Reactions of the iron chelates and the sodium salts of EDTA, DTPA and EDDHA with two alkaline soils, and their effectiveness during growth of barley

The behaviour of the iron chelates and sodium salts of EDTA, DTPA and EDDHA in calcareous (iron deficient) and alluvial (border-line iron deficient) soils and their effectiveness during plant growth were studied. The effectiveness of the iron chelates in maintaining soluble iron and in supplying barley with iron was in the order Fe-EDTA < Fe-DTPA < Fe-EDDHA. Replacement of iron by other cations in Fe-EDTA and Fe-DTPA, rather than adsorption of iron chelate by the soil, was the main factor influencing their effectiveness. The equilibrium of Fe-EDTA with soils was dominated by competition between iron and calcium, whereas that of Fe-DTPA was dominated by iron being in competition with manganese initially and with calcium, zinc and copper afterwards. The relative dominance of the chelate of the competing cation varied with soil type. Fe-EDDHA was neither adsorbed nor was its iron fixed by the soil to any great extent.

The sodium salt of DTPA and EDDHA continually dissolved native soil iron but increased soluble manganese only temporarily. Dissolution of native zinc and copper was least with EDTA and EDDHA and most with DTPA. The sodium salts of the chelating agents were as effective as the iron chelates in supplying barley with iron in the alluvial soil, which was marginally deficient, whereas they were much less effective in the calcareous soil, where iron supply is a greater problem.