Cleaning brick

I have small brick porch in back of my house with a nice tree that gives shade to sit under. It was made from used brick, and looked good. But last year, when they had to open up a trench through it for some pipes to go in the ground, the brick came in touch with soil and, I guess, were lying around "dirty" for a while. When the pipes were in I put the bricks back and washed them, but they still have this muddy look when they dry (look o.k. when wet). I think the mud got into the pores of the brick or something like that. Is there a way to clean them? Marianne