Management of Soybeans on Soils Prone to Iron Deficiency Chlorosis
Field Facts by Paul Gaspar, Pioneer Research Scientist
Introduction of Iron Deficiency Chlorosis
Interveinal chlorosis due to iron deficiency. Soybean "iron deficiency chlorosis" (IDC) is a nutrient deficiency with general symptoms of chlorosis (yellowing) of the soybean foliage and stunting of the plant. This condition is yield-limiting in many soybean fields in the northern and western Corn Belt including western Minnesota, the Dakotas, Nebraska, Iowa and other states. Some experts estimate that losses in income from this problem exceed $100 million. Because soybean acres are increasing in regions at risk to IDC, losses could become even higher unless management practices are implemented to reduce iron deficiency occurrence.
Typically, IDC symptoms begin to appear a few weeks after soybean emergence as interveinal chlorosis on the first trifoliate leaves. Because iron does not translocate in the plant, new growth will be most affected if the deficiency intensifies. Leaves may eventually turn yellow with dark green veins and the plants may be stunted. Under severe iron deficiency, plant leaf edges become necrotic (turn brown) and the necrosis may progress until entire leaves or even plants are dead. The symptoms tend to show up in irregularly shaped spots randomly distributed across a field.
Several management practices have been evaluated to address iron deficiency chlorosis in soybeans. These include variety selection, increased soybean seeding rate, use of inter-seeded small grain crops, delayed planting, avoidance of herbicides that slow growth or cause leaf area loss, and seed-applied, foliar-applied and in-furrow-applied iron. This article will discuss some of these management practices.
Interveinal chlorosis due to iron deficiency
Environmental Factors Causing IDC in Soybeans
The factors that may cause chlorosis are complex and interact with each other to intensify the level of chlorosis. The most dominant factors are carbonate levels, salts, and depressional field areas with poor drainage. Results of past research suggest that free calcium carbonate levels higher than 5% and/or soluble salts values greater than 0.5 mmho/cm indicate a high probability of soybeans expressing iron deficiency chlorosis and reduced yields. Research studies have suggested that decreased temperature, reduction in air-filled soil porosity, increased soil moisture, soil pH > 7.8 and residual nitrogen may also increase IDC symptoms.
Franzen and Richardson (1999) studied the relationship between soil pH, soluble salts (EC), soil Fe, Na, and calcium carbonate equivalent (CCE) within gradients of chlorotic to green soybeans at various locations over three years. These results suggested EC and CCE were the best indicators of the potential for developing iron deficiency chlorosis. Soil Fe, Na, and soil pH did not consistently predict the potential for developing chlorosis.
Managing Iron Deficiency Chlorosis
Variety Selection and Plant Population
Because soybean varieties vary widely for tolerance to IDC, variety selection is the first and most important step in managing this problem. Pioneer Hi-Bred has a significant research effort to screen its soybean varieties in areas with IDC. This screening effort is critical to understand and report current variety response to IDC, as well as identify new varieties that can help growers overcome yield losses to IDC.
Varietal differences in iron deficiency chlorosis tolerance may be extreme.
Pioneer® brand varieties are rated on a 1 to 9 scale where 1 indicates poor tolerance and 9 indicates excellent tolerance. If growers are planting into an area with a history of IDC, they should select varieties with an IDC score of 6, 7 or 8. Along with improving chlorosis tolerance, Pioneer soybean breeders have been able to stack other defensive traits such as SCN resistance, phytophthora and brown stem rot tolerance.
Another management practice that has resulted in increased yields in IDC areas is increased plant population. Scientists suggest increasing plant density to 200,000 plants/acre in 30-inch rows in chlorotic field areas. Using GPS systems to map affected fields, and variable-rate seeding equipment to vary seeding rates in affected vs. non-affected field areas could increase the efficiency of this management strategy.
Cover Crop and Nitrogen
Pioneer and the University of Minnesota have conducted studies evaluating the use of competition crops such as oats or wheat to help reduce IDC. Early work suggested additional roots from oats may reduce soil pH and increase iron availability. However, this did not explain an observation that soybeans in a wheel tracks were typically healthier than those outside the wheel track. To help understand this phenomenon, George Rehm (Extension Soil Fertility Specialist for the University of Minnesota) collected soil samples from the wheel track area and soil adjacent to the wheel track (Rehm, 2008). Rehm identified lower levels of soil nitrate nitrogen in wheel tracks and in the plants grown in the soil from the wheel track in a greenhouse experiment.
