Tools of the Trade — Using Refractometers & Penetrometers

By Gary Digiuseppe

Forage producers can measure the percentage of sucrose and other soluble content of their grasses with the use of a refractometer, although the accuracy of the reading can be dependent on the cost of the instrument.

Martin Capewell, owner of Agriculture Solutions LLC in Strong, Maine, says an analog refractometer costs around $50-$200, while a digital model can run anywhere from $190- $10,000. A “simple, hand held, decent” digital refractometer, Capewell says, costs about $350 and will last a long time provided it’s properly cleaned and maintained and protected from extreme temperatures.

All refractometers work by passing light through sap or juice extracted from a crop, and then measuring the angle of the light as refracted by a prism. The readout, a percentage of soluble content, is called Brix, named for one of the originators of the method, Adolf Brix. The Brix reading is determined by where the line produced by the light refraction crosses the scale on the device. Technically, Brix is the percentage by weight of sucrose in a solution, but the refractometer can’t differentiate between sucrose and other dissolved solids.

The scale is calibrated relative to a temperature of 20°C, and Capewell says refractometers are either temperature compensated or non-temperature compensated. “The great thing about one with automatic temperature compensation is that it gives you the correct Brix reading, and you don’t have to do that formula yourself,” he says.

pentrometer in use in field

An analog refractometer has to be held up to a light source to be read; digital models use an internal light source. “When you click the ‘measure’ button, within about 3 seconds you can get an exact reading to one decimal place,” said Capewell. “It’s much more accurate, quicker and easier to use.”

In order to extract a sample for measurement by the refractometer, the farmer crushes several blades of grass in a vice grip or garlic press that should have a filter so the crusher will not push the solid material through the holes.

Cheryl Pike of Pike Agri-Lab Supplies in Jay, Maine, says there are devices specially designed to squeeze juices out of plants or fruit for sampling purposes. If a garlic press is used, it should be sturdy and made from stainless steel. “Most of the cheaper garlic presses won’t stand up to it, especially if you’re talking about pasture and trying to get the juice from grasses.”

Pike says most recommendations for an acceptable Brix reading are at 12 or higher. “The higher you have that number, it’s going to be a higher quality grass,” she said. “There have been studies showing the feed quality values matching the Brix reading. They are positively correlated — as one goes up, the other goes up.” Some studies indicate higher sucrose levels in pasture grass increase a ruminant animal’s conversion rate of feed to milk.

Temperature is just one of four variables that affect the reading. Concentration of dissolved solids, the atomic weight of the substances in the liquid and the number of covalent bonds, which are higher when components like amino acids and proteins are present also play a part. However, the farmer just needs to know if the meter is temperature compensated and to adjust for temperature if it’s not.

“If you have a very watery substance, it would read lower than something that has a lot of solids dissolved in the liquid,” said Pike.

In short, says Pike, “The more nutrition you have in your juice, the higher the reading will be … the more complex, and the more desirable things that are in our vegetables, fruits and grasses are the things that make the reading go higher.”

To ensure an accurate readout, Pike cautions that some inexpensive instruments they tested from overseas suppliers were not very accurate. “You get what you pay for.”


Another popular tool for farmers is the penetrometer, which measures resistance in soils. In addition to providing farmers with an idea of the tilth and oxygen content of soil, Capewell says it can also be used to determine the depth of the hardpan layer, so the producer can tell how much room a plant’s roots will have to grow.

He says in order to operate the penetrometer, push it into the soil at a constant rate while keeping an eye on the dial that shows how much resistance it is facing. “It might go up to 200 pounds per square inch (psi),” he said. “You’ll know that you’ve got oxygenation as you’re pushing it in, and you’ll certainly hear it go ‘thud’ as you’re hitting a harder layer. You can continue to push it past that point to see how hard it is to push through that, too, but when you hear it go ‘thud’ you can mark on the penetrometer where it met the harder layer, and you can see how deep your topsoil is and how good the tilth is.”

Penetrometers start at $250 with digital models being more expensive. They’re made of stainless steel and should last a number of years, but the tips may need replacing if they’re used very frequently. Capewell explains, “If you’re pushing into something all the time, you may hit rocks, and it may get blunted over time” which would affect the measurement. Luckily, the tips are inexpensive to replace.

Pike adds that plants do not perform well on ground where 300 psi or higher is required to push through the soil. The penetrometer produced by her company has marks along the length of the probe every 3 inches so the operator can record the depth at which the 200 psi, 300 psi mark, or hardpan is reached.

Finding the hardpan would be useful in a situation where the ground has been plowed. She says when that layer has been detected, “People are using different treatments, not necessarily just the plowing, that are supposed to either lighten up the soil or alleviate hardpans.” The penetrometer can be used before and after to gauge the efficacy of those treatments.

Editor’s Note: This article appears in the August 2014 issue of Acres U.S.A.

How To Select Your Soil Lab

By Susan Shaner

Soil lab selection: How does anyone choose the right laboratory? Aren’t they all the same? Should you send a sample to several different labs and average the results? How do you get the samples to a lab and what is the turnaround time? Some homework needs to be done here.

These are all questions that I hear on almost a daily basis. All labs are not the same. This does not mean that one laboratory is better than another. They all provide a different “menu” of services. It is important to find a lab that provides all of the services that you require. Are you just looking for a soil analysis, or do you also need an irrigation water test or tissue analysis?

Laboratories can also choose from a number of methods or “recipes” to obtain results. Which method would be best for your soil type or crop? “Presentation” of results can also vary greatly from one laboratory to another. It is important that you can read the report and make use of the information it provides. These are all questions that you should consider before choosing a laboratory.

Menu of Services

Packages with various soil parameters are usually available, plus some a la carte choices. This will vary greatly from one laboratory to another. I think we all agree now that there is a lot more to soil than pH. Therefore, look at what is included in the soil package you are requesting.

soil sample

Important parameters include pH, organic matter, exchange capacity and base saturation. Also important are the major elements calcium, magnesium, potassium, sodium and phosphorus.

Important minor elements include sulfur, boron, iron, manganese, copper, zinc and aluminum. A complete soil analysis including all of these parameters may cost a little more. More information will provide better insight into your fertility situation. If you base your decision on cost alone, you will probably get what you pay for. An inexpensive analysis may only include pH, phosphorus and potassium.


