Controlling Soybean Cyst Nematode: Do you have a resistance problem?

This article originally appeared in the AGVISE Laboratories Winter 2022 Newsletter

This is the third year of our soybean cyst nematode (SCN) resistance project. Each year, we have flagged spots in soybean fields and collected paired SCN soil samples in June and September. If the SCN egg count increases through summer and into fall, we can quickly learn if the soybean SCN-resistance source, either PI88788 or Peking, is working or failing. University SCN surveys have found that the PI88788 resistance source has begun to lose its effectiveness at controlling SCN populations in much of Minnesota. This is a particular problem because 95% of SCN-resistant soybean varieties still use the PI88788 resistance source.

SCN egg count and soybean yield data from the 2021 AGVISE SCN resistance project. Bars of the graph represent SCN egg count, lines of the graph represent soybean yield. Click on the graph for a higher resolution version.

In 2021, paired soybean variety comparisons with SCN soil samples and soybean yield data really helped us see the difference in these SCN resistance sources. Among the sites, the Peking resistance source always had a lower SCN egg count than the PI88788 comparison, indicating that the Peking soybean varieties had better control of the SCN population at 4 of 5 sites. The Alberta site had similar SCN population control with both PI88788 and Peking resistance sources, so the soybean yield was similar at the site. However, the other sites demonstrated SCN resistance to PI88788, and the resulting soybean yield with the Peking resistance source was better with 7-bu/acre soybean yield increase on average.

For 4 of 5 sites, it is apparent that a Peking-traited soybean variety is the better choice. To learn if you have SCN resistance problems in your field, the simple early-late SCN soil sampling exercise, like we did in this project, is a quick way to learn if your current soybean variety is still controlling SCN and delivering the best soybean yield.

 

 

How much residual soil nitrate is left after the 2021 corn crop?

It’s probably more than you think.

So far, the residual soil nitrate-nitrogen trend following corn is much higher than average across the upper Midwest and northern Great Plains. This follows the same trend set by the 2021 wheat crop. For many growers in the region, the hot and dry growing season has resulted in high residual soil nitrate-N carryover where corn yield was lower than average. An update on average residual soil nitrate-N after grain and silage corn, broken into zip code areas, can be found below (Table 1). This data highlights the importance of soil sampling for nitrate-N, even after high N-requirement crops you may not think of leaving much residual soil nitrate-N behind.

Bar graph showing median residual nitrate-N in lb/acre for fields sampled after grain corn as of Oct. 11, 2021. Results include fields tested in MN, ND, SD, and MB. Fields tested thus far are on pace to set a record for amount of nitrate-N left after corn.

The early soil nitrate-N trend data gives us a snapshot of the soil samples that AGVISE has analyzed so far. The average soil test data is not a replacement for actual soil test results on your fields or your clients’ fields. There is considerable variability within a single zip code area, with some corn fields having less than 20 lb/acre nitrate-N and many other fields that are much higher. Take a look at eastern South Dakota, the Sioux Falls and Watertown areas have over 49% of soil samples with more than 100 lb/acre nitrate-N (0-24 inch soil depth). Considering sky-high nitrogen fertilizer prices (and still rising), it makes sense to soil test for nitrate-N and credit it toward next year’s crop nitrogen budget.

Agronomic considerations for soybean in 2022

One crop that will not benefit from extra residual soil nitrate-N after corn is soybean. Soybean can create its own nitrogen thanks to a symbiotic relationship with nitrogen-fixing bacteria. The nitrogen fixation process takes energy, however, and if there is already ample plant-available nitrate in the soil, soybean will delay nodulation and take advantage of the free nitrate. Delayed nodulation may ultimately lead to soybean yield loss.

High residual soil nitrate-N can also increase soybean iron deficiency chlorosis (IDC) severity.  Soybean IDC is a challenge for growers in the upper Midwest, northern Great Plains, and Canadian Prairies, especially on soils with high carbonate and salinity. If soil nitrate-N is also high, research has shown it can make soybean IDC even worse and result in lower soybean yield. If you plan to grow soybean on fields with high residual soil nitrate-N, seriously consider IDC-tolerant soybean varieties or consider planting them on fields with lower residual soil nitrate-N.

