As decomposition of dead plant matter and animals occurs, nutrients held within the plant matter or animal is released to the soil. This end result of the decomposition is known as humus. This humus represents up to 1 to 5% of the dry weight of the soil. Because it is rich with nutrients, humus can be extremely valuable to the plant. It is also a “sink” to retain moisture.
On the other hand, if there is too much humus present in the soil, the soil may retain too much water, creating unhealthy conditions. Breakdown of humus is carried out by bacteria, fungi and other organisms like earthworms. For example, earthworms will excrete nitrogen when consuming humus. To create rich humus or organic matter, some farmers or growers will create a compost, providing a fertile ground for plant growth.
Some humus is never degraded because it’s bound inside soil aggregates or bound tightly to charged clay particles. The remaining humus is eventually decomposed.
Watch the below TEDx Talk to learn more about Humus!