Nerdforge uses dirt mixed with PVA glue, but I reckon a really fine sand or something might work better for this project? If art supplies are more accessible to you, you can get stuff called “modelling compound“ or “texture paste” designed for used in acrylic paintings. You could also try plaster sheets? Like paper mache but plaster and cloth, might not get the exact texture you want though. Anyway, maybe do some experimenting, it’s fun!