Though iron deficiency symptoms can be visually apparent in most crops, the underlying reasoning for reduced uptake or availability can be more complex. Dedicating time to understanding the soil‐plant environment in each distinctive soil where you suspect iron to be limiting productivity is well worth it.
Iron (Fe) is a nutrient required by all organisms, including microbes, plants, animals, and humans. It was first recognized as a necessary plant nutrient in the mid‐19th century when iron‐deficient grapes were successfully treated with foliar applications of iron‐bearing salts. Iron is a component of many vital plant enzymes and is required for a wide range of biological functions. As the fourth most common element in the earth’s crust, most soils contain abundant iron but in forms that are low in solubility and sometimes not readily available for plant uptake. The concepts of 4R Nutrient Stewardship—understanding how the right source of iron can be applied at the right rate with optimal timing and placement—can alleviate iron deficiencies.
Iron in Soils
Iron is abundant in many rocks and minerals, and as soils develop, there can be either enrichment or depletion of iron. Depletion commonly leads to deficiency, and enrichment can cause toxicity when potentially soluble iron minerals are high and in poorly drained soils. Iron concentration can be present at 50,000 times the crop’s annual demand, but factors that affect availability limit utilization. The main source of iron in soils for use by plants comes from secondary oxides absorbed or precipitated onto soil mineral particles and iron–organic matter complexes.
It is critical to consider that iron occurs in two oxidation states: reduced, as ferrous iron (Fe2+), or oxidized, as ferric iron (Fe3+). Soil pH and water‐filled pore space will significantly affect the form of iron present. In aerated soils, iron is readily oxidized to its ferric state and forms a group of highly insoluble ferric oxides and hydroxide minerals, such as goethite (FeOOH) and hematite (Fe2O3) displayed in Figure 1. The presence of specific iron forms will affect the dominant soil color (Figure 1). Organic matter content (brown to black) and iron (yellow to red) presence have a tremendous influence on matrix soil color (Schwertmann, 1993; Schulze et al., 1993). When iron is reduced, or in Fe2+ form, gray color dominates the soil matrix, as is the case in many oversaturated or hydric soils. If adequate aeration has occurred, Fe2+ will lose electrons and exist as Fe3+ with the characteristic reddish hue.
Determining soil iron concentration is complicated, and multiple methods of soil iron analysis have been established depending on the use of the soil test results. The most widely used extraction procedure is the DTPA (diethylenetriaminepentaacetic acid) method (Lindsay and Norvell, 1978). This method utilizes acid dissolution of iron‐containing minerals and complexes and then chelates to iron in solution for quantification. Other methods that have been developed, but not adopted in more routine soil‐testing programs, include magnesium chloride extraction and a buffered reaction that reduces iron in the soil sample and then chelates with the iron for analysis (Holmgren, 1967). A remaining challenge of determining the need for iron application or the rate to apply iron to a crop is that soil test iron has not been well correlated with crop yield or nutrient uptake. Currently, it is best used as an index for potentially bioavailable iron in the rooting zone.
Iron in Plants
In plants, iron plays an essential role in oxidation and reduction reactions, respiration, photosynthesis, and enzyme reactions. For example, iron is an important component of the enzymes used by nitrogen‐fixing bacteria. Iron requirement, and thus uptake, is relatively low compared with other essential nutrients. Bender et al. (2015) reported 0.76 lb Fe/ac taken up by a soybean crop yielding approximately 52.5 bu/ac, and more than half of the iron was accumulated before seed filling. In corn, total plant iron uptake has been reported to be between 1.25 lb Fe/ac (Bender et al., 2013) and 1.74 lb Fe/ac in a 2000‐era hybrid, which was measured at reproductive stage R6 (Woli et al., 2019).
Plant roots absorb iron from the soil solution most readily as (ferrous) Fe2+, but in some cases, also as (ferric) Fe3+ ions (Kobayashi and Nishizawa, 2012). Plants have developed clever methods of influencing rhizosphere conditions to obtain iron, and generally utilize two ways to access the Fe2+ or Fe3+ ions. The first strategy occurs in dicot species and non‐grass monocot species where Fe3+ ions are reduced to Fe2+ ions before moving into the root across selective membranes (Marschner and Römheld, 1994). This process involves the root excreting a variety of organic compounds and acids into the soil and is employed by most fruit and tree crops. In the second strategy, roots of grass species acquire iron by excreting an organic chelate (siderophore) that solubilizes iron from the soil, allowing enhanced uptake, used by crops such as corn, sorghum, and wheat (Marschner and Römheld, 1994). Chelates are simply organic or synthetic chemical compounds with multiple sites to bond with metals, or in this case, iron.