Should a corn-corn rotation be considered after a drought year and high soil nitrate?

Planting a second corn crop would allow a producer to capture this “free” nitrate-N in the soil profile. However, planting corn on corn has many challenges from soil moisture to insect pressures (e.g. corn rootworm). The 2021 corn crop started the growing season with a full profile of water (due to excessive moisture in 2019 and adequate moisture in 2020) and ended with enough to push the corn crop through harvest. Going into the 2022 growing season, plant available water will be considerably less than the beginning of 2021. If the drought continues into 2022, remember that corn requires more moisture than soybean, so planting corn on corn means putting a higher water-requiring crop on ground that had less water to start with (versus corn following soybeans). Less available moisture, combined with other agronomic pressures, may mean less than expected yield for a corn-on-corn rotation.

Table 1. Residual nitrate trends as of Oct. 11, 2021 from more than 2,500 soil samples taken after corn. Regions with less than 60 soil samples are not included in the table.

Updated Residual Soil Nitrate Trends (Variability is high this year)

The 2022 growing season may seem like a long way off, but spring will be here before we know it. In fact, many growers are already making (or have made) crop choices and seed variety decisions for 2022. One factor that must be considered when making crop and variety selections for 2022 is residual soil nitrate-nitrogen following the 2021 growing season. For many in the northern Great Plains and Canadian Prairies, the 2021 growing season was hot and dry, which resulted in high residual soil nitrate levels following many crops. An update on average residual nitrate levels after wheat, broken down by geography, is below (Table 1). Residual soil nitrate-nitrogen following other crops, including soybean, are also higher than average (Table 2). This highlights the importance of soil sampling, even after crops we do not typically think of leaving high residual soil nitrate behind.

The data in the tables represents a snapshot of the samples we have tested so far this fall. While the average residual soil nitrate-nitrogen for an area may be interesting to talk about, it is not a replacement for actual soil test results from you or your growers’ fields. The data shows that over 30% of the wheat fields in many areas (see the right-hand column of the table) test over 100 lb/acre soil nitrate (0-24 inch depth). Droughts like 1988 and 2021 are very uncommon and leave us in situations that we are not used to dealing with. Using an average soil nitrate level from a region to decide an N rate on an individual field would be like deciding to apply an insecticide on every acre of the farm without even looking at each field to see if the insect is present. You need actual soil test data on each field to make informed decisions.

Table 1. Residual nitrate trends as of Sept. 17, 2021 from more than 20,000 soil samples taken after wheat. Regions with less than 100 soil samples are not included in the table.

Table 2. Residual nitrate trends as of Sept. 17, 2021 for crops other than wheat. Regions with less than 100 soil samples for each respective crop are not included in the table.

High Fertilizer Prices

According to the September 15, 2021 DTN fertilizer price survey, retail fertilizer prices continue to rise. The average price per pound of nitrogen by fertilizer product is $0.61/lb N for urea, $0.46 lb/N for anhydrous ammonia, and $0.66/lb N for UAN-28. This represents a 55%, 73%, and 71% increase in price compared to prices for the same fertilizers this time last year. Long story short, fertilizer is expensive. High residual soil nitrate following wheat may help reduce input costs in 2022, as long as you know what the residual soil nitrate in your fields is and take advantage of it by growing a crop that requires nitrogen fertilizer. If you have a soil nitrate test of 80 lb/acre (0-24 inch) after wheat, that is about 50 lb more than normal carry over. The extra 50 lb/acre soil nitrate is worth $30.00/acre (based on the current urea price).

Sampling Fields for SCN

Soybean cyst nematode (SCN) is a microscopic, parasitic worm that attacks the roots of susceptible soybean and dry edible bean, causing unseen or unexplained yield losses. Soybean and dry edible bean are naturally susceptible to SCN, but through plant breeding, most soybeans have some level of resistance, varying in level from good to poor. The most common source of resistance to SCN in soybean is PI88788, which is about 30 years old, and many soybean growing areas have SCN populations that are becoming resistant to this source. The Peking source is a very effective SCN resistance source but is only available in less than 5% of all soybean varieties.

