You may determine the soil properties and then implement the weighted soil quality index.
to determine soil fertility, you may generate some regression models by exploring the relationship between soil and some agronomic traits such as yield, lai, etc
however, a talk with the supervisor is always a good step as suggested
Also consider determining some biological soil quality indices such as microbial biomass carbon, soil basal respiration, and metabolic and microbial quotients to better understand your soil health and fertility status.