[alancalverd (OP)]

I spent a happy couple of hours today spreading some good home-made organic muck  on the vegetable beds.

Said compost began its career as grass clippings, sawdust, vegetable peelings, paper, fruit skins, fallen leaves, dead flowers, chicken poo, and weeds. None of which was black. But after a couple of years it was uniformly black. Why?   

[chiralSPO]

It's turns into humans *erm* humins

https://pubs.rsc.org/en/content/articlelanding/2018/gc/c7gc03054a#!divAbstract

[evan_au]

Is there a relationship between humus and humins?

[Bored chemist]

Is there a relationship between humus and humins?

Both words are derived from the Latin for soil.

[alancalverd (OP)]

But why black? Most of my victims were white.

[Bored chemist]

Essentially, soil is brown/ black for the same reason that many small children are disappointed by mixing paint.

Each material that you add tends to increase the amount of light absorbed.
Soil is a mixture of lots of things, with different absorptions but, overall, they absorb everything and you get a dark mixture.

[alancalverd (OP)]

Interesting idea, but compost isn't just finely mixed potato peelings and stuff. It has undergone a whole lot of chemistry and even those bits with mesoscopic resemblance to the starting material are black.

How about coal, or peat? They began their transition as white wood or green moss with nothing else added, but end up black after a few million years. We even find "young" brown coal in md-transition.

[Bored chemist]

It has undergone a whole lot of chemistry

Yes.
That's why

Soil is a mixture of lots of things

The things are the products of lots of chemical reactions applied to a fairly complex starting material.

[chiralSPO]

From an electronic structure perspective, carbohydrates are essentially made only of sigma bonding and sigma antibonding orbitals (and some nonbonding orbitals, as lone pairs on oxygen). These are highly localized orbitals, and have very large energy gaps between occupied and unoccupied orbitals, so they will absorb very far into the UV, and are therefore colorless in the visible part of the spectrum.

As the carbohydrates decompose, many of the reactions involve eliminations to form water and pi bonds. The difference in energy between occupied and empty pi bonding and pi antibonding orbitals is much smaller (still well in the UV for single isolated pi bonds). As more water is eliminated, forming additional pi bonds, the molecules can start forming conjugated systems, where pi bonds extend over several atoms. This allows the energy levels to come still closer. At this point the color might start turning yellowish or brownish (think honey, syrup, molasses etc.) As the mixture continues to change, the light absorbed will also change, with growing conjugated systems absorbing longer and longer wavelengths, and more and more pi systems coming into being (so the a greater range and greater proportion of photons can be absorbed). Eventually, the mixture looks black because it absorbs essentially all visible light that lands on it!

[alancalverd (OP)]

Thank you for a very cogent explanation! I'll pass it on to the grandchildren.

[chiralSPO]

Is there a relationship between humus and humins?

Yes, as Bored chemist said, both refer to soil. There is also humic acid. Humins don't only form during composting--they are also an unwanted product when trying to convert cellulose into biofuels, and sometimes the desired product...

[novicepug]

Thank you chiralSPO. I have always wondered why is the soil black and I thought it was a very dumb question. I understand the process now after reading your explanation

[chiralSPO]

Thank you chiralSPO. I have always wondered why is the soil black and I thought it was a very dumb question. I understand the process now after reading your explanation

You're most welcome!

[AnthonyJ]

          Do Black Leaves Matter ?

[axscientist]

According to compostjunkie, "It is stated that due to anaerobic conditions, ammonia and hydrogen sulfide are produced which leaves a black residue in the compost." And compost is not always black.

[chiralSPO]

According to compostjunkie, "It is stated that due to anaerobic conditions, ammonia and hydrogen sulfide are produced which leaves a black residue in the compost." And compost is not always black.

Then it appears that compostjunkie is wrong (or the quote is missing something important). Neither ammonia nor hydrogen sulfide absorbs visible light significantly (so are colorless themselves). Additionally, they are both gasses which are unlikely to be found in the compost. The chemical reactions that release these two may also produce some dark residue, but I doubt that this is the dominant pathway, because the compounds that could decompose to produce ammonia and hydrogen sulfide (mostly amino acids and nucleic acids) are in much, much, smaller quantities than the compounds that produce humins (cellulose, starch, and other sugars).