Humus is a block which is necessary for an Arboretum to function properly, especially when used in conjunction with a Logger. Forestry functions on the principle that growing something using an automatic farm dehydrates the soil; as such, when Humus is used in tree farms and a tree is grown on it from a Sapling, it will revert into Sand, which must be replaced with more Humus for the tree farm to continue functioning.
Humus requires fertilizer of some sort, either in the form of Compost derived from biological materials, and Fertilizer derived from minerals (Apatite). Each allows for eight dirt to be turned into Humus. Due to Apatite being fairly common and due to each Apatite producing 8 Fertilizer, a single gem of Apatite can create a stack of Humus, which can create several stacks of logs and saplings as well as a few dozen Apples. Humus can also be created using a Carpenter using Mulch instead of fertilizer, allowing for the creation of 9 Humus per fertilizing agent. However, this requires the use of water and power from a BuildCraft compatible engine.