Fertilizer chelant

On the third of january I sended this questions to the list and I didn't
get any answers yet. Can anybodi help me and tell me where I might find
the answer to them? It would be really helpful to me.
I would like to know if someone can inform me what the best chelants to
make a fertilizer are and what is the diference between them.
Tropica uses HEEDTA and DTPA. Is there any specific reason to use this
ones instead of more comon ones? I also read it was better to use more
than one chelant. Can anyone let me know how to choose a combination?
Thank you for any kind of information.
Joao Falcato Pereira