The Issaries blessing "Spare Grain" is listed as "Obtains extra food via trade." On first read, I thought this meant bonus food only when buying food via caravans.
Does "Spare Grain" give you food automatically or does it require using caravans?
The Spare Grain blessing is useful even with no trading partners. And I can’t think of a blessing that requires you to do something in particular, so you don’t need to send caravans.
Spare grain blessing give you more food. It's more effective if you have more trading partners.
So yes it requires from you to use trades.