The use of iron chelates in compound fertilizers containing trace elements

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Abstract
The chemical behavior of Fe chelates, incorporated in compound fertilizers with inorganic micronutrient compounds, was studied. Pot experiments were conducted to evaluate the effectiveness of these products in controlling Fe deficiency. A commercial compound fertilizer, containing 0.09% Fe as Fe-DTPA [diethylenetriaminepentaacetic acid] 0.12% Cu, 0.16% Mn and 0.04% Zn as sulfates, proved ineffective in preventing Fe chlorosis in Chamaecyparis lawsoniana ''Alumii'', growing on sphagnum peat at 2 lime levels. The same fertilizer formulation with 0.09% Fe as Fe-EDDHA [ferric ethylenediamine di(O-hydroxyphenyl)acetate] prevented Fe chlorosis at both lime levels. Fe-EDTA, incorporated in the compound fertilizer, gave good results at the lower but not at the higher lime level, due to the low stability of Fe-EDTA at high pH. Data from laboratory experiments showed that Cu replaced the chelated Fe in the compound fertilizer containing Fe-DTPA, causing the Fe to precipitate. The strong competition between Cu and Fe for the organic ligand is due to a specific affinity of Cu for DTPA, resulting in a Cu chelate with high stability constant and a molar ratio of Cu to chelating agent of 2:1. In the case of Fe-EDDHA and Fe-EDTA the competition between Fe and Cu is much weaker. In contrast to Fe-DTPA, these chelates remain rather stable when incorporated in fertilizers containing micronutrients.