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.

Fallow Syndrome: Preventing Phosphorus Problems

Some crops that do not support mycorrhizal fungi (left to right: sugar beet, canola, radish).

Producers in the northern Great Plains and upper Midwest need to consider the risk of fallow syndrome in their crop nutrition plans. You are probably asking, what is “fallow syndrome” and why should I care? After all, summer fallow is not that common anymore! But the greater number of Prevented Planting acres in 2019 and 2020 meant that we have had many unintended fallow fields, making fallow syndrome a serious and widespread concern for the next year.

Fallow syndrome is an induced phosphorus deficiency caused by a lack of mycorrhizal fungi in soil. Some plant species, like corn and wheat, rely heavily on mycorrhizae to colonize the plant root system and help acquire important nutrients like phosphorus and zinc. If soil is lacking sufficient mycorrhizae to colonize plant roots, a case of fallow syndrome will increase phosphorus fertilizer needs and even cost crop yield potential.

Understanding mycorrhizae

Mycorrhizae fungi occur naturally in soils and readily colonize plant roots. Upon root colonization, mycorrhizae fungal filaments act as extensions of the root system and increase the soil volume available for plant water and nutrient uptake. The combined root-mycorrhizae surface area can be up to 10-fold greater than roots without mycorrhizae. Mycorrhizae depend on living plant roots to support stable mycorrhizae populations. However, not all plant species host and support mycorrhizae growth. Some common field crops are non-host species and their planting results in rapid drops in mycorrhizae populations.

Summer fallow or unplanted cropland, such as Prevented Planting in 2020, is a classic example of providing no or few living plant roots in soil to maintain mycorrhizae populations. In addition, some crop species do not support mycorrhizae, such as those in the goosefoot family (sugar beet) and mustard family (canola, radish, turnip). Following a classic case of summer fallow or a non-mycorrhizae supporting crop, the mycorrhizae population in soil will quickly drop. A cover crop mix that included a grass species (e.g. barley, rye) should still support mycorrhizae and prevent fallow syndrome concerns.

Preventing fallow syndrome

The easiest prevention strategy after fallow is planting a crop species without fallow syndrome risk like soybean, canola, or sugar beet. Avoid planting susceptible crops like corn and wheat. These crops are highly dependent on mycorrhizae to acquire phosphorus, and extra starter phosphorus will be required if fallow syndrome risk is present.

To reduce fallow syndrome risk in corn or wheat, extra phosphorus fertilizer must be placed with or near the seed. Applying more broadcast phosphorus or relying on high soil test P will not prevent fallow syndrome. The starter phosphorus rate should be 20 to 40 lb/acre P2O5. In some university research trials, up to 60 lb/acre P2O5 with 2×2-band placement near the seed was needed to prevent corn yield loss to fallow syndrome.

For wheat, these phosphorus rates are typically seed safe with monoammonium phosphate (MAP, 11-52-0). Most corn planters can safely apply 20 lb/acre P2O5 (5 gal/acre ammonium polyphosphate, APP, 10-34-0) in the furrow. For medium/fine-textured soils with good soil moisture at planting, you can generally apply up to 10 gal/acre 10-34-0 (40 lb/acre P2O5) safely in the furrow at 30-inch row spacing. Higher 10-34-0 rates may exceed seed safety limits on dry soils or coarse-textured soils and require 2×2-band placement to maintain seed safety.

Complete liquid fertilizers, such as 6-24-6 or 9-18-9, are not suggested for preventing fallow syndrome. Compared to 10-34-0, the products have lower P concentration that result in less applied phosphorus, even if used at maximum seed safe rates. The extra N + K2O in “complete” liquid fertilizers increases the salt index and lowers the seed safe rate.

Field Variability Screaming in Your Ear? Precision Soil Sampling is the Answer

Your land is variable. Each fall, you watch the combine yield monitor go up and down across the field. You know where crop yield will be the best in wet years and dry years. So, why do you still use a whole-field composite soil test to manage fertilizer inputs and ignore the obvious field variability affecting crop yield potential?

Precision soil sampling, using grids or zones, divides whole fields into smaller units for soil sampling and creates more accurate and useful soil test information. It tells you exactly where you need to apply more or less fertilizer within each field, unlocking untapped crop yield potential and fertilizer input savings. Grid soil sampling, which is the most detailed approach, typically breaks a field into 2.5- to 5.0-acre grid cells. The more adaptable approach is zone soil sampling, which divides the field into productivity zones that can be managed to their needs. A well-designed zone should represent the smallest practical management unit that still accurately represents the area (e.g. 20-40 acres). Zones are commonly created using data layers such as crop yield, satellite imagery, soil survey, topography, salinity, drainage, or a combination of several data layers.