Methodology is the most confusing area when comparing laboratories. There are several different methods for almost every parameter on a soil analysis. Laboratories choose methods that are best suited for the geographical area that they service. Most labs will offer different methods upon request to accommodate most customers; you will have to know what to request first.

Sending the same sample to several labs for comparison will be quite confusing unless you do your homework to determine what methods are used. I have talked with several customers after they have submitted the same sample to different labs without understanding the differences.

They have been very unhappy and disappointed with the outcome. Let’s look at a good example of different methods — for example, phosphorus. There are nine phosphorus test methods that I am aware of. All of these methods were run on a specific soil sample and produced results anywhere from 10.5 to 656 ppm. If you know how to interpret the results for each of these tests, you should come up with the same recommendation. If you do not have the correct threshold levels for the method provided, you could make a big mistake in interpretation.

Presentation of Data

What units of measure do you feel comfortable with? Do you prefer graphic results, high and low distinctions or an actual value found?

Soil reports come in all shapes and sizes. Some reports are very colorful and show your results in a graphic form. Various reports show values as low, adequate, or high. A number of reports show actual values found for each parameter. Reporting styles also vary regarding the reporting of desired levels, sufficiency levels and base saturation percentages. What style are you most comfortable with? A combination of these styles may be most helpful.

Units of measure can vary from parts per million, pounds per acre, pounds per 1,000 square feet, or kilograms per hectare.

It is very important that the laboratory is aware of your sampling depth if you will be receiving your results in a pound per acre or pound per 1,000 square feet format. The sampling depth will affect the value reported. It is vital to be aware of units when comparing reports from different laboratories. You have to compare apples to apples. Looking at a phosphorus example again will explain this. A laboratory may report phosphorus at 50 ppm P and another may report it at 229 pounds per acre P2O5. These two results are the same; however the units are the difference.

Does the soil report offer a recommendation? Where did it come from? Some recommendations are generic computer recommendations that give a ballpark range for optimum levels of nutrients. These may or may not be for a specific geographical area. If you are growing a unique or exotic crop, then you may need some specific advice. Inquire about the services of an independent agronomist.


How do you get your sample to the lab? Most labs will provide a soil sample bag or suggest a suitable alternative. Soil laboratories receive hundreds of samples each day. Be sure to acquire the appropriate paperwork from the laboratory to submit with your sample. Incomplete information will only delay the processing of your samples. Packing your samples for shipment is very important. Be sure to pack the samples tightly in a box. Pack newspaper or other packing material around the samples to keep them from bouncing around in the box.

If samples can move around during shipment, they sometimes break open and can be destroyed. Resampling will add to your cost in time and money.

How long will this process take? Turnaround time in the whole soil testing process is imperative. Laboratories under stand that your test results can be time sensitive. Don’t hesitate to contact the lab if you have an emergency situation and need “rush” service. To determine your approximate turnaround time, consider the time it takes to get the sample to the lab (two-three days) and perform the analysis (three-four days).

Turnaround time varies from one lab to another and also varies by season. You may want to contact the lab to inquire about their current turnaround time. How do you get your report? You do not want the report held up somewhere after you have already waited through shipping and testing procedures. Reports are usually emailed or made available on the internet on the same day the testing is completed. Be sure your correct email address and/or a fax number is submitted with your samples to get your results as soon as possible.

This is just the tip of the iceberg when looking at differences in laboratories. Laboratory instrumentation is continually improving. More parameters can be detected in a short amount of time and detection limits keep getting smaller. More efficient and “green” procedures are always being investigated.

Embrace the advancements. Visionaries like Dr. Albrecht are still being cited in soil analysis circles. If he was continuing his research today, I believe he would embrace the latest technology and tools available.

So, the question still looms … which laboratory is best for you? Take the time to do your homework. It will be worth the investment and you will receive the value that you expect. Explore laboratory websites, call a lab and ask some questions, ask your friends about their experiences. Make sure you acquire the appropriate paperwork and instructions from the lab that you choose.

When you have selected a laboratory that meets your needs and you are comfortable, stick with it. Jumping from lab to lab will only discourage you on your quest to improving soil fertility.

Editor’s Note: This article appeared in the October 2011 issue of Acres U.S.A.

Plant Stress & Proline

By Larry Zibilske, Ph.D. 

In his 1995 State of the Union speech, President Bill Clinton highlighted a USDA program addressing plant stress as an example of wasteful “pet project” government spending. We knew then, and know even more now, that plant stress is a very significant yield-and-quality-robbing factor in agricultural crops to which little attention has been paid.

We try to optimize fertility, irrigation, weed and pest management practices to achieve the best production under the constraints of environment and economics. However, it has become clear that plant stress comes from everything we try to control; it is additive and it can be cumulative — resulting in loss of yield and quality potential.

For the grower, visual detection of plant stress often comes too late to do anything more than damage control by preventing further loss of yields and quality for the season. One visually obvious “too late” example is dropped squares and bolls in cotton. Another is shed flowers and pods or a predominance of two and three-bean pods in soybeans if stress is present early on, or empty pods if stress occurs later.

In plant health, as in human health, there are signs, although they may not be obvious, that stress is present. The trick is in detecting and interpreting those signs – ideally, before they can be seen. This is evidenced by many of us having annual physical check-ups and blood (in the case of plants, sap) tests to detect “hidden” problems. Certain biological signals accumulate in the plant during periods of stress. They are produced in response to environmental stresses such as water, light, temperature and salinity. Their appearance signals that something is hindering normal plant growth and development with consequent loss of yields and quality.

Test tubes in a row of plant proline samples
Samples ready for instrument analysis. Even without precision and absolute instrument analysis, comparative differences in proline (plant stress) levels can be clearly seen.

One of the common stress molecules in plants is proline. Proline is an amino acid normally produced during protein production. In times of stress, plants over-produce this molecule which can be measured with whole leaf analysis. Elevated proline indicates stress responses have been initiated by the plant. The excess proline stimulates metabolic changes in the plant to cope with the stress.

