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Agronomic Crops Network

Ohio State University Extension


Smoke from Wildfires Affecting Ohio Agriculture? Or Some Other Stressors?

Figure 1. Waterlogged soils causing reduced nodulation, stunting, and yellowing. (Delaware County, June 29, 2023)

Wildland fires in Canada this year have so far burned over 20 million acres with approximately 4 million of those acres in Quebec. In spite of these fires being hundreds of miles away from us, smoke generated has been driven by northern winds impacting the northeastern and midwestern US. There has been some growing concern about the impact this smoke will have on agriculture in Ohio. Of particular concern are toxins in the smoke.   

Wildland fire smoke is made up of small particles, gases, and water vapor. Water vapor makes up the majority of smoke. The remainder includes carbon monoxide, carbon dioxide, nitrogen oxide, and small amounts of other compounds. Among the particulates in smoke are many compounds which do not turn into a gas during the fire, such as calcium, sodium, and magnesium. As particulates, these substances can be carried down-wind and have a minor fertilization effect. Another compound produced by wildland fire is dioxin. Dioxins are compounds produced as byproducts of some processes including smoking cigarettes, paper production, industrial and structural fires, and wildland fires.  Dioxins are known to be cancer causing. The quantity of dioxin produced from wildland fire is highly variable and will depend largely on the population of fungi in the forest. 

While the smoke from Canada’s fires is reducing air quality and visibility across the Midwest, it is highly unlikely that these fires are going to have any kind of impact on Ohio agriculture. Ohio is a significant distance from the fire locations, and the smoke has dispersed hundreds of miles in all directions, thereby reducing the quantity of smoke and its various components. Combined with the low quantity of toxins in wildland fire smoke to begin with, we can see that there is very little to worry about in terms the impacts of this smoke on crops or livestock.

Although wildfires are not likely to be the cause, there are several soybean issues popping up in areas of the state:

1. Poor nodulation. Yellow soybeans that can also be somewhat stunted are often indicative of poor nodulation (Figure 1). Nodules are small knots found on roots, often near the top of the root system. Nodules are the result of a symbiotic relationship between soybean and bacteria (Bradyrhizobium japonicum). These bacteria convert nitrogen into a form that is usable by the soybean plant. Nodulation and nitrogen fixation by Bradyrhizobium japonicum is reduced in wet soils. Plants should be able to recuperate nodule function when normal (aerobic) conditions are restored. To determine if a nodule is actively fixing nitrogen (i.e., converting nitrogen to a usable form), split the nodule with your fingernail and examine the inside. If the inside of the nodule is pink or red, nitrogen is being fixed.

2. Overall slow growth and poor root development. While cooler weather in April and May limited crop stress symptomology to some degree, dry conditions can lead to slow root growth and poor nodulation in soybeans. Root exploration is key to moisture acquisition and nutrient uptake. Dry conditions early (paired with cooler temperatures) may have slowed initial root development and formation. Planting deeper (as may have been done to reach moisture depending on location and planting date) could also affect root development by slowing the accumulation of early GDDs; in corn, soil accumulated GDDs affects early-season growth more than air temperature GDDs.

In many parts of the state the dry conditions were replaced within the span of a few days with the other extreme in the form of waterlogged soils and excess water conditions. Very few studies have been published examining subsequent stresses of drought followed by flood, but one article from cabbage suggests drought followed by flooding was worse for crops than drought alone. In corn, roots will form arenchyma in central cortical cells to cope with waterlogging. Soybean aerenchyma formation, though, requires creation of a new cell layer near the outside of the roots that contain the internal air pockets. This will typically occur near the water line for flooded plants (Figure 2).

Another typical symptom is that when stressed soybeans are removed from the soil the outer cortical layer may easily slough off when handled (see this article from 2021 to help discern flood damage and root rots). The characteristic “rat tail” cortex remaining is a key indicator that flood damage occurred. The rapid change from drought to waterlogged conditions paired with below average nodulation is a likely contributor toward our yellow soybeans this year.

Figure 2. Soybean roots from plants exhibiting yellowing in 2023, showing adventitious root formation and symptomology consistent with waterlogging. Photo credit: Dr. Horacio Lopez-Nicora

3. Potassium deficiency. Potassium deficiency may be observed when soil test K levels are low or could be induced by dry weather (Figure 3). Yes, it’s strange to discuss both waterlogging and drought conditions in the same article, but various areas of Ohio have seen one (or both) of these extremes this year. [We reported more on potassium and drought in this article.] Soil testing will help determine if a potassium deficiency is due to low soil test potassium or dry weather. If the cause is dry weather and soil moisture is replenished, these symptoms should go away with time.

Figure 3. Yellowing around leaf margins is an indication of potassium deficiency (Madison County, July 5, 2023)

4. Diseases. We will expand on diseases in an upcoming article. However, there have been several disease issues noted within Ohio. Many plants have been collected and sent to the Department of Plant Pathology and are currently being tested for pathogens.

Be sure to watch the weather and crops over the next few weeks to see how they respond. Many soybean plants may start to create new roots as the soil dries, and it would be good to check for new nodule formation. If corn height still allows for sidedress N application to supply the remaining seasonal N budget still plan on making that application once soil moisture allows. In recent work from Ohio State, corn still responded positively to N sidedress application after 3 days of flooding or after repeated 3 day floods (3 days flooded, 3 days drying followed by 3 additional days flooded).


Kunert et al. (2016). Drought stress responses in soybean roots and nodules. doi: 10.3389/fpls.2016.01015

Thomas et al. (2005). Aerenchyma formation and recovery from hypoxia of the flooded root system of nodulated soybean. doi: 10.1093/aob/mci272

Barber and Muller (2021). Drought and subsequent soil flooding affect the growth and metabolism of savoy cabbage. doi: 10.3390/ijms222413307

Novais et al. (2023). Elucidating how N management practices and excess water conditions affect corn N uptake and grain yield.

Swan et al. (1987). Estimating corn growth, yield, and grain moisture from air growing degree days and residue cover.

Crop Observation and Recommendation Network

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.