I have a question about soil humus.  A few texts I've read say that
humus is composed of 50-60% carbon, around 5-6% nitrogen and smaller
amounts of other elements.  Since humus is, by definition, made up of
only decomposing plant materials, I am curious as to what the remaining 30 or so
percent of humus is made up of.  All I've been able to come up with
are waxes, lignins, fatty acids, etc (which are all carbon based
compounds).  Perhaps you can solve this mystery?

The remaining mass is mostly oxygen and hydrogen, combined with the
carbon and nitrogen in various complex, and poorly
characterized/understood compounds.  See, for example,
http://www.ar.wroc.pl/~weber/kwasy2.htm.

Thanks for the info- exactly what I was looking for!