adding negative numbers only in a column

I have a worksheet that is used for tracking clients balances. Each column has both positive and negative amounts based on whether the client is paid up or has payment due. I need to sum only the negative numbers in each column showing balances due so I can see what the total due is. How do I do accomplish this? I can SUM the entire column but it is calculating the positive amounts in with the negative ones so the totals (balances due) are not correct.

Help!! It can't be that difficult can it?? :)

Thank you.