NMDC Soil environmental medium value set soil nitisol Nitisols are deep, well-drained, red, tropical soils with diffuse horizon boundaries and a subsurface horizon with more than 30 percent clay and moderate to strong angular blocky structure elements that easily fall apart into characteristic shiny, polyhedric (nutty) elements. Weathering is relatively advanced but Nitisols are far more productive than most other red, tropical soils.