Soybean cyst nematode cysts each harbor hundreds of eggs. Cysts and eggs of SCN can survive in the soil and remain viable for many years even without a soybean or dry bean host. Any activity that moves soil around will move SCN, meaning that areas with a history of soybean production likely have or will have this pest. Soybean cyst nematodes were first reported in Minnesota in 1978, South Dakota in 1995, North Dakota in 2003, and Manitoba in 2019.

During the growing season, the developing SCN cysts containing the eggs can be seen on susceptible plant roots, as seen in the picture below. To get an accurate assessment of the infestation level of the field, you need to collect soil samples and submit them to a laboratory to get a measure of the SCN egg count.

Photo of soybean roots with SCN cysts. Photo courtesy of NDSU.

Sampling strategies

If you have never tested for SCN before, you will want to sample fields intended for soybean or dry bean for the presence of SCN and gather a baseline SCN egg count. The best time to collect this sample is at the end of the growing season, right before harvest or just after (before any tillage). Sampling in the fall coincides with the highest egg levels in the soil and typically falls in the months of September and October. Collect 10-20 soil cores (6 to 8 inch soil depth) right in the soybean row from areas of the field that are likely to have SCN. Since SCN is a soil-borne pathogen, it moves wherever contaminated soil can enter the field. Therefore, the areas you will want to collect samples from are field entry points where soil can be transferred on equipment and tires, places where blown soil accumulates (e.g., fence lines), ditches and flooded areas, and locations in fields with consistently low soybean yields. Mix the soil cores together and take a subsample to fill a soil sample bag.

If you know you have SCN, you will want to sample soybean fields twice during the year: once in June to get an initial SCN egg count and then again in the fall to get a final SCN egg count. The early and late SCN samples allow you to measure if SCN populations are being effectively controlled (i.e., no increase in SCN egg count) or if the soybean variety SCN resistance source is failing (i.e., SCN egg count increases). Choose a single point in the soybean field and collect 8-10 soil cores (6 to 8 inch soil depth) taken within the soybean row at that spot. Mix the cores together and fill a regular paper soil sample bag. Mark that point with a flag and collect its GPS coordinates. Come back to that exact spot in the fall and collect a second sample. This will help you assess how your SCN management strategies, including the soybean variety SCN resistance source and soybean seed treatment, are working in the field.

Preparing and sending SCN samples to AGVISE Laboratories

You can submit SCN samples via paper form or online through AGVISOR. AGVISE provides special paper forms for SCN sampling and special stickers for online AGVISOR submission at no charge. The bright yellow forms and stickers help us sort samples and ensure samples submitted for SCN analysis are not dried and ground. All SCN samples analyzed by AGVISE Laboratories are analyzed at the Benson, MN laboratory. You can either send the SCN samples directly to the Benson Laboratory (see addresses below) or to the Northwood Laboratory, where they will be routed to Benson for analysis. AGVISE Laboratories reports SCN results in “eggs/100 cc” of soil and provides interpretation on our reports informed by university research.

Helpful links:

Soybean Cyst Nematode, ISU

Plant Disease Management: Soybean Cyst Nematode, NDSU

Soybean Cyst Nematode (SCN), UMN

Soybean Cyst  Nematode in South Dakota: History, Biology, and Management, SDSU

The SCN Coalition

 

Is PI 88788 Working in Your Soybean Fields?

Soybean Cyst Nematode (SCN) is the number one yield-reducing pest in soybeans. Potential yield loss to SCN is expected to rise as more and more populations of SCN overcome the PI 88788 source of resistance. The Peking source of SCN resistance is not near as common as the PI 88788 source but is used in several soybean varieties.

If you want to see how the SCN resistance source in your soybeans is holding up this growing season, you can do an early and late SCN soil test. If the egg count increases substantially between the early and late SCN sample, your SCN resistance source is likely failing.

Here are the 4 steps to this simple test:

Early SCN sample (June): 

  1. Choose a spot in a current soybean field
  2. Collect 8 to 10 0-6″ soil cores taken within the soybean row at that spot
  3. Mark that spot with a flag or GPS so you can get back to that spot to sample later in the season

Late SCN sample (mid to late August): 

4. Go back to the same spot you collected a soil sample from in June and repeat step #2

Once you’ve conducted this simple test, you will get an idea of whether or not the SCN resistance source in your soybean variety is holding up or if it is time to change the resistance source in next year’s varieties. AGVISE completed a field project using a similar procedure in 2019 and 2020. The data showed that the PI 88788 trait was not preventing SCN populations from increasing in some field sites tested in Minnesota. You can read more about our project here.