However, proline is not only a signal but also directly helps maintain internal cell turgor, preventing electrolyte losses by supporting good osmotic balance. Proline also reduces the undesirable reactive oxygen species molecules (ROS) which, in times of stress, can overcome the natural ability of plants to dispose of these harmful molecules that are normally produced during metabolism.

Good crop nutrition is fundamental to warding off harmful effects of environmental stresses. Therefore, proline is both a plant stress response and a signal. Increased proline production allows a plant to marshal its physiology to accommodate the stress. But generally, the internal changes the plant makes to manage stress reduce its productivity in favor of survival advantage.

In short, high proline values are simply symptoms of larger problems in the plant. The primary use of the proline data is to formulate a plan to ameliorate or manage the stresses causing the proline response. With high proline, the plant is telling us that something is wrong.

stressed plants in the field
A dramatic field example of extreme plant stress resulting in unusually high proline levels, with the leaves on these sugar beets having collapsed. While this is a remarkable situation, the point of proline testing is to prevent unplanned crop performance-robbing stresses long before they happen.

We don’t want to eliminate stress, but control it so that the plant can continue to grow and produce well during the season. Elimination of stress is not a goal because the plant uses certain stresses as timing functions for several important metabolic processes.

While the overall concern is in reducing chronic or unanticipated stresses, there are certain conditions during crop development under which induced and carefully controlled plant stress can be of great financial benefit to the grower: Scheduled and managed stress can result in higher nutrient content of the crop (premium quality), greater yields, or both. This is demonstrated by field results.

healthy sugar beet leaves
For comparison, healthy sugar beet leaves.

Accordingly, the goal is not to minimize stress across the board throughout the season, but to carefully time the inducement, control and manage it.

Regardless, carefully managing overall stress responses during growth promotes better total plant performance. Your lab’s in-season recommendations will include steps to minimize destructive unplanned stresses and manage desired ones detected by increased proline production. As an example, for a low-moisture stress, the lab may recommend increasing potassium fertility to increase internal water use efficiency. Additionally, based on other plant tests, other nutrients may be recommended to maintain canopy structure and function and to reduce or eliminate disease and insect pressures.

The plant lab uses the proline test to monitor total stress in crop plants. If stress levels are high, the results, together with sap and tissue nutrient test results, guide the lab in prescribing changes to your management that will control those stresses. Remember, elevated proline levels indicate stresses that are most often caused by multiple factors. Your lab will consider those factors to formulate a complete remediation plan that addresses all stresses.

Proline may have other effects on plants as well. Collection of proline data has been ongoing for some time. Differences have been seen between reproductive tissues from stressed plants and those of low-stress plants. Therefore, if you hold seeds, tubers or bulbs over to plant yourself or sell them for planting, controlling stresses may influence future crops as well. These effects continue to be evaluated.

Editor’s Note: This article appeared in the December 2018 issue of Acres U.S.A. magazine.

Larry Zibilske, Ph.D., is vice president of research at TPS Lab in Edinburg, Texas. Zibilske holds degrees in microbiology and soil science from Texas A&M and a Ph.D. in soil microbiology from the University of Missouri-Columbia.

Acres U.S.A. magazine is the national journal of sustainable agriculture, standing virtually alone with a real track record — over 45 years of continuous publication. Each issue is packed full of information eco-consultants regularly charge top dollar for. You’ll be kept up-to-date on all of the news that affects agriculture — regulations, discoveries, research updates, organic certification issues, and more.

The Basics of Identifying Crop Quality

By Arden B. Andersen

Field trips in agriculture often are more like dog and pony shows than extended classroom experiences. Typically, the circulating tour group is paraded around at a distance, being shown various test plots, tillage systems, and management practices. One very important group experience that is rarely included in traditional field tours is the evaluation of crops, weeds, and soils in a hands-on manner in the field. Keen observation of field and crop details can tell the observer more than any soil or plant-tissue analysis. This is a key point because the farmer and consultant must take note of field observations to determine whether progress is being made and whether soil and plant-tissue tests are providing accurate pictures of the situation.

The Field Trip

Upon approaching any field, notice the overall appearance of the crop. Is it droopy? Rigid? Does it smell fresh? Are there any peculiar odors like methane gas, mold, ammonia, or pesticide? Is the overall color of the crop a deep blue-green? A mild-green or pale-green color? Do the leaves have a glossy sheen or a typical absorbent appearance? Are any birds nearby? Are they singing? How does the field feel to you? In other words, what sense do you get about the field?

As you move into the field, notice what types of weeds are growing and whether they or the crops are being eaten by insects. Ask whether a herbicide was used to control weeds and what type of weeds are most bothersome. Check the refractometer reading of both the weeds and the crop; write down the value of each. As the soil improves, the weeds will decline and the crop will increase in brix readings. Pull some weeds and slice open their stems lengthwise with a knife. Look at the pith of the weed. What color is it? Is the stem hollow? The healthier the weed, the higher the brix reading, the more solid the stem and the more pearly white its pith, and the less insect damage it will have. The same, of course, holds true for the crop. The healthier the weed, the more conducive your fertilization practices are to growing weeds rather than crops unless weeds (herbs) are your crop.

Notice what type of root structure the weed has. Grasses often have shallow, dense root systems that are attempting to loosen compact soil. Broadleaf plant generally have long taproots that are attempting to relieve hardpans and gain access to nutrient reserves at lower depths in the soil, as well as to extend the electrical circuit of the soil/plant complex to greater depths.


Dig up a corn plant. Notice the amount of root mass present and in which direction the roots grow. Are they pretty much growing out in the top two or three inches, or are they mostly growing down? Are there many or any live roots directly below the middle of the plant? Where does the soil structure change from a loose, crumbly structure to a platey, blocky one? Most soils in America have a crumbly structure for only the top one-half to two inches of the top soil and a platey structure from there on down. This means there is only a one-half to two-inch aerobic zone; the rest is predominately anaerobic, for all practical purposes.

