Welcome to part six of the column series on soybean micronutrients. The last couple of weeks I discussed boron, chloride, cobalt, and copper. I will quickly cover how iron (Fe) is essential to soybeans, known deficiencies and toxicities in the region, factors of iron availability in soil, soil and plant tissue testing, and management recommendations. Micronutrients for Soybean Production in the North Central Region is a good regional publication that addresses iron that I utilized as a resource for this news column.
In soybeans, iron is a key micronutrient involved in respiration and chlorophyll for photosynthesis. Iron is not very mobile within soybeans, so symptoms show up in new growth first. Iron deficiency or more commonly called iron deficiency chlorosis (IDC) include symptoms of yellow leaves with green leaf veins, called interveinal chlorosis. There is documented iron deficiency chlorosis in southeast Nebraska on high pH soils, areas with poor soil drainage, and saline soils. The Bulter-Gayville silt loam soil in the Big Blue River and Turkey Creek floodplain can be hot spots for iron deficiency chlorosis. Eroded hillsides with exposed subsoil or hillside seep areas can exhibit iron deficiency chlorosis in Saline, Jefferson or Gage counties. Lastly, Kipson and Benfield soils derived from weathered limestone and shale in portions of Gage and Jefferson counties could cause iron deficiency chlorosis. The appearance and severity of iron deficiency chlorosis and corresponding yield loss will vary between years due to several factors.
Soybeans and grain sorghum are more susceptible to iron deficiency than corn and winter wheat. Plant species have different mechanisms to extract iron from the soil. Soybeans excrete acids and reductants to uptake the reduced iron form needed (Fe2+). Soil availability of iron by soybeans is influenced by soil pH, free carbonates, moisture and aeration, and nitrate. Along with other routine soil test analysis, the DTPA extraction method for iron is helpful. DTPA iron soil test values less than 4.5 ppm would suggest increased chances of a fertilizer or management practice response in soybeans. Unlike past micronutrients I mentioned, tissue analysis of soybean leaves for iron is not particularly useful due to iron contamination from soil dust, equipment, and few other challenges.
Research in Midwest and Great Plains have found yield increases from iron fertilization along with some other management practices. The most successful fertilizer option is in-furrow ortho-ortho EDDHA chelated iron fertilizer (e.g. Soygreen®). Some of my research in Kansas included using this fertilizer as a seed treatment, which also proved to be effective at alleviating iron deficiency chlorosis, but there is a not a protocol for commercial seed treatment at this time. The use of cover crops can also help reduce iron deficiency chlorosis through reducing soil nitrate levels. Selecting varieties more tolerant to iron deficiency chlorosis is another important management practice. Increasing seeding rates in wider row spacing is another practice used in the past to reduce early season iron deficiency chlorosis. With precision agriculture technology, hots spots for iron deficiency chlorosis in fields can be better managed by using multiple-variety/hybrid planters and variable rate in-furrow fertilizer prescriptions.
I look forward to writing about manganese in next week’s column. You can share or read this news column online through my local website for Saline, Jefferson, and Gage counties at croptechcafe.org. Know your crop, know your tech, know your bottom line.
