A miscommunication between the flight deck and ground personnel resulted in a CRJ-200 taking off with only half of the required weight and balance ballast aboard.

Narrative

I performed a manual CG calculation using the CG calculator. The flight was full with 50 passengers. I determined that ballast would be needed to meet forward CG limitations; and decided that 300 pounds would be sufficient. I requested 400 pounds to allow for any errors made with the CG calculator. I spoke directly to a Ramp Agent and requested 400 pounds of ballast. After takeoff ATC informed us that Operations had contacted them about an incorrect amount of ballast loaded. Apparently the Ramp Agent thought I was asking for four bags of ballast; which is only 200 pounds. I may not have been perfectly clear when I requested the ballast. Further; I did not verify that the correct amount had been loaded before closing the main door. Also; I entered the ballast weight on the weight and balance page myself. Apparently the station noticed the amount that I entered after we blocked out; and they were unable to reach us by radio since we had the radios tuned to CTAF and the FSS. After being notified of the information by ATC I contacted Operations via radio. They explained that only 200 pounds had been loaded. The aircraft was flying normally so we continued to destination without further event. I should have made my request more clear; verified the correct loading after they were finished; and shown the corrected weight and balance document to the Ramp Agent after I made the changes.