To confirm greenhouse results in a field environment, a nitrogen rate by competitive crop experiment was conducted at two locations in Minnesota. The results suggested N rate had a very negative impact on yield and the use of oats reduced the impact of nitrogen (Table 1). This was very evident at the Kandiyohi County locations. Yields were near zero when a competitive crop was not used vs. 40 bu/acre with oats planted. In these locations oats were seeded at a rate of 1 bu/acre. The oats were killed with glyphosate when they reached 12 inches in height.
Table 1. Influence of nitrogen fertilizer and a competition crop on soybean yield when iron deficiency chlorosis is a problem. Source: University of Minnesota.
Iron Chelate Treatments
Investigators have looked at various methods of addressing iron chlorosis with an iron chelate, including seed-, foliar- and soil-applied treatments. Because using chelates can be cost prohibitive, researchers consider seed treatments as an efficient means of getting iron to the soil for plant uptake. Fe-EDDHA is the chelate growers can use to deliver iron to the soil. Fe-EDDHA is a dry powder that can be mixed with water and applied to the seed.
To determine the value of seed-applied iron, the University of Minnesota and Pioneer conducted a three-year study. The application of the Fe-EDDHA on the seed was at a rate of 0.06 lb Fe per acre. The seed-applied treatments tended to improve early plant health at V3 but their effect diminished by V6. The seed-applied Fe did not significantly increase grain yield. Within this experiment two foliar applications of ferrous sulfate were made at a rate of 0.5 lbs/acre at V3 and V6 with and without a seed-applied Fe-EDDHA. The foliar applications did significantly improve plant health in this experiment. When Fe was seed-applied and foliar-applied plant health was improved. However, the foliar treatments alone and in combination with seed-applied Fe did not significantly increase grain yield.
In a study conducted by Rehm in 2004, Fe was seed-applied and foliar-applied. The seed-applied Fe-EDDHA significantly increased yields (Table 2). The foliar applications did not increase yields above the seed-applied treatments. Based on this work and Pioneer research, seed and foliar applications of Fe have been questionable at best.
Table 2. Soybean yield as affected by coating the seed with iron and foliar application of iron. Chippewa County, MN.
* 1 lb iron (ferrous sulfate)/acre; ** 2 lb iron/acre
Iron uptake from the soil occurs primarily at the root tips. This would suggest delivery of Fe in-furrow would be advisable. Historically, soil-applied iron chelates were very expensive and could not be justified. However, a new Fe-EDDHA, Soygreen®, entered the market in 2006. Soygreen is a dry water-soluble powder with 6% Fe-EDDHA chelate. The cost of Soygreen is approximately $10.00 to $14.00/acre.
In field observations by Pioneer agronomists, Soygreen® applied in-furrow has shown some promise. In 2007 and 2008, Dr. John Lamb at the University of Minnesota evaluated Soygreen. He reported Soygreen increased soybean yields from 30 bu/acre to 49 bu/acre when it was applied at 3 lbs/acre. He also found when Soygreen was used in combination with a competition crop (oats), additional improvements in yield were observed.
In 2008, Pioneer Agronomy Sciences evaluated Soygreen at two locations with moderate to low chlorosis pressure. When evaluated across varieties with moderate to high chlorosis tolerance, a positive response to the application of Soygreen was observed (Figure 1).
Figure 1. Effect of Fe-EDDHA on soybean grain yield.
A significant variety by Fe-EDDHA treatment interaction was found in this study. The two Pioneer® brand varieties with an iron chlorosis score of 7 did not respond to the applications of Fe-EDDHA. The varieties with a Fe-EDDHA score of 5 had a grain yield response of 2.8 bushels/acre. Pioneer will continue testing in 2009 in locations with historically higher levels of chlorosis.
This recent research suggests growers may have some new tools to manage iron deficiency chlorosis. Soygreen and interseeding competition crops have provided the most consistent means of improving yields in fields with a history of chlorosis. However, the first step to success involves the selection of Pioneer soybean varieties with very good tolerance to iron chlorosis. When considering these management options, contact your local Pioneer sales professional to assist in the selection of the best Pioneer® brand products and management practices for your field.
Pioneer iron deficiency chlorosis research plot.
Franzen, D.W., and J.L. Richardson. 1999. Soil factors affecting iron chlorosis of soybean in the Red River Valley of North Dakota and Minnesota. Pioneer Crop Management Research Award Report. Pioneer Hi-Bred Int'l, Johnston, IA.
Rehm, G., N. Hansen, and P. Gaspar. 2003. Management of soybeans on high pH and SCN soils. Pioneer Crop Management Research Award Report. Pioneer Hi-Bred Int'l, Johnston, IA.
Rehm, G. 2008. Major progress w/ iron deficiency chlorosis in soybean. agbuzz.com. Minnesota Farm Guide and University of Minnesota Extension Service. Online at: http://minnesotafarmguide.com/blog/?p=302