Precision soil test data can reveal previously unknown production problems, which were otherwise masked in a whole-field composite soil sample. For example, more zone soil sampling has uncovered more and more low soil pH zones (below pH 6) in the long-term no-till areas of central South Dakota, southwest North Dakota, and north-central Montana. Previously, the whole-field composite soil sample had blended the low and high soil pH zones together and everything looked okay. But now, the zone soil samples are revealing where low soil pH is causing serious crop yield loss and where soil pH can be corrected with lime to improve crop yield. This is a good example of precision soil sampling revealing a long-hidden problem and showing us how to fix it.

If you break a field into smaller and smaller units (i.e. more zones), you will learn more and more about field variability. To illustrate the concept, we pulled soil test data from 23,000 zone sampled fields in 2020 and calculated the average soil test range (difference) between the high and low zones within each field. The summarized data is presented in the table.

Average soil test range within a field (high zone – low zone)
Number of zones per field Nitrate-N

lb/acre, 0-24 inch

Olsen P

ppm

K

ppm

pH Soil organic matter

%

3 27 9 88 0.57 1.10
4 38 14 108 0.76 1.52
5 45 17 137 0.89 1.73
6 55 21 164 1.12 1.68
7 61 23 184 1.25 1.59
8 65 24 183 1.26 2.04

As the number of zones increases in a field, the range in soil test values (high zone – low zone) also increases and highlights the true variability across the field. The trend is clear not just for soil nutrients like nitrogen, phosphorus, and potassium, but also for soil properties like pH and organic matter. This tells us that one whole-field “average,” was missing the highs and lows that occur naturally in many fields.

Precision soil sampling is the first step in understanding what is really happening in your fields. You can gain a clearer picture of what plant nutrient deficiencies might be occurring and where you can improve crop yield potential. The next step is creating variable-rate prescriptions for seed, fertilizer, lime, and even herbicides (consider soil pH and organic matter). These tools can help you improve crop yield, optimize crop inputs, and increase profitability within each field on your farm.

Prevented Planting Acres in 2020: Maximizing Cover Crop Effectiveness

In 2020, there are again widespread acres of Prevented Planting (PP) in North Dakota and northwest Minnesota. Farmers are now making plans to plant cover crops on unplanted cropland in the next few weeks. It is important to establish cover crops on PP fields because growing plants help reduce the chance these fields will be PP fields again next year.

Let’s look at the major reasons why cover crops are valuable tools on Prevented Planting acres.

Soil Water Use

A field without any growing plants is a fallow field. Before no-till, summer fallow was a widespread soil water conservation strategy in dryland agriculture. Actively growing plants transpire (use) a lot more water than evaporation from the soil surface alone does. Cover crops help fill the water-use void by transpiring a lot of water, helping to dry the soil surface and lower the water table before the following year. This also opens space in the soil profile for summer and fall rains to leach soluble salts from the soil surface and reduce salinity in the root zone.

Soil Erosion Control

Tillage is a popular weed control tool, but it also destroys crop residue and leaves soil exposed and vulnerable to water and wind erosion. Planting cover crops protects the soil surface from rain and wind, keeping soil firmly in place. Just because you cannot grow a cash crop on the field this year, you should not let your soil blow into the next field, letting your neighbor farm it next year.

Weed Control

An established cover crop can compete with weeds, helping suppress weed growth and weed seed production. For fields with problematic broadleaf weed histories, a cover crop mix containing only grass species is preferred. In grass cover crops, you can still use selective broadleaf weed herbicides to control the problematic broadleaf weeds of conventional or no-till systems such as Canada thistle, common ragweed, kochia, volunteer canola, and waterhemp while not killing the grass cover crop. For fields with low weed pressure, a cover crop mix containing grasses, brassicas, and legumes will provide more soil health benefits.

Soil Biological Activity

Have you heard about “fallow syndrome” before? Fallow syndrome is an induced nutrient deficiency, often seen in corn following fallow, when the population of mycorrhiza fungi is insufficient to colonize plant roots and help them acquire water and nutrients. Mycorrhizae are especially important in plant uptake of phosphorus, so plants with fallow syndrome often show phosphorus deficiency symptoms. Fallow syndrome is a major concern in corn following summer fallow or Prevented Planting without cover crop.

During the Prevented Planting year, it is important to include grass species in the cover crop mix to support and maintain the mycorrhiza population through next year. Brassica species, like radish and turnip, are often included in cover crop mixes for their deep taproot architecture and high forage value for grazing livestock, but brassicas do not support mycorrhizae. You do not want a cover crop mix consisting of brassica species alone because fallow syndrome might occur next year.

 

In June 2020, excessive rainfall slammed some parts of the upper Midwest and northern Great Plains, drenching soils with 3 to 15 inches of rain over a couple days. On summer-flooded fields, cereal rye is an attractive soil management tool. You can plant or fly on cereal rye well into August or mid-September, and it will continue to use soil water through late summer and fall. Next spring, the overwintered rye will grow again, using more soil water and maintaining soil structure, providing you with a much better chance to plant the field. If soybean is the next crop, you can plant glyphosate-tolerant soybean into green cereal rye then terminate the cereal rye with glyphosate later. This practice has become more and more popular on difficult fields.

