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Frequently Asked Questions
Q: Is there a difference between humus
and compost?
A: The definition of humus is a bit slippery. Most scientist agree
that 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 around loosely by gardeners to
mean the organic material that makes soil brownish, not all of which
is humus. In nature, humus accumulates in soil because it lasts for
hundreds or thousands of years.
(Trivia: In a typical northern deciduous forest, most of the organic
material in the forest is in the soil NOT the living trees and most
of the organic material in soil is humus. That's because humus hangs
around for so long.) Humus often lasts in soil until a hot fire
burns through the soil.
"Compost" is a matter of location and planning. Anything living starts decomposing when it dies. You call it compost when you put it in a pile and WATCH it decompose. "
Some definitions: The following definitions were taken from www.gardenweb.com.
Organic matter - stuff that came from biological sources (should be biological matter. Diamonds and graphite don't decompose!) This includes humus, mature compost, immature compost, freshly cut plants, live plants, etc.
Humus - (soil science) that portion of the soil that has fully broken down and is thus stable. This stability is important because it allows you to remove it from consideration in a lot of investigations, and this is why soil sciences define it this way. This, incidentally, is also why its constituents have not been subjected to rigorous scientific investigation. It is not that it is somehow mysterious or has magical properties that elude investigation, it is just that it is only recently that anyone has really bothered to think about applying modern analytical techniques to this fraction of the soil. Contrary to common belief, a large number of its constituents ARE known. Their relative proportions vary from implementation to implementation, however, and thus no simple answer is really forthcoming as to "what humus is," as it is a dynamic mix of substances. humus - (colloquially) The organic portion of the soil (you can see how this definition has use for the gardener, but is too impercise for much usefulness in an investigation of soil properties.)
Compost - organic matter in a purposeful state of partial decomposition. The purposeful part is important. Dead stuff on the ground is NOT compost, just decaying orgainc matter. It is the controlled, or semi-controlled conditions that make it compost.
Immature Compost - compost that has not undergone enough decomposition to be of maximal benefit. This definition is thus use-specific, but usually implies insufficient pathogen destruction, lack of friability, poor moisture retention, active generation of metabolic gasses. Primarily still in bacterial stage of decomposition.
Mature Compost - compost that has decomposed to the point of maximal usefulness. Usually in the fungal/actinomycete stage.
So all mature compost is organic matter, but not all organic matter is mature compost. If what you are after is soil conditinoing alone, then the terms humus and mature compost are all but interchangeable."
FIELD:
"One more important attribute of humus should be listed, in addition to improving soil structure (or texture) and water retention. Humus also has a high cation exchange capacity, which means it acts as a veritable storehouse for plant nutrients, something that can be especially important for those with sandy soils."