Alert for low inventory level

Does anyone know how to setup an alert based on the quantity available field for inventory items?  Specifically, if the qty available drops below 50 pcs.

I have a search created for items that have dropped below a certain quantity available.  I can schedule this search to go out weekly, but the recipients would have to compare it to the prior week's list to see what items have been added to the list. 

To simplify things, I figured I could notify customers for each item when it drops below 50 pcs.  I tried the "Send Email Alerts When Records are Created/Updated" option and entered a sales order to reduce the qty available for a specific item.  The e-mail alert did not go out but the item is now on the results of the search.  Do the alerts only work when a user actually goes in to a record and keys in a change?  The qty available is a system field that subtracts qty on order from qty on hand as orders are entered.