Exchange of iron from chelate in the fertilizer against copper, manganese, and zinc in peaty substrates

To overcome iron (Fe) deficiency in horticultural crops, particularly for those crops very susceptible and exhibit marked leaf chlorosis, a new base fertilizer for peaty substrates was introduced with an Fe content (as EDTA chelate) of 0.35%. The fertilizer used normally had 0.09% Fe. With the new fertilizer, higher Fe contents in water extracts were found than before, but also unexpectedly high extract levels of manganese (Mn) and zinc (Zn) were recovered, although the Mn and Zn contents in the new fertilizer were unchanged. The copper (Cu) content of the new fertilizer has been doubled, but the levels recovered in extracts were far more than double. The assumption that accounts for this phenomenon was that the chelate extracted Cu, Mn, and Zn from the peat and the lime. To prove this hypothesis, Fe-EDTA was added to a black and white peat in amounts comparable to those used in horticultural practice with different lime additions. Iron, Mn, Zn, and Cu were analysed in the standard 1:1.5 volume water extract. In order to test the stability of the EDTA chelate, the more stable EDDHA chelate was also used. When Fe chelates were added to peaty substrates, the Mn, Zn, and Cu contents in the water extracts were increased, even without the addition of these micronutrients. This effect was more pronounced when: a) the pH was high: b) a white peat was used rather than a black peat, and c) more with Fe-EDTA than Fe-EDDHA. So the hypothesis proved to be of value. The addition of chelates in fertilizers should not exceed certain limits and their pH stability should be considered because they may act as an extractant, thus elevating the water soluble levels of Mn, Zn, and Cu recovered from the substrate and perhaps to toxic concentrations that could affect plant production. Furthermore, the phenomenon should be taken into account with the interpretation of Fe, Mn, Zn, and Cu contents in water extracts of substrates which have been fertilized with chelates.