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Causes of yellowing soybeans

Corn isn’t the only crop that’s turning fields yellow right
now.  A number of soybean fields
are showing a slight yellowing of leaves, and may need additional
fertilization, according to Fabian Fernandez, University of Illinois Extension
specialist in soil fertility and plant nutrition.

This time of year if soybeans are showing signs of
yellowing, it is likely the result of stressful growth conditions.  “Many soybean fields are starting a
rapid shoot growth phase. 
Meanwhile, the root system, which in some instances has not developed
very well, is not yet able to meet the increasing nutrient demands, causing
what is likely a temporary nutrient deficiency,” Fernandez said.


Wet soil
conditions

The wetter-than-normal soil conditions are to blame again—this
time for unhealthy soybeans.  Water
is critical for dissolving nutrients and transporting them to the soybean
plant, but in excess, it can block other important processes.

“Temporary nutrient deficiencies can be observed when excess
water in the soil depletes oxygen and builds up carbon dioxide levels.  Although oxygen is needed by roots to
grow and take up nutrients, high carbon dioxide levels are toxic and limit root
growth and activity,” Fernandez said.

Low sunlight conditions usually occur with wet soil
conditions, which further exacerbate the health of soybeans.  Clouds reduce photosynthetic rates and
nutrient uptake by the crop.


Dry soil
conditions


The opposite condition—dry weather—can have similar negative
effects on soybean health, although it is not as much of an issue this
year.  Fernandez says when the
surface layer of the soil is too dry, the root system of the plant is small and
shallow, and limits nutrient movement to the rest of the plant.


Reduction
of Nodulation

Another explanation for yellow soybean leaves is the
reduction of nodulation and/or the number of active nitrogen-fixing
nodules.  Fernandez says soybeans
depend on a symbiotic relationship with these nodules to supply needed
nitrogen, but under stressful growing conditions, they have a reduced amount of
nitrogen available.


Soil compaction


Crop Talk contributor and northwestern Ohio planter, Pupdaddy, said deficiency symptoms in
his crops are likely due to lack of Manganese and calcareous soil (high
calcium) conditions that tend to have a high pH.  “I’ve got a field of lake-bed clays that I’ve no-tilled for
20+ years, and while the surface has nice tilth to it, the subsoil has actually
gotten so tight that I see Mn deficiency every time I plant soybeans on it,” he
said.

When the environment is to blame
for harming crops, Fernandez said there is normally little to be done to solve
the problem.  Fortunately, adverse
environmental conditions are often temporary and unlikely to result in yield
loss.

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