In general, the roots will be concentrated primarily in the aerobic layer. You will notice that the greatest concentration of fine, fuzzy hair roots will be in the aerobic zone; this is because microbes and root hairs need oxygen. The microbes and root hairs make up the rhizosphere, which is the area of greatest nutrient exchange. Notice the color of the corn roots. Are they pearly white and soft, or are they brown and brittle? Pinch the roots slightly and pull to check whether the root bark sloughs off easily. If it does, there is a salt problem. Check for hardpan using a penetrometer, shovel, or brazing rod. This condition can be present in a sandy soil as well as in a clay soil. Notice that the roots pretty much stop at the hardpan.

Cut the corn stalk lengthwise and look at the plant’s plumbing system. Is the bottom of the stalk base, the pons, brown and hard? In general, we find that it is. This is congested or plugged tissue caused by toxins in the soil. These toxins might be pesticides, or they might be metabolic by-products from anaerobic breakdown of crop residue and manures. These could be products like formaldehyde and alcohol. Notice that the corn stalk has sprouted brace roots above this area. This is the back-up or by-pass plumbing system functioning to keep the plant alive. You may find that the node where these first brace roots sprouted is also brown and hard. In this case, a second layer of “by-pass” roots has sprouted. Brace roots are the plant’s rescue response when the plumbing system below gets plugged. If the plant did not sprout brace roots, it would die. In this case, as David Larson and others have reported, covering the brace roots, through cultivation, can increase corn yields by 5 to 15 bushels per acre. This allows the brace roots to contact the soil and, consequently, to interact with nutrients. In some areas the soil conditions are so toxic that the brace roots curl or burn off upon touching the soil surface. Compare the conditions of the plumbing systems of corn stalks between fields and note the differences in corn quality and refractometer readings.

Cut the corn stalk horizontally. The stalk should be round, not oblong or teardrop shaped. Out-of-roundness is an indication of a calcium deficiency. Look at the veins. Ideally, they should be pearly white and packed so tightly that it is difficult to count them. The pith should also be pearly white. Pull off the leaves from the stalk and notice that at each node, on opposite sides as you progress up the stalk, there is a baby or embryonic ear. This represents the plant’s true potential, which will be realized only when the soil is regenerated to the point that the plant’s plumbing system remains clear throughout, and the roots have an aerobic zone of 10 to 12 inches in which to grow.

About the Author

Arden B. Andersen

Dr. Arden B. Andersen apprenticed with Dr. Carey Reams, worked as a research assistant with Dan Skow, D.V.M., and as a field researcher with Philip Callahan, Ph.D. In addition to his B.S. in agricultural education, he holds a Ph.D. in biophysics and a D.O. in medicine. He currently works in general preventative and nutritional medicine while still consulting to some of the largest farming operations around the world.

Do Legumes Really Fix Nitrogen in the Soil?

By Margaret Smith, Ph.D., Forage Agronomist, Sponsored by Albert Lea Seed

Farmers have known for millennia that beans and other food legumes provide benefits when grown with grain species or when grains or vegetables follow beans in rotation. But do legumes really add nitrogen to the soil?

Most of the legumes important in agriculture are known as nitrogen fixers, but the plants themselves don’t really fix nitrogen from the atmosphere. These legumes can form a mutually beneficial association with rhizobia bacteria that “fix” nitrogen from the air and share it with their host plant.

Rhizobia bacteria are free-living soil bacteria that, during a portion of their life cycle, can infect the roots of legumes (in a good way) and form nodules on the plants’ roots. During this portion of the bacteria’s life cycle, their numbers increase. It wasn’t until 1889, when rhizobia bacteria were isolated and identified, that we knew what caused that legume “advantage.”

Nodules on legume roots help fix nitrogen
Root nodules occur on the roots of legumes that associate with symbiotic nitrogen-fixing bacteria (known as rhizobia)

Rhizobia bacterial species co-evolved with their legume hosts, and many of them are specific to individual legume species. In fact, most of our legume oilseed, forage, and cover crops aren’t native to the U.S. and neither are their specific companion rhizobia species. As an example, for alfalfa plants to nodulate and fix nitrogen, a specific rhizobia species (Sinorhizobium meliloti) must be present in the soil or introduced with the seed. This rhizobia can also colonize sweetclover as its host species, but not red clover, which needs a different bacteria (Rhizobia leguminosarum (biovariant) trifolii) to develop its nitrogen-fixing capabilities.

Soybeans require yet another bacteria species (Bradyrhizobium japonicum) to nodulate and fix nitrogen. This bacteria forms round nodules on soybean roots, compared with the knobby or irregularly shaped nodules that form from rhizobia on most other forage legume roots.

Because of this species-specific symbiosis, any legume new to a cropping system should be inoculated to provide the specific rhizobia species needed for nitrogen fixation. Where legume species are repeatedly grown in a crop rotation, you may not need to inoculate each time the legume is planted. Factors that affect rhizobia survival in the soil in years where their host legume isn’t grown include low pH (less that 5.5-6.0) and extremely hot or extremely dry soil conditions.

We recommend inoculating your legume species if:

  • The legume has never been grown before in your cropping system; for example, hairy vetch, dry beans or sunn hemp.
  • The legume was grown in the past, but you aren’t sure that the plants nodulated well. Was performance poor?
  • The legume was grown in the past, but only in a small proportion of the total crop mix, such as lentils in a cover crop mixture.
  • The legume crop has not been grown for several years. In this situation, rhizobia levels in the soil will decline with time. For example, if soybeans have not been grown for three or more years, you should inoculate the next soybean crop.

How to Inoculate Legumes with Beneficial Bacteria

A hundred years ago, farmers were advised to inoculate a “new” field by transferring soil from a field where their preferred legumes had already been grown. Fortunately, inoculation is far easier today.

An inoculant is a formulation of a carrier and the live rhizobia bacteria. Commercial inoculants may be powdered (peat-, clay-, or talc/graphite-based), granular or liquid. They are formulated for either application directly to seeds or to drop in the seed furrow at planting.

Peat-based inoculants contain the most bacteria per unit of carrier, but the bacteria in this formulation is very short lived. After opening a package and applying to seed, the seed should be planted within 24 hours. Granular applications are formulated for ease of application to apply directly in a seed furrow, rather than on the seed. Individual planters and drills may not be equipped for this type of application. Clay-based inoculants are applied to seeds and maintain viable rhizobia for a year or more.

Organic growers have access to many OMRI-approved inoculants specific to each legume species.

