Iron fertilization with FeEDDHA : the fate and effectiveness of FeEDDHA chelates in soil-plant systems

Abstract

Iron deficiency chlorosis is a nutritional disorder in plants which reduces crop yields both quantitatively and qualitatively, and causes large economic losses. It occurs world-wide, predominantly in plants grown on calcareous soils, as a result of a limited bioavailability of iron related to the poor solubility of iron at high soil-pH (7.5-8.5). Iron fertilizers based on FeEDDHA (iron ethylene diamine-N,N'-bis(hydroxy phenyl acetic acid)) chelates are among the most efficient in preventing and remedying iron deficiency in soil-grown plants. FeEDDHA based fertilizers comprise mixtures of FeEDDHA components which can be divided into racemic o,o-FeEDDHA, meso o,o-FeEDDHA, o,p-FeEDDHA and rest-FeEDDHA. Both the composition of FeEDDHA based fertilizers and the properties of the FeEDDHA components differ considerably. The fate and effectiveness of FeEDDHA chelates in soil-plant systems were examined in order to facilitate a more efficient use of FeEDDHA fertilizer. Upon interaction with soils, racemic and meso o,o-FeEDDHA largely remained in solution, whereas o,p-FeEDDHA and rest-FeEDDHA were largely removed. In plant experiments it was found that, on soils in which plants from the blank treatment became chlorotic, the iron concentration in soil solution, governed by racemic and meso o,o-FeEDDHA, determined iron uptake by plants. When introduced into soil-plant systems, the concentration of racemic and meso o,o-FeEDDHA underwent an initial concentration drop due to adsorption, and gradually declined further. In particular for meso o,o-FeEDDHA, this gradual decline was not related to iron uptake by plants. The gradual decline could not be explained by biodegradation, which did not significantly affect the concentration of FeEDDHA components. From mechanistic multi surface modeling and batch interaction experiments it was concluded that there is a basis for assuming that under soil conditions iron can be displaced from o,o-FeEDDHA components by copper. This displacement reaction was closer examined in goethite suspensions, and it was found to predominantly take place on reactive surfaces rather than in solution. Factors enhancing adsorption also enhanced the rate of the displacement reactions. The observed concentration behaviour of meso o,o-FeEDDHA in soil-plant systems could be explained from the displacement reaction. Furthermore, the effectiveness of EDDHA ligands in chelating and mobilizing iron from soil, after delivering iron at the plant root (the so-called “shuttle mechanism”) was examined. Plant experiments provided experimental support for a re-chelation mechanism, but results from batch interaction experiments indicated that the efficiency in chelating iron from soil is probably low as a result of complexation of other cations, in particular copper. In conclusion a conceptual model for the behaviour of FeEDDHA components in soil-plant systems was composed. The essence of the model consists of three processes: 1) FeEDDHA adsorption, 2) iron displacement from FeEDDHA by copper on a soil reactive surface followed by release of CuEDDHA into soil solution, and 3) re-adsorption of CuEDDHA. Clay content, iron(hydr)oxide content and copper content were identified as soil characteristics substantially compromising the effectiveness of FeEDDHA components