Do not forget about soil fertility and plant nutrition for cover crops. A modest application of nitrogen will help cover crop establishment, plant water use, and competition with weeds, as cover crops with adequate nitrogen will grow faster and larger than those without nitrogen. Around 46 lb/acre nitrogen (100 lb/acre urea, 46-0-0) should be enough to establish nice cover crop growth. Prevented Planting fields, being wetter than those successfully planted in spring, lost some, if not most, soil nitrogen via nitrate leaching or denitrification. Although additional nitrogen may have mineralized from soil organic matter during May and June, excess precipitation in June may have caused additional soil nitrogen loss. The best way to know is collecting 0-12 or 0-24 inch soil samples for nitrate-nitrogen analysis.

As you choose the appropriate cover crop mix on Prevented Planting fields, you must consider the pros and cons of each cover crop species and how each will help accomplish your goals. These are some helpful resources that will provide additional information on what cover crop options will work best on your fields.

Plant Analysis Sampling Guide

Plant nutrient analysis is a useful management tool. The AGVISE Plant Analysis Sampling Guide discusses the importance of plant analysis in proper crop nutrition and how to collect plant
samples. It is important to collect the correct plant part at the correct plant growth stage to ensure accurate interpretation of plant analysis results. To ensure we have enough plant material to analyze in the laboratory, please collect a minimum number of whole plants or leaves per sample as instructed in the guide. To obtain the most accurate plant analysis information, follow the instructions on plant growth stage, plant part, and minimum sample size.

View the Plant Tissue Sampling Guide

 

High Soil Nitrogen following Drought: How to manage next year

From time to time, moderate to severe droughts hit the Great Plains. Such is life in semi-arid climates. When a drought occurs, it is normal to find higher residual soil nitrate-nitrogen after harvest. Since the widespread adoption of soil testing in the 1970s, we have seen this phenomenon in all major drought years: 1988, 2002, 2006, 2012, 2017 (Figure 1). The lack of precipitation and exhausted stored soil water reduces crop growth and yield, meaning much of the applied nitrogen fertilizer remains unused, showing up in the residual soil nitrate-nitrogen test. In 2017, very high residual soil nitrate-nitrogen was observed across wide geographies of western North Dakota and South Dakota (Figure 2).

Figure 1. Residual soil nitrate-nitrogen following wheat on the northern Great Plains.

 

 

Figure 2. Residual soil nitrate-nitrogen following wheat on the northern Great Plains in 2017.

 

Following a drought, we often get the question, “Can I count on all the soil nitrate in my soil test for next year’s crop?” The simple answer is yes; you can count on the amount of soil nitrate-nitrogen in the soil test, but you must consider additional factors. Even in drought, some parts of each field will produce higher crop yield than other parts because the better soils have higher water holding capacity (e.g. higher clay content, higher organic matter). In the high yielding zones, there is less residual soil nitrate remaining in the soil profile. Drought will create more variability in crop yield and residual soil nitrate, mostly driven by topography and soil texture.

Let’s imagine you had a wheat crop severely affected by drought, but some parts of the field still had 50% normal yield (maybe lower landscape positions, greater water holding capacity). Following harvest, the whole-field composite soil test showed 140 lb/acre nitrate-N (0-24 inch). You were skeptical about that very high residual soil nitrate level, so the crop consultant resampled the parts with better crop yield, which then had 80 lb/acre nitrate-N (0-24 inch). Using the whole-field composite soil test result of 140 lb/acre nitrate-N (0-24 inch), you would only need to apply some starter nitrogen fertilizer for next year’s crop. However, if you only applied starter nitrogen, the high yielding parts of the field with only 80 lb/acre nitrate-N (0-24 inch) would be under-fertilized, costing crop yield and profit next year, on the best soils in the field.

If you only have a whole-field composite soil test result, you must consider spatial variability in residual soil nitrate across the field. You will want to apply a base nitrogen fertilizer rate to cover the parts with lower residual soil nitrate than the field average. The base nitrogen fertilizer rate may range between 30 to 60 lb/acre N, depending on spatial variability and risk tolerance. If you do zone soil sampling, you have a much better idea of spatial variability and nitrogen fertilizer needs in all parts of your fields. Through productivity zone soil sampling, you know the residual soil nitrate level in each management zone, and you can choose different nitrogen fertilizer rates across the field.

If you only soil sample the surface soil depth (0-6 inch), you are missing 75% of the plant-available nitrate-nitrogen pie. To make good nitrogen decisions, you should collect 0-24 inch soil samples for soil nitrate-nitrogen analysis. In drought, plant roots explore deep for stored soil water and uptake whatever nitrate is found along the way. There is no way to model how much soil nitrate remains in the soil profile after drought. Following drought, the best strategy is 24-inch soil sampling and breaking fields into several management zones to determine the proper amount of nitrogen fertilizer required.