The layer above the topsoil in a northern forest soil is called the organic horizon, or organic layer. It is composed of decomposing plant material, such as leaves, twigs, and branches, as well as dead roots and other organic matter. The organic horizon is formed through the process of decomposition, which is the breakdown of organic matter by microorganisms, such as bacteria and fungi.

The rate of decomposition in the organic horizon is influenced by a number of factors, including temperature, moisture, and the availability of oxygen. In northern forests, the cold climate and high levels of moisture slow down the rate of decomposition, which allows the organic horizon to accumulate. The organic horizon is an important part of the forest soil ecosystem, as it provides nutrients for plants, helps to hold water in the soil, and provides habitat for microorganisms and other small organisms.