n. A layer of moderately to highly decomposed leaves, needles, fine twigs, and other organic material found between the mineral soil surface and litter layer of forest soil.
The duff layer is classified by degree of decomposition as upper duff and lower duff. Lightly to moderately decomposed upper duff comprises the Oe soil horizon, which is also called the fermentation layer or F-layer. The more highly decomposed lower duff comprises the Oa soil horizon and is also called the humus layer or H-layer. Duff load, or weight per acre, is a function of duff depth and density. See table below for several duff classifications that are currently in use. See Soil Classification Working Group 1988, Soil Survey Staff 1975, and Soil Survey Staff 2006 for further discussion.