Wikipedia is not the final authority but I found this entry interesting:

"In soil science, humus (coined 1790�1800; "In agriculture, humus is sometimes also used to describe mature compost, or natural compost extracted from a forest or other spontaneous source for use to amend soil[3]. It is also used to describe a topsoil horizon that contains organic matter (humus type,[4] humus form,[5] humus profile).[6]"

So, it's utterly completely broken down stable organic matter, OR compost, which is not most of those. Take your pick.

This should be an interesting thread. [dons raincoat] :-D

"Can anyone define what is truly considered "humus" and what is considered "compost?""

I doubt it. Even "compost" has about a bazzillion different definitions.

".... is it a degree to which the material in question is broken down?"

That is what I think.

Actually, I really don't care about the definitions and the nitty gritty science stuff. Add half to an inch of good quality, mature compost to your soil each year and all is good.

Lloyd

Humus is more aged then compost. Compost is actually better, because when you use compost the natural processes turn it into humus, providing more benefits along the way. Humus could be considered overly aged and therefore lacking in fiber.

From what I've learned about the value of an ACTIVE biological community, I would agree that fresh compost is better. BUT the question was about how each is defined, not which is better.

I tend to think of humus as the ultimate end of the line, and compost as 90% of the way there and on a long tail of that last 10% of decomposition.

tropical thought, is that your opinion or have you got links? I'm very interested in this stuff at the moment and from what I've read, humus's stability in mineral soil has many benefits.

This may help.

Here is a link that might be useful: Soil Humic Substances

I think the more I read, the more confussed I become.
So I am going to say compost is old Organic Matter.
That humus is old compost.

First type of humus is characteristic for podzolic soils, grey brown soils and lateric soils
under forest communities. In this humus predominate humic acids, thus humic acid/fulvic acid ratio is below 1. Humic acid indicate small extent of aromatic rings condensation and they are approximate to fulvic acids. Considerable hydrophilic properties of humic acids favor to creation of chelates with polyvalent cations and ability to displacement deep into profile of soil. Considerable mobility of this humus favor process of podsolization.
Second type of humus is characteristic for phaeozems, rendzinas, black earths and
brown soils. Humic acid/fulvic acid ratio is upper than 1, Extent of aromatic rings condensation is high in humic acids, which cause their hydrophobic properties and inability to creation of chelates. Humic acids are strongly connected with mineral portion of soil in this type of humus.
Third type of humus is characteristic for semidesert soils. In this humus predominate
fulvic acids fraction, whereas arise of humic acids is limited. Beyond this, humic acids are largely bounded with mineral portion of soil.

Think of Humus are the residual organic matter in soil, what is left after the Soil Food Web digests what they like.
Compost, when added to soil, contributes to the humus level after a while by feeding the Soil Food Web.

At the stores, I think they define compost as being more expensive than humus :-)

Compost is cheaper then humus. If you could find humus for sale, it would not be more expensive. Maybe you are thinking the other way around? It takes a lot of compost to create humus, therefore as you can see humus is more expensive.

You could ask these guys how much a 50 lb. bag of humus costs.

Here is a link that might be useful: Black Earth

Thanks all, I understand now and ultimately compost will stabilize into humus. Neither would be bad to add to the soil! LOL!

If you look more careful into the humus link from canada, it says "Our humus material is of oxidized sub-bituminous coal from Alberta, Canada, which is identified as humalite." So, that is actually a coal and not really as if someone made compost and let it break down to humus. I would take the free sample, but they probably won't send it to the USA. I don't know if coal would do my soil any good. I don't think so. What is the differences between maybe adding some charcoal or adding some of that coal? Is not coal just more aged charcoal?

What is sold in stores as "humus" or "humates" is probably not really something someone would really want to add to thier soil.
Coal, and charcoal, could be defined as humus, the undigested residue of organic matter in soil. However, neither really add much of anything worthwhile to soil. Given enough time, and pressure, charcoal could be made into coal. Given enough time, and pressure, most any organic material could be made into coal. But coal is not a good soil amendment.

The newest research on the humus molecule is being done at the National Laboratories at Los Alamos and Sandia, New Mexico. I have posted extensively on humus vs compost a few pages back on this forum. See the thread by bob1016 on humus and also the thread on mycorrhizae. Google soilsecrets.com, soilsecretsblog.com and my website, thedoublevictorygarden.com for more info. Michael Melendrez of Soil Secrets LLC probably knows more about humic substances (lay term humus) than anyone that I know of.

Here is a link that might be useful: The Double Victory Garden