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I often see the term "carbon in the soil" (example) but I cannot figure out what it is. It's a positive goal too - people want agricultural techniques that do not deplete it.

What form does this carbon exist in and what are the major sources? How does it help plants and other organisms who use the soil?

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  • $\begingroup$ CaCO3 can be a major constituent of soil, i.e. Chalk, so mountains can be made of carbon in large amounts. Environmental Scientists study mountain weathering's effect on the climate, with sulphuric acid and absorption of CO2 carbonic acid into alcaline soils. Carbon in the soil conversely, refers to carbon fixing by plants adding to the soil, and plant matter being consumed and returning to the air. There is mineral and organic carbon. soil technically is not often mineral, it is normally a living matter of organic and mineral. $\endgroup$
    – bandybabboon
    Commented Oct 8, 2017 at 6:02

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It refers to organic carbon, i.e. the carbon present in proteins, carbohydrates and other molecules as consequence of the metabolism of living creatures. Carbon rich soil is an indication that microorganisms, insects, and worms are living in the soil. They are the basis for the soil ecosystem and boost the flow of nutrients that ultimately will support plant growth. A soil with low content of organic carbon indicates that it's a harsh environment where living organisms struggle.

Some articles on the matter:

https://en.wikipedia.org/wiki/Soil_microbiology http://www.nature.com/nature/journal/v501/n7468_supp/full/501S18a.html http://mbio.asm.org/content/6/2/e02527-14.full

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  • $\begingroup$ How does agriculture deplete it? $\endgroup$
    – Jesvin Jose
    Commented Aug 8, 2017 at 6:10
  • $\begingroup$ Why the downvote? $\endgroup$
    – alec_djinn
    Commented Aug 8, 2017 at 6:56
  • $\begingroup$ @aitchnyu It depends on many factors. The use of chemicals for pest control of fertilizers may affect the ecosystem, for example, causing a cascade of reactions that it's hard to predict. Also, intensive cultivation may deplete the soil nutrients too fast, "starving" the biota. $\endgroup$
    – alec_djinn
    Commented Aug 8, 2017 at 7:01
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If you take an average soil sample, it contains lots of organic matter, unless it is completely mineral like desert sand. Oganic matter always has C, H, O elements, and often N. Peat bogs for example, are classed as a type of soil and can contain almost only organic matter, with many kilos of carbon per cubic meter, rainforest soils are sometimes meters of peat, it can catch fire due to carbon content. Humus in forest soils is also mostly organic. If carbon fixing life is removed from a carbon rich land, soil carbon will often slowly return to a mineral state, often as a function of temperature (hot soil mineralizes faster) carbon products get turned into simpler more volatile lower energy compounds, by being eaten by biota and respired as CO2.

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Carbon in the soil would be what's left of anything that died in there, or any animal feces.

Carbon in the soil may possibly benefit bacteria that live in the soil, which may fix or otherwise convert nitrogen, but the plants don't use it.

However, let's imagine a dog is walking around and dies. It will rot into a gooey paste as maggots eat it, and then will liquefy into a putrid mess and eventually what's left of it will turn to dirt.

The dirt will have lots of carbon in it, which bacteria will probably use, The dirt will have lots of minerals in it, which the plants DO use

They get their carbon from co2.

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