Humus

Humus is the magical dark stuff of soil that helps plants grow. Or is it?

We know that soil is made up of organic matter (1-5%), minerals (45-49%), water (25%) and air (25%). Organic matter includes living things and the remains of things that used to be alive. 

Until recently, humus was seen as a mysterious dark material that didn’t break down easily in the lab. Better technology and more strict research methods are bringing the whole concept of humus into question. Until scientists sort things out, we can assume that humus is nothing more than organic matter. Whatever we call it, organic matter in the soil is important for plant health.

Microorganisms in the soil facilitate the decomposition of plants, bug bodies, and other living things in a process called humification. Humification occurs in the soil and in compost piles. Along with microorganisms, worms, nematodes, and other tiny critters help this transformation along. The black, rich, earthy smell from premium soil is the humus, or organic matter. (The black color is from carbon.) ​

Humus is the dark organic matter found in soil. (Wilsonbiggs) CC BY-SA 4.0

When you add organic matter to the garden or compost pile, you are feeding the organisms that make nutrients available to your plants.