Data from the AGVISE SCN field project, 2019-2020

A silver bullet for managing SCN does not exist and will likely never exist. Do your due diligence and figure out if your SCN resistance source is working in your own fields.

You can order SCN submission forms from our online supply store here.

Additional resources:

SCN in Iowa: A Serious Problem that Warrants Renewed Attention

Iowa State University – SCN Resources

 

 

 

Soybean cyst nematode: Failing resistance traits, increasing SCN populations

Originally featured in the Winter 2020-2021 AGVISE Laboratories Newsletter

In 2019, AGVISE Laboratories investigated if popular soybean varieties with PI88788 or Peking SCN-resistance traits were effectively providing protection from soybean cyst nematode (SCN) and found that a number of the varieties failed to do so. We expanded the project in 2020 with cooperation from agronomists in west-central Minnesota.

For over 20 years, PI88788 has been the primary SCN-resistance trait in over 95% of soybean varieties. In the past few years, university research is showing that PI88788 is losing its effectiveness at controlling SCN. Detecting SCN-resistant trait failure with the naked eye is impossible, unlike the detection of failed pesticide control, where you can still see a herbicide-resistant weed that is growing vigorously. Therefore, we wanted to demonstrate how you can measure SCN resistance with soil sampling, even though you cannot see it with your naked eye.

In the project, we had 41 soybean fields with SCN-resistant varieties, 35 with the PI88788 trait, and 6 with the Peking trait. In each field, a location was flagged and soil sampled for SCN egg count in early (June) and late (September) parts of the growing season. From June to September, the SCN egg count increased by 4.9 times on average across all 41 soybean fields (individual field reproduction factor ranged from 1.2 to 12.9). In some fields, the high SCN reproduction rate shows that SCN were successfully reproducing on soybean plants and the SCN resistance trait is failling. We also learned that soybean varieties with the Peking trait had much better control of SCN than those with the PI88788 trait. One cooperator from Benson, MN grew both PI88788 and Peking soybean varieties on his farm. He noted a 2.5 bu/acre soybean yield advantage with the Peking soybean variety (56.5 bu/acre) over the PI88788 soybean variety (54.0 bu/acre).

The project showed that SCN soil sampling in the early vs. late growing season was a simple way to detect a failing SCN resistance trait. The simple protocol only takes a big flag to mark the spot, then a set of soil samples in June and September to compare the SCN egg count results.

 

Scouting Shorts: Soybean Iron Deficiency Chlorosis (IDC)

As soybean plants emerge and add trifoliate leaves, keep your eyes peeled for soybean iron deficiency chlorosis (IDC). Through the upper Midwest and into the Canadian Prairies, soils with high pH and calcium carbonate pose a special problem for soybean plants and iron uptake. If you encounter soybean IDC, you will start to notice soybean plants with distinct interveinal chlorosis (yellow leaf with green leaf veins) in the newest leaves. The unifoliate leaves typically remain green.

Look for characteristic symptoms of soybean IDC (above photo).

When to scout

Right now! Soybean IDC symptoms begin to appear as soybean plants enter the first- to third-trifoliate leaf stage. You will often see soybean IDC symptoms appear after a period of cool, wet weather.

Where to look

Soybean IDC symptoms are usually confined to soybean IDC hotspots with high carbonate and salinity. Soil pH is not a good indicator of soybean IDC risk because some high pH soils do not have high carbonate or salinity, which are the two principal risk factors. The soybean IDC hotspots often occur on landscape positions with moderate to poor drainage, but soybean IDC symptoms may appear across the entire field if high carbonate and salinity are present throughout the field. High residual soil nitrate-nitrogen can also make soybean IDC worse, so take an extra look at fields that were fallowed last year (e.g. Prevented Planting) and had higher soil nitrate-nitrogen than normal.

What soybean IDC can be confused with

Nitrogen deficiency: Pale green and yellowing is uniform across the entire leaf and veins (not interveinal like soybean IDC). Yellowing appears on older leaves. It is sometimes observed when poor inoculation or delayed nodulation occurs. Look at soybean roots for active nodules (bright pink-red center) or take plant and soil samples to confirm.

