Answer: What differences have you noticed about your garden since switching to compost?
Step-by-step explanation:
The follow-up question that demonstrates attention to the topic and point of view of the speaker is "what differences have you noticed about your garden since switching to compost?"
Since the student indicated that the compost is more effective than the fertilizers which are sold at the store, then the student should be asked about the changes that has been noticed when he changed to using compost.