Published on Wednesday, July 24, 2019
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Iron deficiency chlorosis (IDC) is a physiological disorder caused by a lack of iron in the soybean plant which creates the “chlorosis” symptoms. Plants with IDC have yellowing (chlorosis) beginning between the veins and progressing to a generally chlorotic canopy. Other symptoms include reduced plant growth and ultimately, lower yields. Yield reductions from IDC are a primary limitation for some farmers on certain fields.
WHAT IS IDC?
Iron (Fe) is necessary for the formation of chlorophyll, the green pigment in plants. Most soils contain abundant levels of iron beyond plant requirements; however, at a high soil pH (greater than pH 7.5), iron is not soluble (in ferric form) and cannot be absorbed by plant roots. Thus, iron in the ferric form will not be available to the soybean. Other crops, like cereal grain crops, can take in many forms of Fe and do not show IDC symptoms.
In soybeans, Fe is mobile from germination through the unifoliate leaf growth stage. As the first trifoliate leaf emerges, Fe becomes immobile. The plant must continually take up Fe through the season to avoid deficiency. Soybean Fe uptake begins when the soybean roots acidify the soil environment directly around them. The acidic microenvironment surrounding the root surface allows Fe to be taken into the plant.
Iron deficiency chlorosis (IDC) in Soybeans
Mild IDC symptoms on soybean leaves starting at late V1 growth stage
Moderate IDC symptoms on soybean leaves starting at late V1 growth stage
Symptoms of IDC usually appear when the soybean plant is between the first and second trifoliate, growth stage V1-V2. Symptoms do not show up on cotyledon (seed leaves) or unifoliate (single leaf) leaves. Severe IDC situations may turn the leaf tissue yellow or white, and the outer edges may scorch and turn brown as plant cells die (necrosis). Symptoms may increase or decrease in intensity during the season depending on growing conditions. Iron chlorosis in a soybean field often occurs in spots and usually in a defined pattern, depending on chemical and physical soil differences in the field.
Field of soybeans showing iron chlorosis symptoms (Note: wheel tracks are green)
The drivers behind iron deficiency chlorosis in soybeans are complex, with research on-going. At the root of iron availability in the soil is chemical reactions. Iron chlorosis frequently occurs on high pH (>7.5), calcareous (high “lime” containing) soils. IDC of soybeans does not occur on all high-pH soils. In these cases, there are other contributing factors, such as free Calcium Carbonates (CaCO3), elevated salts (high electrical conductivity (EC), compaction in the seedling root zone, excess free nitrate-nitrogen, and saturated soils. These individually or collectively drive the chemical reactions in the soil and around the developing roots.
Image 1: Field with IDC – The wheel tracks are a lighter yellow (Image 4 and 5 are from the same field)
For nutrients and desired root growth, Oxygen is needed in the root zone for plants to take up iron. Soil compaction or excessive rainfall or irrigation can result in poorly aerated soils and reduced iron uptake. In Image 1, the wheel tracks are lighter green to yellow as the result of compaction preplant. This ultimately has reduced root growth and the availability of oxygen to drive the chemical reaction for iron availability to the soybean plant.
As noted, Images 1 and 2 (below) are from the same field. Therefore, all management actions were the same, and images were captured the same time. It was found in this situation that the tile drainage on the part of the field depicted in Image 2 was less than on the part of the field where Image 1 was captured. When the wheel tracks are found to be green, as depicted in Image 2, compaction, either from preplant or the planting operation, has reduced the air pore space substantially – pore spaces that contain excess nitrates. Further, with reduced air pore space and saturated soils, iron can actually be more available to the seedling plant.
Extensive field research conducted by Dr. Dave Franzen, Extension Soil Science Specialist, NDSU, has found, and he shares in Bulletin SF1164, “Iron in well water is reduced iron (Fe++ or ferrous iron). Ferrous iron is very soluble in water. A No. 2 carpenter nail can be dissolved in water if it was ferrous iron. Unfortunately, as soon as the ferrous ion is exposed to oxygen, it oxidizes to oxidized Fe (Fe+++ or ferric iron). Ferric iron is a trillion times less soluble than ferrous iron. Plants, except for aquatic plants such as rice and pondweed, implement Fe uptake strategies to improve Fe nutrition and avoid deficiency."
In soybeans, Fe is mobile from germination through the unifoliate leaf growth stage. As the first trifoliate leaf emerges, Fe becomes immobile in the plant and must be taken up continually through the season to avoid deficiency. The soybean strategy for Fe uptake begins by soybean roots acidifying the soil environment directly around the soybean root. The acid soil environment is necessary for the activity of a Fe-reducing protein that the soybean root secretes. If the root remains acidic, the Fe-reducing protein contacts oxidized iron and reduces it to soluble ferrous iron, making it available to the plant. It has been found by both plant breeders and plant physiologists that those soybean genetics that are more IDC tolerant have increased ability to acidify the soil environment around the root/hairs.
Image 2: Field with IDC – The wheel tracks are darker green (Image 4 and 5 are from the same field)
Management of IDC
A review of research conducted by universities in the “Chlorosis States” indicates that consistent with previous findings, variety selection still is the most important action that can be made when managing IDC. For some high IDC fields, the highest IDC tolerance score for a variety provides the best insurance policy.
