How to Solve Common Soybean Problems

By Dr. Harold Willis

Agriculture tends to be full of unpleasant surprises. Things seldom seem to go right. The weather doesn’t cooperate—too wet, too dry, too hot, too cold. Weeds, pests, diseases, all proliferate. Most of these problems seem beyond the farmer’s control, but there are things you can do to largely prevent or overcome most of the things that go wrong. The following is a list of common problems seen with soybeans, and how to solve them.


Emergence

The first problem likely to strike soybeans is difficulty for the seedling to break through a surface crust. This is an emergency, and you should break the crust immediately with a rotary hoe or cultivator. Driving on emerging soybeans will not significantly reduce stands. The problem can be prevented by getting good soil structure so that crusting will not occur. This can be done by adding compost or light amounts of manure or poultry litter to the soil; that is, increase the soil’s organic matter content. The organic matter should be worked into the upper several inches of soil.

common soybean problems - a healthy soybean
Healthy soybeans depend on healthy soil.

Weeds

Weeds compete with soybeans for moisture, soil nutrients and sunlight, reducing yield. They can also interfere with harvesting, and their seeds contaminate harvested soybean seed. Weeds should be controlled if the probable yield loss (dollar loss) exceeds the cost of control. Weeds can be controlled by management, mechanical and chemical control. Chemical control is rapidly becoming less an option. Heads-up bio-farmers feel themselves secure in the knowledge that weed control is based on fertility management, and not in buying a more powerful poison from Dow Chemical or Monsanto.

For weeds that have germinated, control must be made within the first four weeks after soybean emergence to prevent yield reduction. Weeds that emerge after six weeks will have little effect on yield. Weeds growing before planting should be killed by seedbed preparation tillage. Soybean seeds should have good soil-seed contact to get the seedlings off to a rapid start so that the soybean foliage will shade out weeds.

Deep-planted seedlings grow more slowly than shallow-planted ones. Leaving a rough between-row seedbed will slow down weeds. Narrow rows allow soybeans to shade out weeds more quickly.

Before soybeans are one inch high, rotary hoeing is an effective method of killing emerging weeds. It works best at relatively high speeds (8 to 12 miles per hour) late in the day when soybean seedlings are less brittle. A shovel cultivator is effective on small or larger weeds when soybean seedlings are a little larger. Shovels should be set at a shallow depth (1 to 2 inches) to reduce pruning of soybean roots. A rolling cultivator can also be used effectively for small weeds when operated at 6 to 10 miles per hour. These mechanical methods cannot be used in narrow-row or solid seeded stands.

The use of herbicides to control weeds in soybeans has grown gradually since World War II, and is now both questioned and under fire. Newer, more powerful herbicides have been developed. Some kill broadleaf weeds, others grasses. Some are applied before planting, others after soybean emergence. Some are incorporated into the soil, others are surface-applied. Some are more toxic than others, but they are all designed to kill living plants and thus should be used with great care if used at all. The Acres U.S.A. position is that they do not belong in a sound management program. Many problems can arise, including killing (or damaging) soybean seedlings, spray drift into unwanted areas, carryover into succeeding years, toxicity to beneficial soil organisms, and human or animal toxicity. Since most weeds can be eliminated by having healthy, balanced soil, herbicide use should be unnecessary.

Most of the weeds afflicting soybean growers prefer to grow in poor, out-of-balance, waterlogged or poorly aerated soil. These include quackgrass, giant foxtail, Johnsongrass, smartweed, bindweed and velvetleaf. I know it will be hard for some to believe, but the above weeds, and many others, grow best in “sick” soil; in healthy, balanced, well aerated soil, they grow poorly or not at all. A 2:1 phosphate to potassium ratio (as shown on water-soluble soil tests) will help eliminate weeds. Only a few weeds grow well on good soil. These include lamb’s quarters and redroot (rough) pigweed. Milkweed, purslane and cocklebur grow well on fairly good soil.

Diseases

About 50 diseases attack soybeans in the U.S. with viruses, bacteria, fungi and nematodes (roundworms) being involved as pathogens. There are seed and seedling diseases, root diseases, stem and leaf diseases. Some can cause serious yield or quality losses, whereas others cause little problem. The worst are the fungal Phytophthora rot and the root knot nematode. Some varieties are resistant to these two diseases.

