Nickel has been proven to be essential to plants, and several other elements have been proven to increase robustness and efficiency, even though they are not necessary for plants to reproduce under lab conditions: Silicon and Sodium and Cobalt. So plants probably do best with 20-some elements.
The claim that plants need 90 elements is religio-philosophical: Panglossian rubbish--- Everything that is, is good, because the Creator placed it here for a reason. So drink Radon, Mercury, Lead, and Cadmium and feel sanctimoniously giddy!