"Sixteen Tons" is a song about the life of a coal miner, first recorded in 1946 by A 1955 version recorded by Tennessee Ernie Ford reached number one in the Billboard charts.
According to Travis, the line from the chorus "another day older and deeper in debt" was a phrase often used by his father, a coal miner himself. This and the line "I owe my soul to the company store" refers to the fact that workers were not paid cash; rather they were paid with non-transferable credit vouchers which could be exchanged for only goods sold at the company store. This made it impossible for workers to store up cash savings. Workers also usually lived in company-owned dormitories or houses, the rent for which was automatically deducted from their pay. Lumberjacks in Michigan were often paid the same way. From Wikipedia