The thing about diseases is that they only attack plants that are already under stress. That is what really causes the disease. The so-called pathogens—bacteria, fungi, worms—then move in to destroy the sick and unfit plant. Nonsensically, we treat the symptoms and spray toxic chemicals to kill the pathogens and save the sick, poor quality crop. (Unfortunately farmers feel they have to do this to survive in farming today.)

The simple solution to crop diseases is this: healthy, vigorous plants growing in good, balanced soil don’t get sick. They have the ability to resist disease pathogens in much the same way as our bodies resist germs, as long as we eat right, get plenty of rest and exercise, and avoid stress. Plants often resist diseases by producing substances that prevent pathogens from growing; some of these are called phytoalexins.

Of course, the way to grow healthy plants is to have fertile soil with good structure. As we have mentioned earlier, this means well aerated, loose soil with high levels of humus and beneficial soil organisms, and high, balanced levels of nutrients (high calcium is essential for disease resistance, for example). Some of the beneficial soil organisms actually protect a plant’s roots from pathogenic bacteria, fungi and nematodes. Others channel nutrients and water into the roots, helping the plant to grow.

If your soil is in good shape, the only other stresses that could affect the plant are from adverse weather. But even here, good soil can counteract most weather stresses. Loose, well drained soil will soak up heavy rain. Humus holds water in drought conditions, and the sticky secretions of soil microbes also help drought-proof soil. Friable soil will not crust and will allow plenty of air to reach roots and nitrogen-fixing bacteria. Most disease pathogens cannot live in well-aerated soil.

Pests

The story about soybean pests is about the same as for diseases and weeds. Many species of insects and mites will attack soybeans, although only a few cause economically serious damage. Some eat seeds and roots, others attack stems, leaves or pods. Some chew, while others suck juices. Again, the scientists come to the rescue by spraying toxic chemicals (and a farmer may feel he has to resort to that technology to save a crop), but again, the simple solution is the same—healthy, vigorous plants growing in good soil. Such plants are naturally immune or resistant to pest attack. The pests either avoid the plant altogether or else just take a few nibbles and then go away. As with herbicides, toxic fungicides, insecticides, and other pesticides, their use should become unnecessary after ecological farming principles have been used for a year or two.

Nutrient Deficiencies

If a plant is not getting the proper amounts of nutrient elements, it may develop certain symptoms, abnormal colors or growth deformities. By the time these symptoms appear, it is often too late to do much to alleviate the problem (unless plants are still small or you can foliar feed them), but you can try to trace the cause and overcome it for the future.

Nutrient deficiency symptoms may not necessarily mean one or more elements are deficient in the soil. The nutrient may be adequate but the plant may not be able to take it up, perhaps because of high or low pH, or too much or too little of some other soil element. Or perhaps stress on the plant from drought, wet soil, cold weather or toxic soil conditions causes roots not to absorb the element. Nutrient deficiency symptoms in soybeans are listed in the accompanying box.

soybean symptoms

Sick Root Nodules

Healthy, active nitrogen-fixing root nodules will have a pink or reddish internal color. You should monitor the health of these furnishers of free nitrogen by occasionally digging up a plant and examining its root nodules. They should be abundant and pink when cut open. If the nodules are few in number or greenish or yellowish inside, something is wrong in the soil. Perhaps there are toxic substances (from pesticides or too much raw organic matter) in the soil which are harming the bacteria.

Perhaps the soil is poorly aerated, since nitrogen-fixing bacteria need oxygen and nitrogen from the air. Perhaps there is a molybdenum deficiency or low pH (acid). Perhaps the soil already has plenty of nitrogen, either from past fertilizers or from heavy manure application (this is not necessarily a problem, however). Sick root nodules are just one more symptom that you may be able to use to trace down and solve a problem.

Foliar Feeding

If your soybeans seem to be lagging in growth or standing still, perhaps because of weather stress (cool, cloudy weather), you may be able to give them a “shot in the arm” and pull them out of it. With a high value crop such as soybeans, it may be economically feasible to foliar feed to help the plants along.

Source: How to Grow Super Soybeans