Are the plants otherwise healthy? I suspect that these are the plants own work (or co work) with a nitrogen fixing bacterium. The Fabiaceae (formerly Legumeaceae or Pea family) have I believe in most cases a symbiotic relationship with a particular genus of bacterium that they have developed for mutual benefit to both organisms.

So weather it's the tiniest annual Medick or the largest Acacia tree and all in between, they will have these little nodules on their roots.

ps, when you dig them up, don't. Just cut them off at the base and turn the roots in later on when they have died. The Nitrogen that the bacteria have fixed from the plants hard work will be released back into the soil as the nodules break down.

if you are really interested the bacteria are Rhizobium and they take atmospheric nitrogen and fix it as a nitrate. the plant gains some nitrate and the bacteria gains some glucose from the plant.

Ah, thats the system, thanks Sbg :-)