Dermatology: Skin crusts are dried serum, pus, or blood usually mixed with epithelial and sometimes bacterial debris. A crust protects a skin lesion and enables healing of the skin. A crust forms a hard irregularity in the skin surface, which often irritates or itches and causes picking. We feel repulsed by the look of a skin crust.
Geology: Soil crusts are soil surface layers that are distinct from the rest of the bulk soil, often hardened with a platy surface. Depending on the manner of formation, soil crusts can be biological or physical. Biological soil crusts are formed by communities of microorganisms that live on the soil surface whereas physical crusts are formed by physical impact such as that of raindrops.
Skin crusts and soil crusts both are surface layers formed by different sorts of interaction with their respective surfaces.