Potassium deficiency: Yellowing starts at the outer leaf margin, works its way inward with some brown mottling. Yellowing appears on older leaves during early growth stages and sometimes on upper leaves during pod fill. Take plant and soil samples to confirm.

Soybean cyst nematode (SCN): Aboveground symptoms are virtually invisible during the early growing season. Visual SCN symptoms only occasionally appear in late July or August, or if dry soil conditions occur. Look at soybean roots for small white-colored SCN cysts or take an SCN soil sample including infected root material to confirm.

More information on soybean IDC symptoms, causes, and management: https://www.agvise.com/soybean-iron-deficiency-chlorosis-symptoms-causes-and-management/

Starter Fertilizer Display: How low can YOU go?

When profits are squeezed, more farmers are asking about optimal starter fertilizer rates and how low starter fertilizer rates can be. These questions are the result of wanting to keep fertilizer costs down, to plant as many acres per day as possible, and to take advantage of more efficient, lower rates of banded phosphorus fertilizer compared to higher rates of broadcast phosphorus fertilizer.

To illustrate the role of starter fertilizer rates and seed placement, we put together displays showing the distance between fertilizer granules or droplets at various rates and row spacings. You can see several pictures with canola, corn, soybean, sugar beet, and wheat. We greatly thank John Heard with Manitoba Agriculture for helping with the displays.

The displays show the normal seed spacing for several crops with different dry or liquid fertilizer rates alongside the seed. These displays help visualize the distance between the seed and fertilizer at several rates. University research shows that to achieve the full starter effect, a fertilizer granule or droplet must be within 1.5-2.0 inches of each seed. If the fertilizer granule or droplet is more than 1.5-2.0 inches away from the seed, the starter effect is lost. Some people wonder about these displays, but you can prove it to yourself pretty easily. Just run the planter partially down on a hard surface at normal planting speed. You will see what you imagine as a constant stream of liquid fertilizer, ends up being individual droplets at normal speed, especially with narrow row spacings and lower fertilizer rates.

These displays help illustrate the minimum starter fertilizer rate to maintain fertilizer placement within 1.5-2.0 inches of each seed for the full starter effect. In addition to an adequate starter fertilizer rate, additional phosphorus and potassium should be applied to prevent nutrient mining, causing soil test levels to decline in years when minimum fertilizer rates are applied.

Fertilizing soybean

Soybean acres expanded greatly across the northern Great Plains and into Manitoba through the 1990s and 2000s. Today, soybean occupies a large portion of planted acres and makes a desirable rotation crop in canola, corn, and small grain production systems. As soybean has advanced northward and westward, soybean is often billed as a low maintenance crop, requiring no fertilizer or even seed inoculation. The fact is, if you expect soybean to be a low maintenance crop, you can expect low yield results. Achieving high soybean yields starts with a good, long-term soil fertility plan.

Nitrogen

Soybean yielding 40 bu/acre requires about 200 lb/acre nitrogen, but luckily you do not have to provide all the nitrogen! Soybean relies on nitrogen-fixing bacteria to meet its nitrogen requirements. Legumes, like soybean, form a symbiotic relationship with N-fixing bacteria, housed in root nodules, to provide sufficient nitrogen. Each legume species requires a unique N-fixing bacterium, thus an inoculant for lentil or pea does not work on soybean. Soybean seed must be inoculated with the N-fixing bacteria Bradyrhizobia japonicum. Ensure you have the proper soybean-specific seed inoculant. You can count the number of nodules on soybean roots and verify the presence of active N-fixing bacteria in the nodules with bright pink centers. These soybean plants have enough active N-fixing bacteria to meet soybean nitrogen requirements.

For new soybean growers, the N-fixing bacteria Bradyrhizobia japonicum is not naturally present in soil and seed inoculation is required. During the first few years of soybean establishment, supplemental nitrogen may be required to achieve good soybean yield while the N-fixing bacteria population builds. University of Minnesota researchers in the northern Red River Valley showed that soils with less than 75 lb/acre nitrate-N (0-24 inch) required 40-50 lb/acre additional preplant nitrogen. If successful inoculation and good nodule counts are observed in the first year, then no additional nitrogen should be required in subsequent years.

