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Why is soil brown?

Some quarter years ago, while sitting in a bus, i just wondered:"why is soil brown?".. a pretty common question...
its common because the subject involved is common... but we seldom stop to think about such things..
A scientist/engineer would know why things are coloured:
There is a photon associated with each wavelength of light.
Light of different wavelength are composed of photons having differnt energies.
The lights of different wavelength are perceived as lights of different colors by our eyes.
Hence the different colors of light, are thought of as representing photons of different energy.
Now object on which white light falls, are composed of molecules or ions in a lattice. The electrons in these materials can only move between well defined discrete levels. And while doing that these electrons
absorb (lower energy level to higher energy level) or emit (higher energy level to lower energy level) photons. Since their energy levels are discrete, they can absorb only those photons which have energy higher than the difference of the two energy levels between which the electron transits. If a photon has a higher energy, the extra energy appears as the kinetic energy of electron which has now reached the new higher energy level.

If on the other hand, the electron transits from a higher to lower energy level, it will emit a photon which has energy which is the difference between the two energy levels.. this will form the monochromatic light which appears to be "reflected" from an object when a beam of white light (having photons of all kinds of energies) is incident on it.

There are two ways of producing a color as seen by our eyes:

a) shine a light of a particular wavelength on our eyes so as to produce that color

b) shine light of different wavelengths having different weights (intensity) to form the same sensation in our rods and cones as the single wavelength light above.
But there are some colors like purple and brown which can only be produced by method (b).

The reason for this is as follows:

consider the "Normalized typical human cone responses (and the rod response) to monochromatic spectral stimuli" chart given at and also given below:

We notice that there are certain overlapping regions between the "band pass filtering" type spectrums of the three different cones. Light of single wavelength in these overlapping regions of sufficient amplitude would produce a colour perception in the same way as the lights of two or more different wavelengths (and having appropriate amplitudes) appearing in non overlapping regions of the three curves. Purple for example can be created with a mixture of wavelengths in both the red and the blue. Brown requires a more complex mixture of wavelengths.

L M S are the different kind of cones in our eyes. See the above two links, (especially the wikipedia link) for more info about how the colour is perceived by our eyes. By the way, our eyes are more responsive to green color because our very first food were plants..which reflect green light!.. and why are plants green? .. aw! we know that one...They taught us in school..: Because they have chlorophyll! and according to

"It turns out that eons of evolutionary design have matched the absorbance of chlorophyll to the actual color of the sunlight that reaches the leaves. Sunlight consists of primarily blue and red light mixed together, which are exactly the colors that chlorophyll molecules like to absorb. Light is a form of energy, so the chlorophyll is able to harvest the sunlight with little waste."

Getting back to topic of soil, its probably brown because of the lots of diffrent kinds of stuff mixed in it (dead or alive). See the Image on the left side : I dont remember where i got this article from , but it gives a bit more insight as to why soil is really brown.. For more on Maillard Reaction, see wikipedia

This is like another one of those natural cycles... as for me , I think the soil is brown because its has lots of stuff in it which can reflect light corresponding to different areas of color wheel. Its like water colors, now matter how much or how many colours (except for black color) you dissolve in the water, you always get brown color!

Similarly there so many things in nature which have different colors.. bright yellows sunflowers... green leaves... pink roses... red hibiscus.. orange oranges and mangoes etc etc..

All of these colors eventually mix (dead or alive) to form the soil....

In fact humans, who eat foods of different colors (omnivores) are also brown colored!!!

So when we die the brown of our skin mixes with the brown of the soil... ASHES TO ASHES, DUST TO DUST..

Talking about brown colour reminds me of one last thing.. a PJ..

Haldi (Turmeric powder): a single word which when uttered during the lunch time among my friends who know this joke can make them lose their appetite. I asked my mom what is the function of turmeric powder in our food.. she said she didnt know any major reason except for the flavouring and the antiseptic properties. Now the antiseptic properties have nothing to do with the haldi being used in food and as for flavour.... i am not sure it has one anyway...

So the only reason the chef uses it might be to warn the eater....

You see, when in the morning we are sitting on the sacred toilet (i say sacred because sitting on it has become a ritual and most rituals are sacred... and things which are sacred bring peace to our hearts (in this case, our stomachs)) we observe that what we excrete is brown in color, almost similar to the color of haldi (yellow). So in a way haldi in the food reminds you of the eventual food you are going to eat. Mostly this haldi is used in spicy foods, which is only fair... people need to be warned of the consequences of eating too much spicy food by reminding them of what they would have to deal with the next morning..... in this case the daily ritual would be painful but seem truly "sacred" later on!