Question about test kits...

Hey everyone.

I've had a planted 75 litre tank for just over 2 months now. Some of my
plants weren't doing quite well (turning yellow & brown then rotting). I
upgraded my lighting, bought some liquid fert & I just got a Nutrafin CO2
Natural Plant System. We'll see if all this helps...

So here's my question. If I was to get one main test kit for a planted
aquarium, which one is most important? Iron? Phosphate? Nitrate? (I already
have PH, Nitrite, Ammonia, KH & GH)

Thanks for the help.
Ciao!

Mork.