Plant soybean on soils with less than 100 lb/acre nitrate-N (0-24 inch), if possible. High residual soil nitrate may delay root nodulation with N-fixing bacteria and increase the severity of iron deficiency chlorosis (IDC). Because soybean can fix its own nitrogen, you may recoup better economic return on soils with high residual nitrate with crops that do not fix their own nitrogen like corn or wheat.

Phosphorus

Soybean does not respond to phosphorus as dramatically as grass crops like corn or wheat do. Nevertheless, medium to high soil test P are required to achieve good soybean yields. Soybean responds to broadcast P placement better than seed-placed or sideband P. In dryland regions where soybean is planted with air drills, seed-placed P or sideband P is often the only opportunity to apply phosphorus. You must pay special attention to seed-placed fertilizer safety with soybean. An air drill with narrow row spacing (6 inch) should not exceed 20 lb/acre P2O5 (40 lb/acre monoammonium phosphate, MAP, 11-52-0). Fertilizer rates exceeding the seed safety limit may delay seedling emergence and reduce plant population. For wider row spacings, no fertilizer should be placed with seed.

Potassium

Soybean removes far more potassium in harvested seed than canola or wheat. Soybean yielding 40 bu/acre removes about 60 lb/acre K2O, while wheat yielding 60 bu/acre removes only 20 lb/acre K2O. Pay close attention to potassium removal across the crop rotation. After soybean is added to the crop rotation, cumulative potassium removal greatly increases, and declining soil test K is observed over time.

Do not place potassium with soybean seed; delayed seedling emergence and reduced plant population can occur. Any potassium fertilizer should be broadcasted or banded away from seed.

Sulfur

Sulfur deficiency in soybean is uncommon, yet sometimes observed on coarse-textured soils with low organic matter (< 3.0%). Soybean response to sulfur is usually confined to certain zones within fields. With additional sulfur, soybean can produce more vegetative growth, but more vegetative growth may increase soybean disease severity, such as white mold. The residual sulfur remaining after sulfur-fertilized canola, corn, or small grain is often sufficient to meet soybean sulfur requirements.

Iron

Soybean is very susceptible to iron deficiency chlorosis (IDC). Soybean IDC is not caused by low soil iron but instead by soil conditions that decrease iron uptake by soybean roots. Soybean IDC risk and severity are primarily related to soil carbonate content (calcium carbonate equivalent, CCE) and worsened by salinity (electrical conductivity, EC).

Soybean IDC is common in the upper Midwest, northern Great Plains, and Canadian Prairies, where soils frequently have high carbonate and/or salinity. Within a field, IDC symptoms are usually confined to soybean IDC hotspots with high carbonate and salinity; however, symptoms may appear across a field if high carbonate and salinity are present throughout the field. Soybean IDC severity is made worse in cool, wet soils and soils with high residual nitrate. Soil pH is not a good indicator of soybean IDC risk because some high pH soils lack high carbonate and salinity, which are the two principal risk factors.

Guidelines for managing soybean IDC:

  1. Soil test each field, zone, or grid for soil carbonate and salinity. This may require soil sampling prior to soybean (possibly outside of your usual soil sampling rotation) or consulting previous soil sampling records.
  2. Plant soybean in fields with low carbonate and salinity (principal soybean IDC risk factors).
  3. Choose an IDC tolerant soybean variety on fields with moderate to high carbonate and salinity. This is your most practical option to reduce soybean IDC risk. Consult seed dealers, university soybean IDC ratings, and neighbor experiences when searching for IDC tolerant soybean varieties.
  4. Plant soybean in wider rows. Soybean IDC tends to be less severe in wide-row spacings (more plants per row, plants are closer together) than narrow-row spacings or solid-seeded spacings.
  5. Apply chelated iron fertilizer (e.g., high quality FeEDDHA) in-furrow at planting. In-furrow FeEDDHA application may not be enough to help an IDC susceptible variety in high IDC risk soils (see points #2 and #3).
  6. Avoid planting soybean on soils with very high IDC risk.

