Fertilizer chelant

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?

Joao Falcato Pereira