Soybeans may tolerate drought better than corn, but the reason may be their ability to camouflage their drought stress better than corn. Corn plants will roll their leaves and resemble pineapple plants, but beans just look like beans when the water is shut off. At least until you take a really close look.

Windshield surveys of drought-stressed soybeans will not be very productive. Save the gas. But if you want to determine the degree of stress your soybeans are demonstrating, Purdue soybean specialist Shaun Casteel says you have to look closely at the plant. 

His report on soybean stress from drought is laden with pictures that will help you determine how much your fields are suffering from the lack of moisture.

Casteel says the soybean plant will show stress within the cell and in metabolic processes, which will not be visible to the untrained observer. However, he says physiological development will be noticeable. 

  1. While somewhat difficult to assess in the field, he says the first sign is the reduction in leaf growth and expansion, which is obvious after several weeks in which the plant should have been growing and forming a canopy in the field. The plant is really shifting its internal resources toward root development in an effort to find water, rather expanding vegetative and reproductive functions.
  2. The next sign, which is more obvious is leaf-flipping, which is parallel to leaf-rolling in corn, in an effort to reduce the loss of moisture through transpiration. The more silver colored under side of the leaf will be a dominate indicator of stress as leaves point toward a more upward orientation and keep the top of the leaf protected. However, that reduces the photosynthesis in the plant, but still conserves valuable water resources. Casteel says if leaf flipping occurs early and often, the severity of drought stress is worse.
  3. Soybeans that are severely drought stressed will clamp their leaves together as if forming a taco shell. The outer leaves of the trifoliate will cover the inner leaf in an effort to reduce water loss. These processes can happen at any time in the life of the plant.
  4. Drought stress will also have an impact on flowering, other than just fostering more bloom abortion. Casteel says drought stress can cause young soybeans, even as young as the V-3 stage with just three leaflets to begin flowering, in an effort to produce a seed while the plant is still living. It is in a survival mode and that initial bloom will only be a couple inches off the ground. 

Casteel notes that, “Implications of early season drought can be long-lasting or short-lived in soybean. Many of our soybean fields are limited due to drought because of reduction in photosynthetic factories (vegetative biomass) in addition to limited nodal production.” 

And he adds that soybeans have a longer grace period than corn in terms of responding to a drought, since there is not a brief pollination period as in corn. But he says there may be shorter production of nodes on drought-stressed soybean plants. However, he says beans may compensate for that with flowering, pod development and seed fill.

Soybeans are going through the same drought stress as is corn, but may hide that stress more than the corn plant’s leaf rolling. Soybeans will decrease vegetative growth initially, and then flip leaves away from the sun to reduce water loss through transpiration, and then clamp their leaves together. 

Early flowering in drought-stressed fields is also an indication the plant is in a survival mode.