Zinc

Zinc deficiency in soybean is rare, even on soils with low soil test Zn. Soybean seed yield response to zinc is limited on soils with less than 0.5 ppm Zn. More zinc sensitive crops like corn, dry bean, flax, and potato will respond to zinc on soils with less than 1.0 ppm Zn. If zinc sensitive crops also exist in the crop rotation, you may apply zinc with broadcast phosphorus or potassium during the soybean year as another opportunity to build soil test Zn across the crop rotation.

Soybean Iron Deficiency Chlorosis: Symptoms, Causes, and Management

 

Figure 1. Soybean plants with iron deficiency chlorosis symptoms. Note the newest leaves are yellow with indistinct green veins.

If soybean turns yellow during an early growth stage, you may have a case of soybean iron deficiency chlorosis (IDC). The distinctive yellow symptoms of soybean IDC often appear as soybean enters the first- to third-trifoliate leaf stage. Soybean IDC is characterized by distinct interveinal chlorosis (yellow leaf with green leaf veins) in the newest leaves and may result in substantial yield loss (Figure 1). Soybean IDC is not caused by low soil iron but instead caused by soil conditions that decrease iron uptake by soybean roots.

Soybean IDC risk and severity are primarily related to soil carbonate content (calcium carbonate equivalent, CCE) and worsened by salinity (electrical conductivity, EC) (Table 1). Soybean IDC is common in soybean-growing regions of the upper Midwest, northern Great Plains, and Canadian Prairies, where soils frequently have high carbonate and/or salinity (Figure 2). Within a field, IDC symptoms are usually confined to soybean IDC hotspots with high carbonate and salinity; however, symptoms may appear across a field if high carbonate and salinity are present throughout the field. Soybean IDC severity is made worse in cool, wet soils and soils with high residual nitrate-nitrogen. Soil pH is not a good indicator of soybean IDC risk because some high pH soils lack high carbonate and salinity, which are the two principal risk factors.

Table 1. Soybean iron deficiency chlorosis risk (IDC) risk potential based on soil carbonate content and salinity.
Calcium carbonate equivalent (CCE)
Salinity (EC 1:1), dS/m less than 2.5% 2.6 – 5.0% greater than 5%
less than 0.25 low low moderate
0.26 – 0.50 low moderate high
0.51 – 1.00 moderate high very high
greater than 1.00 very high very high extreme

Figure 2. Soil samples with high risk of soybean iron deficiency chlorosis (IDC) in the northern Great Plains and upper Midwest.

Unlike a nitrogen or sulfur deficiency, soybean IDC is not correctable with an in-season fertilizer application. Foliar application of iron fertilizers, including FeEDDHA, may have short-term cosmetic effects, but foliar iron applications have not consistently increased soybean yield on IDC-affected plants. Chlorosis symptoms often alleviate naturally as environmental conditions improve (e.g. drier, warmer weather), but severe cases can persist and cause yield loss. North Dakota State University research has shown that IDC persisting into the fifth- and sixth-trifoliate leaf stage will significantly reduce soybean yield. For fields with historical soybean IDC problems, you should delineate soybean IDC hotspots for selective management using aerial or satellite imagery.

Guidelines for managing soybean IDC

  1. Soil test each field, zone, or grid for soil carbonate and salinity to evaluate soybean IDC risk potential (Table 1). This may require soil sampling prior to soybean (possibly outside of your usual soil sampling rotation) or consulting previous soil sampling records.
  2. Plant soybean in fields with low carbonate and salinity (principal soybean IDC risk factors).
  3. Choose an IDC tolerant soybean variety on fields with moderate to low carbonate and salinity.
  4. Plant soybean in wider rows. Soybean IDC tends to be less severe in wide-row spacings (plants are closer together) than narrow-row spacings or solid-seeded spacings.
  5. Apply chelated iron fertilizer (e.g., high quality FeEDDHA) in-furrow at planting. In-furrow FeEDDHA application may not be enough to help an IDC susceptible variety in high IDC risk soils (Figure 3).
  6. Avoid planting soybean on soils with very high IDC risk.

Figure 3. Soybean iron deficiency chlorosis (IDC) severity is reduced with iron fertilization. However, IDC-tolerant soybean varieties are more effective. Research from Dr. R. Jay Goos, NDSU, 2000.