Humus is the most basic resource for tree farms. Growing lots of trees in a small space requires a mineral and nutrient rich soil. Growing lots of trees in a small space will deplete any mineral and nutrient rich soil in quite a hurry, so be prepared to fertilize lots and lots of dirt and ship it to your tree farms. Players can produce this quality soil (called humus) by fertilizing plain old dirt.
There are two types of fertilizer: The biological variant, Manure, and the mineral variant, Fertilizer. They can both fertilize the same amount of dirt (eight for every manure or fertilizer) after being created. Alternatively, Humus can be created by using a Carpenter with Mulch instead of fertilizer and get 9 Humus, though this does require some use of water as well.