At a cost of about $1 to $3.50 per acre, inoculation is a relatively inexpensive “insurance” for your soybean, forage and cover crop legumes.

For more information, visit or call 800-352-5247.

What is sponsored content?

This article is sponsored content, also known as native advertising. That means that a sponsoring company wrote the article and paid for placement. However, instead of the information in a traditional ad, the information in a sponsored article is relevant to a specific topic, in which the sponsoring company is an expert. If you’d like to learn more about Acres U.S.A. and native advertising, visit our advertising page here, or call us at 1-800-355-5313.

How Soybeans are Used as Food

By Dr. Harold Willis

The versatile soybean has many uses as human food. It is used in a large variety of prepared or processed foods. Here we will review ways people often use edible soybeans in the home.

Soybeans contain more protein than lean meat. Two pounds of soybeans supply the protein equivalent of 5 pounds of boneless beef, 15 quarts of milk, 6 dozen eggs or 4 pounds of cheese. Soybean protein is the only complete plant protein; that is, it contains all of the amino acids essential for human health. However, it is somewhat low in the amino acids methionine and cystine, but these can be supplemented by eating whole grains (wheat, rye, brown rice, etc.), fish or casein (milk protein).

soybeans as food
If the beans are picked just after the pod is filled out (when the pod is plump and green), they can be cooked as a green vegetable, similar to lima beans. Courtesy How to Grow Super Soybeans

Soybeans are low in calories. One serving (one-half cup) has only about 100 calories, far less than a serving of meat. Soybeans are excellent for a diabetic diet, since they contain virtually no starch (1 to 3%; the carbohydrates they do contain are complex sugars).

Soybeans are low in cholesterol, but rich in polyunsaturated fats. They also contain high amounts of lecithin and linoleic acid, which have been shown to lower blood cholesterol levels. The soluble fiber content of soybeans has also been found to help lower cholesterol (the harmful kinds of cholesterol are reduced, not the beneficial kinds that the body needs).

Cooked soybeans are nearly flavorless, allowing them to be blended into many dishes or used as extenders. Many different textures of soybean products are available, from “milk” to paste to flakes to cake to flour. Sprouted soybeans are rich in vitamin C as well as other vitamins and minerals. The cost is very low compared to animal meat—a true nutritional bargain.

Here are some ways you can use soybeans as food (from The Soybean Book, 1978):

Fresh Soybeans

If the beans are picked just after the pod is filled out (when the pod is plump and green), they can be cooked as a green vegetable, similar to lima beans. They can be shelled or cooked in the pod. Their flavor is rich and nutty. They are rich in vitamin A and the B vitamins. For best food value, cook within two hours of picking. Cook in a small amount of salted water for 10 to 20 minutes (or 25 to 30 minutes in the pod—do not eat the pod).

Dried Soybeans

Dried mature beans must first be soaked before cooking. Soak 24 hours in cold water. Refrigeration is necessary to prevent fermentation. Cook beans in their soaking water (it contains vitamins), either simmer for 4 to 5 hours in a saucepan (add water if needed), cook at 15 pounds for one hour in a pressure cooker, or simmer 8 to 12 hours in a slow cooker.

Cooked soybeans can be used sparingly along with grains or other vege­tables. They can be mashed and used as an extender in hamburgers and meat loaf. They can be put in a blender and added to bread or cookies.

Roasted Soybeans

Dried uncooked soybeans can be spread on shallow trays and roasted lightly in a 300 degree F. oven. They taste like peanuts and can be stored dried for a long time.

Soy Grits

Coarsely ground dried soybeans cook in about half the time of whole dried beans (see above) and have a meat-like texture. By adding meat broth and other flavorings (onion, tomato juice, soy sauce), a good meat sub­stitute can be prepared.

Soy Flour

Soy flour is a fine powder, rich in protein, with almost no starch or gluten. A small amount in wheat flour (no more than ¼ the amount of wheat flour) will keep bread soft and moist. In cookie, cake and pancake recipes, as much as one-half of the wheat flour may be replaced by soy flour. Lower the oven temperature about 25 degrees F., since soy flour browns more quickly than wheat flour. You can grind your own soy flour, but it is easier to buy it at health food stores. Keep it refrigerated in tight containers.

Soy Milk

Resembling cow’s milk, soy milk is good for people who can­not digest or are allergic to cow’s milk. Soy milk contains as much protein as cow’s milk, but less calcium. The easiest way to make soy milk is to gradually stir 8 cups of cold water into 2 cups of soy flour. Let stand for two hours. Heat to simmering in the top of a double boiler, then lower heat, cover and cook 40 minutes. Cool slightly and strain through cheesecloth. Add 4 tablespoons sugar (or honey), 4 tablespoons cooking or salad oil and ½ teaspoon salt; mix thoroughly in a blender. Add cold water to make two quarts soy milk. Keep tightly covered in a refrigerator; use in a few days.

Soy Sprouts

Sprouted dried soybeans are a very nutritious fresh vegetable that can be steamed, fried, creamed, or used fresh in salads, soups, stews or casseroles. Sprouts are rich in vitamin C, protein and minerals, and are easy to grow.

One pound of soybeans will produce six pounds of sprouts, enough to serve 35 to 40 people, so use small amounts (one-third cup of beans will produce two cups of sprouts). Rinse the dried beans with water and put into a suitable container (a glass or plastic jar is fine) and cover with water overnight at room temperature. Pour off the water, rinse with fresh water and cover the container with cheesecloth to allow air circulation but retain moisture. Keep in a dark, warm (70 to 80 degrees F.) place for 4 to 5 days. Rinse with fresh water several times a day and turn the container over to stir up the sprouts. Refrigerate 2 to 3 days before eating. Use within 3 to 4 days.

Besides being used fresh or cooked, soy sprouts can also be dried (on a cookie sheet in a dry, well-ventilated place or in a 150 degree F. oven) and chopped or ground. They have a nutty flavor and can be used to add flavor to many foods.

