What plant are we talking about? Some plants do have nodules on the roots, can you post a photo please.

Members of the pea family and alders form small roundish nodules that hold bacteria that take nitrogen from the air and turn it into forms that plants can use. On many other plants, similar, but more irregular nodules are often the result of root-knot nematodes, common in, but not exclusive to, sandy soils. If a picture isn't possible, you might want to use your favorite search engine to find images and compare them to your roots.