Marl or bog lime,soil, essentially clay mixed with carbonate of lime, highly valued as a dressing or fertilizer. It crumbles rapidly and easily. Marl in which the lime is in the form of invertebrate shells is called shell marl. The term is loosely used for a variety of soils, some of which are low in lime content, e.g., the greensand marl of New Jersey. Marling of soil tends to lighten it, correct acidity, and promote nitrification.