Soy Curd (Tofu)

Soy curd is curdled soy milk made by adding an acid or mineral salts (calcium sulfate) to the milk. You can buy it at health food stores or make your own (use one tablespoon vinegar or lemon juice for each quart of soy milk and leave in a warm place until it thickens; cut into chunks and slowly heat to boiling in a double boiler; cool for 10 minutes and strain out liquid through cheesecloth). The softer curd can be drained or pressed to give a firmer texture, then sliced if desired. Salt, pepper or herbs can be added for flavoring.

When creamed in a blender, soft tofu can be used in salad dressings, pud­dings, dips, pie fillings and sauces. It makes an excellent, digestible baby food (add vegetables or flavoring). Tofu can replace the eggs in quiche or some or all of the ground beef in meatloaf. Sliced drained tofu can be breaded and fried or baked as a meat substitute. Chunks of tofu can be used in soups, casseroles or stir-frys. It can be scrambled like eggs. Frozen tofu has a chewy, meaty texture when thawed and squeezed to remove water.

There is growing concern about excessive reliance on soybeans, especially among children, because of the phytoestrogens present. Nonetheless, the soy­bean is a useful food, particularly when fermented.

Source: How to Grow Super Soybeans

Hay, Silage, Grain, Green Manure: Non-Human-Food Uses for Soybean Crops

By Dr. Harold Willis

Although most soybean producers strictly grow their beans to sell on the market, high quality soybeans are a valuable source of animal food. It is always better to feed crops you have grown to your animals than to risk buying feed of uncertain origin and quality. There are other uses as well outside of just food.

Soybean Forage

During the 1930s and 1940s, soybeans were widely grown in the United States for forage and hay. Even though largely superceded by alfalfa, soybeans are an excellent, high protein source of animal feed. Any variety may be grown, but taller indeterminate varieties are best.

If planted for forage or pasture, use a high seeding rate, about 4 to 6 plants per foot in rows about 8 inches apart. Soybeans may also be interplanted with grasses for grazing or with corn or sorghum for a ready-mixed chopped feed. The soybeans are valuable as a soil builder.

soybean hay
Soybean hay. Courtesy How to Grow Super Soybeans.

Soybean Hay

For maximum yield, soybeans for hay should be cut when the seeds have begun to set but before the leaves turn yellow. Cut on a sunny day after the dew is off. Let lie until the leaves are wilted but not brittle (usually the next day). Rake into windrows and let cure for 4 to 5 days. Soybean hay takes longer to cure than other hays, but it is less susceptible to rain damage and can be stored for long periods without nutrient loss. When baling or handling soybean hay, use care to avoid leaf loss.

Soybean hay is similar to alfalfa in nutrient content. It is slightly laxative, and limited amounts should be fed for the first few weeks (it can be mixed with grass hay to reduce amounts). There may be some animal refusal of the hay. Soybean hay should not be available to the animal all the time.

Soybean Silage

Soybean plants make a palatable component of silage, at a ratio of two parts corn to one part soybeans. The two crops can be interplanted to facilitate chopping or they can be grown separately and mixed. If interplanted, the seeds can be mixed in the planter and planted at about 20 pounds per acre in corn-width rows. If planted separately, the soybeans should be allowed to wilt after cutting to about 70% moisture. Harvest when the plants are green and succulent.

Other silage seed mixtures that work well are:

  • 3 parts corn or: 3 parts corn
  • 1 part sorghum 1 part sunflowers
  • 1 part soybeans 1 part soybeans

Soybean Feed Grain

Raw soybean seeds are difficult to digest because they contain what is called an anti-trypsin factor, a substance that inhibits protein digestion. Also, raw soybeans contain urease, an enzyme that breaks down urea into the more toxic substance ammonia. Both of these are destroyed by heat, so cooked or roasted soybeans are easily digested. One-stomach animals such as pigs, horses, poultry and rabbits must eat cooked soybeans. However, ruminant ani­mals (cattle, sheep and goats) can digest raw soybeans without difficulty, and one-stomach animals can be fed grain mixtures with up to 10% raw soybeans. All animals prefer the taste of cooked soybeans.

The most ready source of processed soybeans for animal feed is the soybean meal that results from beans being processed to extract the oil. Currently, soy­bean meal is the most commonly used source of protein supplement in animal feed. Heat used in processing destroys most of the anti-nutritional factors. Stan­dard meal is 44% crude protein meal, with the hulls being included as fiber. Since poultry do not digest the hulls, a 48% meal is also available. For use in poultry feed mixtures, soybean meal should be toasted to eliminate all of the anti-nutritional factors. Since soybean protein is low in the amino acid methio­nine, poultry feed mixtures need a source of it. The amount of soybean meal used in the feed depends on the animal’s stage of growth and protein needs for lactation. Vitamin and mineral supplements are often added. Soybean oil is also used in feed mixtures, but generally only if economically feasible.

Whole cooked or roasted soybeans can be a valuable addition to animal feeds. Relatively inexpensive cookers or roasters can be obtained for large batches. For small amounts, you can cook them in the kitchen (simmer fresh beans in a small amount of water for 15 to 20 minutes, or until tender; dried beans must be soaked in cold water 24 hours and simmered 4 to 5 hours). Use them within a few hours or refrigerate. To roast soybeans, put in a 300 degree F. oven until light brown (they may be stored in a closed container for a long time).

Soybean Green Manure

Soybeans tilled into the soil when they are green and lush make an excellent green manure. Besides the nitrogen from the root nodules, the rest of the plant adds organic matter and various nutrients to the soil. As mentioned earlier, increased organic matter improves soil structure, water holding capacity, drainage, and has other beneficial aspects. An acre of soybeans can add 175 pounds of nitrogen, 95 pounds of potassium and 20 pounds of phosphorus—and you can use seed you grow yourself.

Soybeans can be planted in late summer or early fall after another crop has been removed. High plant populations are best. Don’t worry about weed control. When plants are green but before pods form, till into the upper several inches of soil. If the weather is still warm, they will decompose within six weeks. If a green manure crop is incorpo­rated into the soil in the spring, wait several weeks (until it decompos­es) before planting another crop, since the decomposing organic matter temporarily ties up nutrients.

