Some of the problems concerning iron nutrition of plants after four decades of synthetic chelating agents

Abstract

Even though considerable progress has been made in the past ten years with the six iron (Fe) symposia, many problems concerning Fe nutrition still remain. The differential Fe stress response mechanisms in plants appear to be more complicated than earlier thought and may be related to degree of stress. Differential cation‐anion uptake by plants which can regulate, at least in part, the pH of the rhizosphere is only vaguely understood. Iron stress, region of plant root, age of plant, different nutrient substrates, differential plant species and cultivare can give different responses. Nitrate‐N nutrition, ammonium‐N nutrition, and dinitrogen fixation‐nutrition, although varying with species, at times can be more important than cultivar for differences on pH in rhizosphere. The critical level of Fe (and other micronutrients) in nutrient substrates in chelate‐buffered solutions probably requires an expanded definition of critical level to include the reservoir. A critical value may be valid only if it can be expressed as a ratio to other needed nutrients. Improvement in analytical techniques during the forty years that synthetic chelating agents have been used for plants has not made it possible to solve all the problems involved. Considerable progress has been made, however, especially with use of computer programs, such as GEOCHEM‐PC. Among troublesome problems which remain are opposite effects on micronutrient uptake which occur for soil vs. solution culture; why levels of metal chelates that are adequate for solution cultures are inadequate for the soil solution; what problems arise if synthetic metal chelates are not 100 per cent excluded by roots but only 99 per cent excluded and how important is passive entry through breaks in roots; is there reason for apparent inconsistent results for use of FeEDDHA; are all sources of EDDHA similar, especially those that result from different manufacturing techniques now versus 30 years ago; is EDDHMA really as effective as EDDHA which sometimes appears to happen but not always; can a plant root sometimes remove Fe from an Fe chelate without the Fe being in the free ionic state; and are metal chelates bridged to clay in soil through polyvalent cations. Synthetic chelating agents have probably failed to reach expectations as micronutrient carriers with soil, but they have contributed to the knowledge of plant nutrition.