Need serious rethinking on the use of Zinc-EDTA or other chelates for plant nutrition - Bio-chemical issues
I think the scientific community – agriculturalists, biochemists, plant physiologists, fertilizer specialists – need to give serious thought to the use of micronutrient chelates as a source of micronutrient nutrition for crops.
There are several fundamental questions that raise doubts on how chelates can ever be considered as a nutrient carrier.
I would say the fundamental issues are the following :
1. Taking Zn-EDTA as an example, Zn from this chelate can be used to produce plant proteins or enzymes if and only if the plant protein / enzyme can remove the Zn ion from its tight binding with EDTA – i.e., the forward reaction
Zn-EDTA + Protein -> Zn-protein + EDTA
must occur. By chemical laws, this can occur only when the dissociation constant of Zn-protein is lower than the dissociation constant of Zn-EDTA (i.e., only if the Zn-protein binding is tighter than the Zn-EDTA binding). The dissociation constant of Zn-EDTA is around 10-16 M whereas zinc containing metalloproteins have way below this (at around 10-9 M to 10-12 M). What this means is that through the plant would have Zn as Zn-EDTA circulating within its system, that Zn would not be ‘useful’ Zn – it cannot be used to synthesise biochemical proteins.
2. Even if some proteins did extract the Zn from the Zn-EDTA then there would be free EDTA circulating within the cells & that would make matters even worse. The EDTA would bind to any other metal ion like Fe and cause further damage.
3. Tissue analysis would indicate ‘sufficiency’ of zinc but that is all in a useless form.
4. EDTA as a carrier is never recommended for animal nutrition – why? Simply for the same reason that the chelated form that goes in will be of no use in biochemical synthesis of essential proteins. In fact EDTA is used as a treatment for removing toxic levels of metals from the human / animal body.
In summary the point is this :- if the science behind a process is not correct how do we recommend its widespread use?
We really need to look into this deeply!