Well I didn’t know about this one. A very impressive story.
Andhra folklore states that once Karna while riding his chariot in his kingdom of Anga, encountered a child who was crying over her pot of spilt ghee. On asking her the reason for her dismay, she stated that she feared her step mother who would be angry over her carelessness. Being generous enough Karna told her that he would give her new ghee (clarified butter). But, the child insisted that she wanted only the ghee that was mixed with the soil and refused to take the new ghee. Taking pity on the girl, Karna took the soil mixed with ghee in his fist and squeezed it with all his might to extract the ghee and pour it back into the pot. During this process, Karna heard the agonized voice of a woman. When he opened his fist, he realized that the voice was that of Bhoomidevi – the Earth goddess. Furious, she chastised Karna for inflicting enormous pain on Mother Earth for the sake of a mere child. So, the Earth goddess cursed him that in a very crucial battle of his life, she would trap his chariot wheel in the same way that he held the fistful of soil, thereby making him vulnerable to his enemy.
Well this story clears my doubt about Karna’s strength. Karna was as strong as Bhima. For he was undefeated in mace battle with bhima in the battle of Kurukshetra. In addition, he was amongst the few men such as Bhim, Duryodhana, Dau (Krishna’s elder brother) and Bhishma who could have killed Keechaka (a mighty and powerful warrior who was killed by bhima during agyatvasa). His knowledge about this fact led the Kauravas and Karna to attack Matsyadesh where the Pandavas were spending the year of their agyatvas.