When reloaded the mod show in the logs how many crops were modified in a message like the following one:
Successfully reloaded IMU soils and modified soils for X plants
If the reload of the mod failed you will see Failed to reload IMU soils: followed by an error that must be reported on Github
By default the configuration file is an empty json.
You can modify soils by adding the block id followed either by an array of block ids or just a block id