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In the present study, five insoluble Fe-humic substance (HS) complexes (Fe-IHS) extracted from Leonardite were synthesized and characterized by Infrared (IR) spectroscopy, scanning electron microscopy (SEM), and dynamic light scattering (DLS). The efficiency of Fe-IHS complexes in curing Fe chlorosis was compared with two soluble Fe sources (Fe complexed by a water-extractable humic fraction [Fe--WEHS] and the synthetic Fe-chelate, Fe-o,o-EDDHA]) and a poorly soluble mineral form (Ferrihydrite). The IR spectra revealed coordination of the Fe ions with the carboxylic and phenolic groups of the oxygen-rich humic substances. The SEM and DLS data suggest that the Fe-IHS complexes were large aggregated particles with diameter of about 500 to 1500 nm formed by different nanoparticles with very small size ranging from 50 to 200 nm. Chemical extractions show that the Fe pool is dominated by poorly ordered Fe phases strongly associated with IHS. The efficiency of Fe-IHS complexes in curing Fe chlorosis symptoms in Fe-deficient plants has been demonstrated, being, when used in high concentration, comparable to those of Fe-WEHS and Fe-o,oEDDHA. The capability of cucumber (Cucumis sativus L.) plants to acquire Fe from poorly soluble Fe forms was also confirmed when a poorly crystalline radiolabeled Fe-oxide (59 Ferrihydrite) source, used to represent a short-range order Fe phases associated with Fe-IHS complexes, has been used. The results of this work point out the possible use of environmental-friendly Fe insoluble high- molecular weight complexes (Fe-IHS) as an effective product to correct the Fe nutritional disorder. [ABSTRACT FROM AUTHOR]