The Scientific Meaning of ‘Organic’

Use of the word ‘organic’ in chemistry does not imply ‘healthy’, ‘good for you’, or ‘best for the planet’ — those are relatively recent meanings used in the broader community, and are not used in science.

To a chemist, the old-school meaning of ‘organic’  was simply ‘contains carbon’, while ‘inorganic’ meant ‘doesn’t contain carbon’. The modern definition of an organic compound is now one which ‘contains carbon-hydrogen bonds’.

Warfarin and manufactured herbicides are as much organic substances as are fruits, vegetables and meat.

‘Organic compounds’ and ‘carbon compounds’ are pretty much synonymous terms in the chemistry world. Organic chemistry is in fact a very important industrial field involved in the manufacture of plastics, fertilisers, fuels, pharmaceuticals, and many other products of modern life.

Having said that, chemists make exceptions for simple carbon-containing compounds such as carbon dioxide (CO₂) and the carbonates (those containing the CO₃^2- carbonate ion), and consider those inorganic. Hydrogen cyanide (HCN) was once regarded as organic, for example, but is now borderline organic/inorganic as it is a simple molecule with a single hydrogen-carbon bond.

Carbon in Soil

The presence of carbon in soil implies an origin from living and once-living organisms. Carbon-rich coals, charcoals, natural gas and crude oil all originated from once living organisms. (Hence ‘fossil fuels’.)

However, this isn’t always the case, and soil chemists divide soil carbon into inorganic carbon and organic carbon (almost a tautology!).

Soil Inorganic Carbon (SIC)

Soil inorganic carbon (SIC) is from sources which were never alive.

SIC materials are primarily carbonate minerals (with the CO₃^2- carbonate ion) from weathered rocks. These are also typically the simple carbon-containing compounds that chemists would regard as inorganic generally.

Soil Organic Carbon (SOC)

Soil organic carbon (SOC) is that in living and from once-living organisms, and tends to be more complex in structure.

SOC in living organisms includes that from proteins, fats, carbohydrates (sugars, starch, cellulose) and lignin.
SOC from once-living organisms are from the decomposition of those same proteins, fats, carbohydrates and lignin.

SOC, along with other elements (hydrogen, oxygen, nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, iron, manganese, zinc, copper, molybdenum, boron, nickel, cobalt, silicon and chloride) makes up soil organic matter (SOM).