You need to perform following steps
1. Add +AUDITLOG() to UI_INFO of your custom field
2. Publish schema changes.
3.Create new activity notification (activity type) e.g. zfield1_update.
4. Create new association type and relate there your zfield1 with activity type zfield1_update.
5. Create notification rule.
6. Go to your zfield1_update activity notification and associate it with the notification rule You just created.
This will trigger notification when zfield1 is changed, You can also associate more than one field with your custom activity notification, if you don't need separate notifications for them.