Of or containing iron or iron rust. A ferruginous soil is a very deep, zonal soil found in warm temperate climates without a dry season, or in tropical savannas/bushlands. The A horizon is dark red-brown with a weak crumbling structure; the B horizon is stained red by ferruginous gravel. In US soil classification, a ferruginous soil is an ultisol. See B. Warkentin (2006).