What is the real difference between Compost and Humus?

captaincompostal(z7 AL Bham)January 1, 2013

This discussion has come up many times on this site on the Soil/Compost and Organic Gardening forums. When is organic matter "compost and when is it humus?

Here are some of the responses by some of our OG experts and friends on this site. The following are just a few resonses in a recent discussion:

BILL_G:

"Humus is the stable, long lasting remnant of decaying organic material. It improves soil structure and increases water retention. It's nutritive qualities include trace elements and several important organic acids but do not include nitrogen or phosphorus"

IANN:

"Mature compost is still organic matter and can be used when your planting instructions request it. Organic matter that hasn't decayed as far as compost shouldn't normally be used directly on plants because the nutrient balance and pH can get all out of whack as it decays (too much nitrogen in fresh manure, or nitrogen depletion as woodchips decompose) and because it may still contain toxic substances (fungal diseases, weed seeds, or toxic bacteria).

You are right, humus has no nutritive value. It's also impossible to get hold of since it can't be manufactured. It also can't be produced from decaying organic matter in a reasonably pure form on any sensible timescale. Lastly, we don't know exactly what humus is chemically so while you may find stuff sold as "humus" or "humates" or something similar, its buyer beware, you really have no guarantees about what is in the bag.

Humus is good because it has extremely high absorption abilities. It can hold and release water and nutrients as needed. It also improves the physical structure of soil so that it is crumbly and aerated instead of a nasty clay mess or an arid beach."

KELLY_CASSIDY:

"A lot of confusion is created by different uses of the words "organic" and "humus."

To a scientist, an organic molecule is a molecule that contains carbon. All life is based on carbon compounds, so living and dead cells are loosely refered to as organic material, even though they also contain inorganic molecules like water. Leaves, mushrooms, trees, live rats, dead rats, fish, etc. are "organic material." "Organic gardening" is a vague term that means many things to many people. The source of the term is probably that "organic gardeners" tend to avoid adding nutrients in their inorganic form.

To a soil scientist, "humus" is the organic, non-cellular, long-lasting component of soil. It is organic because it is composed of chemicals containing carbon. It is mostly extremely stable carbon compounds with no phosphorus or nitrogen. Their stable form makes them difficult to break down by microorganisms. If humic compounds had N or P, micro-oragnisms would try harder to attack them, but since they don't, they aren't worth the effort. Humus is non-cellular because everything else in the cell breaks down and gets recycled into other organisms, leaving the humus in the soil but no longer within a cell. The term "humus" gets tossed...

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