Strategic Farming: Field Notes Addresses Iron Deficiency Chlorosis (IDC) in Soybean

Jul 14, 2022

By Liz Stahl and Angie Peltier et.al

On the July 6, 2022, Strategic Farming: Field Notes episode, Dan Kaiser, Extension Nutrient Management Specialist, and Seth Naeve, Extension Soybean Specialist, joined moderators and Extension Educators Angie Peltier and Anthony Hanson in discussing iron deficiency chlorosis (IDC) in soybean. Yellow soybeans are showing up in fields throughout western Minnesota this year.

What is iron deficiency chlorosis (IDC)?

Iron deficiency chlorosis symptoms
in soybean. Photo by Angie Peltier

IDC does not occur due to a deficiency of iron in the soil. IDC is caused by the inability of some plant species to extract iron from the soil in a plant available form. This results in yellowing, or chlorosis, of leaves between the leaf veins while the veins remain dark green. A plant requires iron to make chlorophyll, the green pigment vital to photosynthesis. A large amount of iron is usually in the soil, but it is not in the soluble form required by plants. IDC is particularly a problem in higher pH soils and in soils with large amounts of calcium carbonate.

Conditions that favor IDC

Kaiser and Naeve discussed soil conditions that can affect the availability of iron, such as high soil pH, compaction, high soluble salt concentration and poor soil drainage. Environmental conditions also play a role in the development of IDC.

The amount of bicarbonate in the soil correlates with IDC in soybean - the more bicarbonate in the soil the higher the occurrence of IDC. Bicarbonates are formed in the soil typically when water-logged soils trap carbon dioxide. Bicarbonates aren’t stable, so when the soil is exposed to aeration, bicarbonates break down to carbon dioxide, which can then leave the soil.

There is also a linkage between high soil nitrate content and IDC. IDC issues can be greater where residual nitrates are higher in the soil. Kaiser noted there may be more IDC in some fields this year due to higher levels of residual nitrates after last year’s drought conditions, particularly in the Red River Valley. Naeve also noted the importance of considering soybean root depth. Water, nitrate and the amount of salts in the soil can change throughout the profile, impacting the incidence of IDC as soybean roots grow deeper in the soil profile.

Managing IDC

Planting a tolerant variety is the number one defense against IDC in soybean. Research shows that tolerant varieties have a higher yield potential than less tolerant varieties where IDC is a problem, but an even greater yield potential may occur when combined with other practices that reduce IDC.

Additional practices shown to be effective in managing IDC include using an ortho-ortho EDDHA iron chelate source applied in a band directly on the soybean seed at planting. The greatest economic advantage can occur when those areas of a field with a history of IDC symptoms are targeted with an in-furrow iron-chelate at planting. Foliar applied chelates however, may only green up tissue that comes into contact with the product. New vegetative growth after treatment may still be yellow, as iron is not mobile in the plant. Placement of an ortho-ortho iron chelate with the seed has provided the best return on investment where levels of IDC have been moderate or high.

A companion crop of oats with soybean has also been shown to help reduce IDC. The oats help take excess nitrate and moisture from the soil. In fields where IDC has been a problem, soybeans grown with oats have greened up and yielded better than soybeans without an oats companion crop. It is important, however, to terminate the oats before they reach more than about 10 inches in height, as competition with the soybean crop for water could be an issue when moisture is limiting.

Attempts to acidify the soil may sound good in theory, but in practice, the majority of our soils have a high buffering capacity, which means they have high resistance to a pH change. It would take a large amount of elemental sulfur to result in a very small drop in pH, and pH levels would likely then rise back to where they were within two to three years.

Source : umn.edu

Video: Turning Plant Defense Into a Management Strategy

Turning Plant Defense Into a Management Strategy

Understanding how a plant responds to stress is one thing.

Using that knowledge to make better management decisions is another.

Systemic acquired resistance, or SAR, is the plant's more direct defense response. When stress or infection occurs, the plant begins signaling throughout its system and preparing defensive compounds.

But if we know that response exists, can we help prepare the plant before the stress arrives?

The answer starts with understanding what triggers the response and what the plant needs to carry it out.

The Trigger and the Fuel

Salicylic acid plays an important role in triggering the SAR pathway.

Think of it as turning the truck on.

The engine may be running, but it still needs fuel to do the work.

In this case, manganese plays an important role in supporting the enzyme systems involved in the plant's defensive response.

This makes manganese status an important part of the conversation. Whether a producer is using tissue testing, sap analysis, or simply scouting for visible deficiency symptoms, the goal is to make sure the plant has adequate manganese available.

Manganese is required in relatively small amounts, but that does not make its role small.

If the plant receives a signal to defend itself but lacks the nutrition needed to support that response, it may struggle to carry out the process efficiently.

The trigger matters.

The fuel matters too.

Prepare Before the Stress Arrives

The best time to think about stress management is before the plant is overwhelmed.

Once a crop is already struggling, management can quickly turn into a game of catch-up.

This is similar to nitrogen management. Once a plant becomes severely deficient, correcting the problem does not necessarily erase the time and yield potential already lost.

Plant defense can work the same way.

Low-rate, targeted approaches designed to support the SAR pathway may fit best ahead of an expected stress event rather than after significant damage has already occurred.

That requires producers to think about predictable stress.

We may not know exactly what the weather will do tomorrow, but we generally know summer heat is coming. We know certain field conditions increase disease pressure. We know a herbicide application can temporarily stress a crop as the plant processes the chemistry.

Even a properly timed and labeled herbicide application can create a temporary response in the plant.

That does not mean the herbicide is bad.

Weeds can create significantly more yield loss than the temporary stress caused by controlling them.

The question is not whether we should eliminate every stressor.

The question is whether we can better prepare the plant to manage necessary and predictable stress.

Not All Stress Is Bad

Stress is a normal part of plant growth.

A perfectly stress-free environment does not exist in the field.

In fact, some stress is necessary for normal plant development. A plant responds to wind, temperature, moisture, sunlight, and countless other environmental signals throughout the season.

The environment is stress.

The plant's job is to manage it.

Problems begin when the stress load becomes greater than the plant's ability to respond.

Extreme heat, drought, high salt concentrations, disease pressure, and even certain management practices can add to that load.

This is where understanding SAR becomes useful.

Instead of waiting until the plant is visibly struggling, producers can begin identifying periods when stress is likely and make management decisions around those windows.

ISR Starts With the Soil

While SAR is a more direct defense response, induced systemic resistance, or ISR, brings the conversation back to soil health.

The longevity of a farm is closely connected to the health of its soil.

Carbon plays a major role because it supports biological life within the soil. Bacteria, fungi, and other organisms interact with plant roots and influence how the plant grows, accesses nutrients, and prepares for stress.

This is why soil health cannot be reduced to one product or one application.

It is a system.

Keeping living plants in the field longer can support biological activity. Cover crops may fit some operations. Better water management can improve soil conditions.