Cooperative research conducted in the Red River Valley of Minnesota and North Dakota indicates that in-furrow iron treatment provides the best in-field management practice for return with the lowest risk. No yield reductions were measured from the in-furrow application of the EDDHA-Fe fertilizer source. Yield responses were greatest in areas severely affected with IDC (soybean plants would generally die off during the season) and generally were still positive in areas that would show significant yellowing of leaves with some stunting of plants and subsequent recovery of IDC. There was no yield response in areas with little to no IDC pressure. This would follow as if not deficient; then there is no need for additional Fe.
The cooperative studies found that the chelated EDDHA-Fe is the only current, in-furrow iron source to provide tangible benefits. Also, work conducted by both the University of Minnesota and NDSU would indicate that this iron source needs to be ortho-ortho-EDDHA Fe chelate. The ortho-para-EDDHA is not active enough for managing IDC. Iron is a metal ion that reacts with oxygen and hydroxide. When this reaction happens, the iron is useless to plants, as they are not able to absorb it in this form. To make iron readily available for plants a chelator is used to protect the iron from oxidation, prevent it from leaching out of the soil, and keep the iron in a form that the plants can use. Chelated iron fertilizers are available in granules or powders. These two forms can be used as water-soluble fertilizers; however, do not mix the chelated irons with other liquid fertilizers. Companion research studies have also demonstrated that foliar chelated iron sprays are not effective in correcting iron deficiency chlorosis in soybeans.
Another in-field management practice is the seeding of oats in the spring ahead of planting. The goal is to reduce saturated soils and excess nitrates. There have been mixed results with the oat companion crop. The greatest risk with this strategy was reductions in yield if the oat crop was not terminated at the correct time – targeted when oats are 10 inches tall. In excessively wet springs, such as 2019, getting the oats crop seeded is equally challenging. Field research has demonstrated that the companion oat crop can increase yield, but the increase is typically seen only for the susceptible variety. Yield without oat for the IDC tolerant variety was similar to the IDC susceptible variety with oat. Grower experiences have found that a fall-seeded cover crop, especially following vegetable crops, and then terminating after soybean emergence has been equally effective. In some very high pH and CEC soils where small grains are the primary cereal crop, the residual volunteer growth is often allowed to proceed, and then the soybeans are planted reduced-till the next spring.
In-field research conducted by Dr. Dan Kaiser, Extension Soil Fertility Specialist, University of Minnesota, through a 2011-2013 project found that precipitation in June seemed to be the greatest factor influencing the extent and degree of IDC in a given subject field. With reference to past research, Dr. Kaiser notes that the major driving force for IDC symptoms is soil moisture content. He notes, “If you take soil from a field area affected by IDC, the severity of the problem has been shown to increase with increasing level of water saturation of the soil.” From agronomist’s observations, system and/or pattern tiled fields, do not experience the extent or degree of IDC and subsequent yield impact.
A recent four-state study that NDSU led found that the highest yield for a soybean field having soils with and without susceptibility to soybean IDC would be best managed by seeding a high-yielding IDC-intolerant variety in non-IDC soils and an IDC-tolerant variety in the IDC-susceptible soils. To reduce IDC pressure, the soybeans should be seeded in wider than 15-inch rows. Experience has demonstrated, in lieu of additional at-plant management, row widths of 22” demanded a plant population of 175-180k seeds per acre to optimize production with moderately tolerant soybean varieties.
Although the exact mechanism for reduced IDC symptoms in denser stands is not known, many growers have seen this effect when the planter stops within the field and leaves a high strip of seed behind when it resumes planting. Soybeans in densely seeded areas are taller and have less IDC symptoms, compared with the normally seeded fields. A similar reduction in IDC symptoms are seen as soybeans are seeded closer to each other in wider row spacings or higher seeding rates. The causes of the denser-seeding IDC reduction could be related to reduced soil moisture under the row, higher root-zone acidity that would favor activity of the Fe-reducing substance secreted by the soybean root, or other unidentified mechanisms.
At Beck’s, the Product Management Team frequently utilizes soybean clump testing for IDC (see Image 6). These clumps of different soybean varieties are planted into known IDC hot-spots. The product specialists then can observe the varietal difference in growth and IDC symptomology throughout the growing season.
Beck’s Soybean Variety Clump Test
In summary, an IDC-tolerant variety should first be selected. An effective IDC-prevention strategy could also include the at-plant, in-furrow application of ortho-ortho-EDDHA but on a comprehensive approach to the condition. The integration of variable rate planters and precision application equipment can enable spatial application of an in-furrow EDDHA-Fe product, maximizing placement and minimizing cost.
Additional Information on IDC and Soil Conditions:
Most soils contain abundant levels of iron beyond plant requirements; however, at high soil pH (greater than pH 7.5), iron is not soluble (Ferric) and cannot be absorbed by plant roots. Iron in this form will not be available to the plant unless soil chemistry conditions change.
Chlorosis of soybeans does not occur on all high-pH soils. Even within a field with high pH soil analysis, areas of chlorotic and non-chlorotic soybeans can be found. Researchers have noted that conditions in the subsoil may vary. Subsoil in areas with chlorotic soybeans are often poorly drained, higher in pH, and higher in soluble salts and excess lime (carbonates) compared to areas where soybeans do not have IDC.
Author: Mike Blaine
Categories: Agronomy, Agronomy Talk
Tags: soybeans, Agronomy, Agronomy Update, IDC, iron