimes aerial seeding was used. Conditions must be just right. Examples include planting soybeans in standing small grain, small grain into growing soybeans, ryegrass or clover into growing soybeans, alternate strips of corn and soybeans, corn and soybeans in the same rows, and early soybeans into a growing late variety. Interseeding grasses or legume-grass mixtures into soybeans at the leaf-yellow or leaf-drop stage will provide an excellent erosion reducing ground cover over the winter that can be worked into the soil next spring.

Row width. 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, the same may be true if good weed control is achieved. To produce maximum yield, soybean foliage should completely cover the space between rows by the time flowering begins. The plants tend to do this anyway, producing more branches in wide rows (but if you use wide rows, be sure to plant a bushy variety). The faster the foliage covers the ground, the less weeds are a problem, but one cannot cultivate weeds with narrow rows.

In recent years row widths have decreased, averaging about 18 inches and sometimes as small as 7 inches (and experimentally even 2 inches). Newer planters will plant narrow rows, or older planters can often be modified. It is recommended that soil fertility for narrow rows be increased 10 to 20% over levels for wide rows.

Linear feet per acre at different row widths

(From Modern Soybean production, 1983, p. 90)

Row width (inches) Linear feet of row per acre Row width (inches) Linear feet of row per acre
40 13 068 22 23 760
38 13 756 20 26 136
36 14 520 18 29 040
34 15 374 16 32 670
32 16 345 14 37 337
30 17 424 12 43 560
28 18 668 10 52 272
26 20 105 8 65 340
24 21 780 6 87 120

Population. Soybeans can adjust to a wide range of plant populations. Yields remain fairly constant within a range of 70,000 to 180,000 plants per acre. For wide rows, about 150,000 is a good target, with 175,000 for narrow rows (solid seeding). 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.

Seedbed preparation. An ideal seedbed for soybeans should provide adequate moisture and warmth for rapid germination and seedling establishment. Soil should be friable and not crusted. Germination of weed seeds should be delayed or prevented.

Soybeans need a lot of moisture to germinate (50% of their weight). Soil moisture must be sufficient at planting depth. There should be good soil-seed contact. If soybeans get off to a rapid start, young weeds can be shaded out. One way to discourage weeds is to prepare an ideal seedbed only in the rows and leave the soil rough and cloddy or covered by residue between the rows. Another approach is to prepare the seedbed well ahead of planting, let the weeds germinate, then refill just before planting to kill sprouted weeds.

Most people use herbicides to control weeds, but - such chemicals may have their deleterious environmental effects, and their use can be avoided as we shall see later in these articles.

Planting. As mentioned earlier, use good quality seed of high germination rate (80 to 90% or more). If soybeans have not been grown on that soil 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. The bacterial inoculant can be applied to the seed just before planting time or in the row during planting (the latter requires soil inoculant)

Seed can also be treated with fungicide, but unless the soil is cold, if the germination rate is over 85%, there is little advantage in this. Lower germination seed may have a 5 to 10% increase in emergence if treated.

Early planting usually gives higher yields, but only if a good stand is obtained. Cool weather will delay germination and allow root diseases or pests to get a start. Soil and air temperatures of 55 to 60 degrees F. are needed for good germination and seedling establishment. Germination rates increase at warmer temperatures, and high quality seed is more likely to produce a good stand. The predicted weather is probably the most important factor to consider, along with your local soil conditions. Adequate moisture is essential for germination.

Planting rate. To achieve a desired population, you need to calculate the number of seeds required. Some seeds will not germinate, and some that germinate will not become established because of weather, pests or disease. Generally, if the seedbed and planter are good, about 90 to 95% of germinated seedlings will become established. To figure planting rate, use this formula:

desired population per foot of row = seeds needed per foot of row
% germination x % expected establishment
 
For example: 6 / .80 x .95= 7.9 per foot of row

You need to plant 7.9 seeds per foot of row to get six plants per foot. Since soybean seed is usually sold by weight rather than by number of seeds, you -need to know the number of seeds per pound to figure pounds needed per acre. If the seed dealer cannot give you number of seeds per pound, weigh a few one-ounce samples on a postage scale to get an average figure.

The number of linear feet of row per acre can be found from the accompanying table. Then figure the pounds of seed needed per acre:

seeds per foot of row x total feet of row = pounds of seed per acre
seeds per pound
 
7,9 x 29 040 / 3500 = 65.5 pounds of seeds

Calibrate your planter accordingly and check seed drop in the field regularly.

Planting depth. Seeds should be planted deep enough to absorb enough moisture to germinate, but not so deep that they have trouble emerging from the soil. Some varieties can emerge from greater depths than others. Typical planting depths are I to 1 l/: 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 if there is no danger from herbicide.

Planting method. Best results are obtained using a unit planter or grain drill to plant in rows. Drills usually do not handle rough seedbeds as well as planters. Broadcasting or aerial seeding followed by light tillage to cover seed often results in uneven emergence and stands that are too thin in some areas and too thick in others.

Replanting. If a stand of soybeans is reduced by disease, pests, hail, flooding, herbicide injury, etc., replanting may be considered. If the loss is covered by crop insurance, consult your insurance agent first. If most of a field is lost, be sure enough growing season is left for beans to mature.

If the surviving population is 75% or more of the desired population, replanting is not necessary (unless weeds will be a problem) since the surviving plants will branch out to fill in gaps.

When replanting, you may want to use shallow tillage to kill young weeds. Do not apply herbicide. Use a variety with maturity date appropriate for the later planting date, increase the seeding rate by 10 to 15% and plant in narrow rows to increase yield. Above all, rely on the eco-principles set forth in The Rest of the Story, The Coming Revolution in Agriculture, and in An Acres U.S.A. Primer under topics such as soil, air, water and decay management.

 


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