Wire for 50A 220V Service - 110' run

The wire currently in use is old - it was there when we bought his place 27 years ago. At the time, it did not extend into the barn. There was no electricity at in the barn since its primary purpose was for hay storage. The wiring exits the pump house and is exposed to sunlight for about two feet. The rest of the cable is buried and surfaces inside the barn where I have the breaker box. Although I've had no problem, I was a bit afraid that the unprotected cable may have deteriorated due to exposure to sun and the elements. And since I had the other wire on hand, it looked like a possible fix. Having read all the replies, it seems logical to install two 30amp circuits using the existing four wires. At present, one of those four is not connected to anything.

Many thanks to those who know a lot more about sparks than I. Also many thanks to those who know even less than I about sparks, but were willing to give it a shot... :-)

GM