Thanks @ron_horne. The model I am using has nominal factors as fixed effects as well as crossed:
plant richness (1, 2, 3, 4);
Mic_dil (D0, D2, D6);
Drought (W, D);
plant richness x mic_dil;
drought x plant richness;
drought x mic_dil.
The model also has random effects.
If they would be continuous I would understand how to standardize them but I am not sure how to in this case? Thank you for your suggestion.