Source: How to Grow Super Soybeans

Soybean Plant Chemistry, with a Focus on Soil and Plant Calcium

By William Albrecht / Edited by Charles Walters

Normal development of soybean plants, like any other growth performance, is distinctly a matter of proper nutrition. Nodule production on the roots of the soybean plant and its use of nitrogen from the atmosphere to let this crop serve as a protein-producing, and a nitrogen-fixing factory on the farm, are determined in the main by the nutritional levels or the fertility conditions of the soil.

Of the 14 chemical elements required to construct plants, 11 must be supplied by the soil in cases of the non-legumes. One less, or 10 are demanded from the soil by the legumes. The legumes, in the same manner as the non-legumes, use carbon, hydrogen and oxygen provided by air and water. In addition, and quite different from the non-legumes, they can take a fourth nutrient—nitrogen—from the air provided they are operating in cooperation with the proper bacteria on their roots commonly sup­plied as inoculation.

Calcium in soil chart

Much of the attention to the behaviors of legume plants and their accompanying bacteria has centered about the fact that legumes can draw, on the weather, as it were, for four of their nutrient requirements, while non-legumes are limited to three. Little attention has gone to the fact that legumes must still obtain ten (possibly more) nutrient elements from the soil. Demand made on the soil by the legumes for these elements is greater than by the non-legumes because the mineral content of legume forages are of higher concentrations. These demands are more significant because on these increased mineral content drawn from the soil fertility store there depends the effectiveness with which the root nodule producing bacteria will work.

Pulling Nitrogen from the Air

Because the soybean can go, via bacteria, to the atmosphere for its nitrogen supply, we must not fix attention so completely on this escape from one responsibility as to forget the ten others that still lie in the soil. Studies to date have not given sufficient importance to all the soil-borne plant nutrients as these influence inoculation, nodule production and nitrogen fixation by legumes. Critical attention has gone to some, namely: calcium, phosphorus, magnesium and potassium, the four most prominent in the soil fertility list. Consider the importance of these in connection with the soybean.

Liming and Soybeans

Since long ago the art of agriculture has been pointing to the need for lime by many soils if they are to grow legumes. Nodulation of soybeans is generally improved by the practice of liming. It is only recently that science has begun to understand the function of liming for better cooperation between the plants and the bacteria. The scientist’s first suggestion as to the role of liming soils in giving better legume growth was that lime was effective because it removed soil acidity.

This explanation is about to lose its adherents, in the face of the accumulating evidence that liming serves because it supplies the plants with calcium, one of the foremost soil requirements for both legumes and non-legumes.

Inoculation and Soybean Seeds

Legume bacteria, too, have been considered sensitive to soil acidity. Failure of inoculation has often been ascribed to injury to the bacteria by the soil sourness. Successful inoculation, or ample nodule production, however, involves more than the idiosyncracies of the plant and bacteria. It involves, most decidedly, the soil as it nourishes both of these properly and sufficiently to make their joint activities result not only in a crop of larger tonnage but one of increased concentrations of proteins and minerals.

Since the soybean must be provided with its specific nodule bacteria when seeded on a soil for the first time, naturally the practice of inoculation of the seed is a recommended one. Failure of inoculation to produce nodules in many instances has brought blame on the bacterial culture, which, like water over the wheel, was past recovery or beyond defense when once distributed throughout the soil. It seemed a logical hypothesis that defective plant nutrition because of soil fertility deficiency might be prohibiting effective inoculation.

Yield and Calcium treatment

In order to test the nutritional value of liming for the soybean plants as compared to the role of lime in neutralizing soil acidity as these two effects encouraged better nodulation, calcium as a chloride was drilled with soybeans in comparison with similar drilling of calcium hydroxide.

Though the latter neutralized soil acidity while the former did not, but treatments brought about effective nodulation, deeper green color and larger plants of more stable cell structure. The nodules were not necessarily located in the soil areas into which the calcium compound was deposited. Roots in the acid soil areas bore nodules. Here was evidence that liming was improving the results from inoculation because lime was providing calcium.

Calcium and Soybean Production

In order to separate the nutritional value of calcium for the plant from those for the bacteria, more detailed tests of the soybean and its calcium needs were undertaken. It was readily demonstrated that calcium was more important than magnesium or potassium in the early life of the soybean plant.

A deficiency supply of calcium encouraged attacks on the plants by a fungus resembling “damping off,” and brought failure of inoculation.

To determine the minimum amounts of calcium required per plant for effective establishment of the stand, calcium was used in the solution form and in the form absorbed on colloidal clay. The latter method permitted variable amounts of clays at different degrees of acidity (pH). It thus permitted controlled amounts of calcium at any pH or degree of acidity desired. These trials demonstrated that the soybean’s early growth was dependent on a significant supply of calcium more than on a particular degree of soil acidity. Nodulation could not result later unless liberal levels of calcium were provided early to carry the plant to the inoculate age. More detailed separation of the calcium as a nutritional element from its role in modifying the soil’s reaction as this influences nodulation was undertaken by using clay neutralized to different pH values or degrees of acidity through titration with calcium hydroxide.

Constant amounts of calcium were provided at different pH values by taking the proper amount of clay at a particular pH. Thus by placing these different amounts of the clays of different pH values into sand for soybean growth, there were provided soils of variable pH but of constant supplies of exchangeable calcium. Plant growth and nodule production showed clearly that even though the soybeans reflected their response to differences in soil acidity, they reflected far more their growth and nodulation response to the amount of calcium provided.

Nitrogen fixation, or an increase of nitrogen in the crop over that in the planted seed, did not occur even in a neutral soil unless the supply of calcium was ample. It occurred in acid soils containing liberal supplies of exchangeable calcium. Here, then, was distinct evidence that if inoculation of soybeans is to be effective in making this crop serve for soil improve­ment, the soil must deliver calcium to these plants.

We may well imagine competition between the soil and the plant for the lime. The absorbing power of the soil for nutrients like calcium, potassium, magnesium and other substances is appreciable. It was demonstrated by means of better soybean growth, nodulation and nitrogen fixation, that placing the calcium on a small smount of soil to saturate it highly is more effective than is placing it on much soil to increase the soil saturation only slightly.

