What differences have you noticed about your garden since switching to compost?
The student who switched to compost indicates that she has found it to be more effective fertilizers sold at the store. The appropriate follow up question would ask her further information about how she knows it's more effective. Does she have a higher yield? Larger plants? Fewer insets and grubs? What differences did she notice once she switched to compost? She may not know the answers to the other questions based on her change from fertilizer to compost because they don't directly relate to the action of switching.