Abiotic Soybean Problems

Herbicide injury

With nearly all herbicides, there is a certain risk of crop injury when they are used.  For instance, application of a post-emergence herbicide may injure plants stressed by cool, wet weather or hot, dry conditions.  Soil pH can increase the activity or decrease the rate of breakdown of some herbicides resulting in crop injury.  Soybeans can be damaged by herbicide carryover, excessive rates of application, application of the wrong chemical, overlaps, uneven application, incorrect incorporation or drift from nearby fields.

Growth Regulators

Growth regulator herbicides include some of the more effective chemicals for perennial broadleaf weed and brush control and include such products as 2,4-D, Clarity, Banvel, Tordon, Stinger, etc.  These herbicides are known as growth regulators because they mimic the natural growth hormones of the plant, and thus, upset the natural hormone balance in plants. The killing action of growth-regulating chemicals is not caused by any single factor, but rather by the disruption of several growth processes in susceptible plants. 

With 2,4-D containing products, young leaves will be thickened, puckered and have a strap like appearance due due to the parallel growth of the leaf veins.  Leaves will often be cupped downwards as well. Injury is most severe when there is direct drift from sprayed ditches, adjacent pastures and chemical fallowed land or if spray tanks have not been thoroughly cleaned from a previous application.  In these instances, serious losses may occur. Because 2,4-D readily  volatilizes, it can occasionally drift several miles before injuring a soybean field.  In these cases, yield loss is usually negligible.

2,4-D injury is sometimes confused with soybean mosaic virus symptoms.

Dicamba containing products such as Banvel and Clarity will cause a shortening of the veins and the leaf tip that will result in an upward cupping of the leaf. Symptoms from products such as Tordon, Stinger and Crossbow will be similar to dicamba.

Triazine injury

The most common triazine herbicides used are atrazine and metribuzin (Sencor).  These herbicides are classified as photosynthetic inhibitors.  They work by disrupting photosynthesis. They are absorbed both through the roots and the shoots.  Plants are not affected by the herbicide until after they emerge and begin photosynthesis.

Plants will exhibit interveinal chlorosis and necrosis beginning at the margins of the oldest leaves and progressing toward the center of the leaves. Symptoms are likely to be worse in areas of high pH, or where application overlaps occurred. In the severest of cases, the plants may turn brown and die.  Symptoms are similar to iron chlorosis, but iron affects the newest leaves first.

Amino Acid Synthesis Inhibitors (ALS)

The acetolactate synthase (ALS) inhibiting herbicides (e.g., Glean, Classic, Pinnacle, Beacon, Peak, Scepter, Pursuit, FirstRate, etc.) have a broad spectrum of selectivity and are used at low rates as soil-applied or postemergence treatments in a variety of crops. ALS herbicides are readily absorbed by both roots and foliage and translocated in both the xylem and phloem to the site of action at the growing points.

Look for patterns in the field that follow application patterns or changes in soil pH. Injury symptoms caused by ALS inhibiting herbicides are not apparent until several days after treatment, although susceptible plants stop growing almost immediately.  Affected plants can exhibit stunting, interveinal chlorosis, red leaf venation, purpling, root pruning (also called "bottle brushing"), and gradual death.  The risk of crop injury with some of the products is more likely on high than low pH soils.

Nutrition problems

In Kansas, soybeans have relatively few nutrition problems.  The most common are iron chlorosis in the western areas of the state where calcareous soils with high pH tie up iron, and potash, which is common in east central and southeast Kansas.  Where fields have not had a recent history of soybeans and a nitrogen fixing bacterial inoculant has not been applied, nitrogen deficiency may also occur.

Iron

Potash

Potash deficiency symptoms regularly occur in east central and southeast Kansas soybean fields.  While not as common, they can occur in other parts of the state, particularly when soils are excessively dry.

Deficiency symptoms usually first appear on lower leaves as pale green to lemon-yellow leaf margins.  The discoloration may occur on just the the edge or extend inward to as far as half-way to the middle of the leaf.  Under severe deficiency, there may be browning or necrosis of the leaf margins.

Potash deficiency symptoms are often temporary, particularly when induced by dry weather.  They will often disappear following rain.  Symptoms are also similar to those caused by soybean cyst nematode and some ALS inhibiting herbicides so caution should be taken to rule these out as potential causes of the problem.

Nitrogen

Soybeans, because they are a legume, can form a symbiotic relationship with rhizobial bacteria in the soil that allows them to provide their own nitrogen.  Nitrogen deficiency symptoms can occur however in fields that lack a population of rhizobial bacteria.  This most often occurs in fields that have never been planted to soybeans or fields that have not been planted to soybeans in many years.  The other situation where N deficiency occurs is during times when soils are very wet.  The rhizobial bacteria do not function in water saturated soils do to a lack of oxygen.

Soybeans deficient in nitrogen are generally a pale green.  As the symptoms progress, the leaves will become uniformly yellow.  Some stunting may also be present.  When roots are examined, there will be few if any bacterial nodules.

Soil compaction

As growers manage larger acreages, they are often forced to do field operations under less than ideal conditions.  This means that it is often more wet than desirable when fields must be entered and significant soil compaction can occur.

Soybeans growing in compacted soils will show a general stunting.  This can easily be confused with soybean cyst nematode damage or herbicide problems.  The diagnosis of compaction problems will require the digging up of plants.  When compaction is present, the tap root will show lateral or twisted growth at the point where the compacted layer occurs.