These effects from variable degrees of saturation of the clay by the plant nutrient demonstrated similar results regardless of whether the variable calcium was accompanied by acidity or by neutrality. The soybean growth proved that it was not the acidity that disturbed plant growth, but that it was the deficient soil fertility commonly present when soils become acid. Likewise it demonstrated the more efficient use by the plant of the applied calcium in a soil more highly saturated by it. It also suggests a higher efficiency for drilling soil treatments than for broadcasting them.

That calcium is needed for the legume bacteria as they live indepen­dently of their host has become a well known recognized fact. With limited lime supply they become abnormal, and fail to inoculate. But given plenty of calcium, they grow well and are effective inoculators.

Unless both the plant and the bacteria have access to calcium, effective inoculation cannot be expected. Lime for a legume—even an acid tolerant plant like the soybean—plays a helpful role because it nourishes the plant rather than because it removes soil acidity. Even an acid soil must supply lime for successful inoculation and growth of the soybeans.

Phosphorous and Soybeans

Phosphorus, like calcium, is a requisite if soybeans are to be active in nodule production and in nitrogen fixation. But its importance and behavior are closely related to the amount of calcium. Unless calcium is amply supplied, soybeans are poorly nodulated, and are poor nitrogen fixers for soil improvement. In fact they may even lose phosphorus back to the soil, so that the final crop will return less phosphorus when harvested than was in the planted seed.

Magnesium and Soybeans

That magnesium should be helpful toward better soybean inocula­tion has not come to our attention because relatively little magnesium is required by the plant, and most soils are not seriously deficient in this nutrient. This element is effective on soybeans, but probably indirectly as well as directly. It makes calcium more effective and thus illustrates the fact that fertility elements work together. These effects suggest an interstimulation among the elements, so that the final results can not be considered merely as additions of the values of their effects when applied singly.

Potassium and Soybeans

Improved inoculation may be also dependent on the potassium supply in the soil. Experimental studies demonstrated increased nodule production and better growth as potassium deficiencies of the soil were remedied. With more liberal amounts of potassium, however, particularly in contrast to the amount of calcium, inoculation may be less effective and give reduced nodulation and nitrogen fixation. Excessive potassium in relation to calcium makes the soybeans produce more tonnage, but they fix less nitrogen and become more clearly non-legumes than legumes. They move into the class of woody vegetation and out of the class of vegetation with high protein content of high nutritional value as animal forages.

Chemistry and Soybean Summary

Inoculation, or the introduction of nodule bacteria with the seeding of the soybeans, is not necessarily a practice that will compel the plant to accept the companionship of the bacteria. The latter cannot use cavemen tactics. Rather, the plant and bacteria will unite in their efforts toward getting their necessary nitrogen out of the gaseous supply in the atmosphere only when the soil provides liberally of all the nutrient elements required by both the plant and the bacteria.

Successful soybean growth on soils of declining fertility cannot be guaranteed simply by the introduction of particular pedigree microbes. The plant must first be healthy because it is well fed with calcium, phosphorus, magnesium and other soil-contributed elements. Unless it is well nour­ished by the soil, the inoculating bacteria will not associate with it to give it the one distinguishing character so desirable in legumes, namely, nitro­gen fixing capacity.

Inoculation, or the introduction of the bacteria, is no substitute for the high levels of soil fertility that are demanded for successful legume crops.

Here is the takeaway on calcium testing done by Dr. William Albrecht to soybeans:

  1. An experimental study was made of the effect of calcium on nodula­tion of soy beans on certain acid soils, with the hope of contributing to the knowledge of the role calcium plays in inoculation.
  2. The divided root system of soy beans, grown in acid soil on one side and calcium-treated soil on the other, gave differences in nod­ule production to as great an extent as those produced when plants were grown wholly within these same soils. This indicates (1) that calcium plays some physiological role in favoring nodulation; and (2) that its effects are local or restricted in increasing the number of root infections, at least within the periods of time used in this experiment.
  3. The addition of lime carbonate to an acid soil of pH 5.4, and already infected with legume organisms, gave a very marked increase in nodule production. It suggests that the effect of liming is not neces­sarily one of keeping alive the bacteria applied as inoculation, since in this case liming increased the number of nodules by organisms originally present in the soil. Evidently the lime carbonate exerted a physiological effect on the plants, and possibly on the organisms, to bring on the greater nodule production.
  4. The addition of small amounts of calcium chloride to an acid soil (pH 5.5) increased the viability of the legume organism, radicicola, of soybeans, applied to the soil by pure cultures, and stimulated nodulation of the host plant.
  5. Calcium taken up by the plant in its early growth influenced nodulation, since there was a difference in the nodulation of 10-day old calcium-starved and calcium-bearing seedlings when replanted to an already well inoculated lime-deficient soil of pH 5.4.
  6. This functioning of calcium within or through the plant to produce increased nodulation may have a fundamental histological or physi­ological basis in the plant, since running parallel with the effect on nodulation by calcium given the seedlings, there is a distinct differ­ence in the plant cell wall structure suggested by differences in ease of obtaining micro-sections of the 10-day old calcium-starved and calcium-bearing soy bean seedlings.

Source: Albrecht on Calcium

Soybean Plant Biology: History, Plant Structure & Growth Cycles

By Dr. Harold Willis

The soybean is a truly amazing and versatile crop plant. It is one of the oldest food plants, domesticated by 1100 BC in northeastern China. Its ancestor is a wild vine-like plant that 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 was grown and evaluated in the United States during the 1800s. The primary use for the crop was for forage, hay and green manure.

Soybean pods
A soybean plant at harvest time. Courtesy How to Grow Super Soybeans.

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 of Soybeans

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 corn belt 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.

Soybean 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 was 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.

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.

Soybean 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.

A young soybean plant. Courtesy How to Grow Super Soybeans.

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 where 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 few 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 fall off. This is nothing to be concerned about, since the plant is just getting rid of unproductive leaves.

Soybean 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.

Soybean roots.
Soybean roots.

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.

 Soybean 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 “semi­dwarfs.” There are also some intermediate varieties, called semideterminate, 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 from 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.

Soybean Light & Temperature

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 I 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.

Soybean 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 slow down rather suddenly (called senescence). The leaves slow down their photosynthesis and begin to turn yellow, eventually dropping off. Root nodules cease produc­ing nitrogen.

Soybean 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.

Source: How to Grow Super Soybeans