Iron-deficiency chlorosis is a common problem in soybeans grown on soil where iron availability is reduced due to high pH or other soil conditions. Most soils contain adequate levels of iron, however, at high soil pH (greater than 7.5), the iron is not soluble and cannot be effectively used by the soybean roots.



In IDC-prone fields, soybean yield losses are variable and can be significantly reduced in areas of the field that are poorly drained, higher in pH or higher in soluble salts. It has been estimated that about 4.4 million acres in the North Central Region (Minnesota, Iowa, Nebraska, Kansas, South Dakota and North Dakota) have high pH soils where IDC could be a major soybean problem.



Management options to minimize IDC losses include planting tolerant varieties and the use of seed coatings, foliar sprays or placing iron compounds near the germinating seed at planting. These management practices are expensive and the effectiveness has been inconsistent.



Based on preliminary results obtained at the University of Minnesota and grower experiences, a research project was initiated to: 1) Determine the association between IDC resistance and soybean yield under non-IDC conditions; and 2) Identify the best management practices for growing a competitive crop (oats) to reduce the nitrate nitrogen (NO3-N) level in soil and reduce the severity of IDC. A team of researchers from the University of Minnesota, North Dakota State University, South Dakota State University, Iowa State University, University of Nebraska and Kansas State University designed several studies to thoroughly investigate the two objectives.



In the first study, 20 high-yielding Roundup Ready commercial soybean varieties of locally appropriate maturity were planted on paired sites (one chlorotic and one non-chlorotic) within or near the same field at nine locations in five states. IDC scores and soybean yields were obtained. The research team reported substantial variability in IDC scores at each location and between locations. No consistent response was found between the paired sites. The inconsistency was most likely due to variability in soil properties. Yield comparisons between the chlorotic and non-chlorotic sites were also inconsistent. Average yields for the commercial varieties were similar where chlorosis symptoms were classified as "mild"; whereas, yield was depressed on four of the seven chlorotic sites. Based on this experiment, at these locations, they did not find a strong relationship between IDC symptoms and soybean yields.



To test the theory that IDC severity is related to excess nitrate nitrogen, a field study was conducted to evaluate the ability of oats in removing nitrate nitrogen from the soil. Nitrogen was applied at three rates (0, 100 and 200 pounds per acre) with and without seeding oats. The oats were sprayed with glyphosate when reaching a height of 9-12 inches. The competing crop removed the results indicated between 52-109 pounds of nitrogen per acre.



To further evaluate the theory, studies were placed at five locations to evaluate the effects of oats and nitrogen on the severity of IDC and subsequent soybean yields. The soybean yields at three of the five sites were affected by limited moisture in the 2006 growing season. The researchers concluded from this study that averaged over nitrogen treatment, the use of the competition crop increased soybean yields; nitrogen applications reduced yields; and the results generally supported the theory that reducing nitrate nitrogen levels in the soil may lead to less IDC severity and less effect on soybean yields.



The bottom line is that IDC is a difficult disease to study. The field study results are dependant on several factors (soil components, the genetics of the soybean varieties, environmental conditions and other components) that stress the growing soybean plant and reduce the soybean's ability to utilize iron. Additional research is needed to clarify the abiotic and biotic factors that alter IDC's impact on soybean growth, development and resulting yields.



SOURCE: NCSRP.