Mor humus

Mor is a humus form occurring mostly in coniferous forests and ericaceousheathlands. Mor humus consists of evergreen needles, ericaceous twigs and foliage and woody debris that litter the forest floor. This litter is slow to decompose, in part due to its chemical composition (low pH, low nutrient content, high lignin, tannin or terpenic content), but also because of the generally cool and wet conditions where mor humus is found. This results in low bacterial activity and an absence of earthworms and a low presence of other soil fauna. Because of this, most of the organic matter decomposition in mor humus is carried out by fungi.

Mor humus is one of three current classifications of forest floor humus, along with Moder and Mull. Each class corresponds to a scale of increasingly colder conditions, decreasing biological diversity and activity, and decreasing nutrient availability. Mor humus ranks at the bottom of this scale and is characterized by very slow decomposition and accumulation of plant material. In the seminal work of Peter Erasmus Müller on Danish forest soils, which described mor for the first time, this humus form was called beech peat by analogy with sphagnumpeat.