Home » Carbon and Carbon Dioxide

Carbon and Carbon Dioxide

This excerpt is brought to you by Book of the Week – offering you a glimpse between the pages and an exclusive discount of a new book each week. Get the Book of the Week email newsletter delivered directly to your in box! This week’s Book of the Week is Secrets of Fertile Soils, by Erhard Hennig.

Finnish Nobel Prize winner Artturi Virtanen ended his well-received speech at an international convention in Lindau, Germany, with the words: “The biological process of compounding nitrogen and collecting and using bacteria is, apart from the assimilation of carbon dioxide, a process of fundamental significance not only for plant nutrition but for the whole of life on earth.”

This statement by Virtanen could serve as the maxim for the following treatment of carbon and carbon dioxide.


Carbon refers above all to the gas carbon dioxide (CO2) and to the soil-borne carbonic acid upon which all biological activity in the soil depends.

Carbon is, like atmospheric nitrogen, no article of commerce and cannot be bought!

All plant and animal matter consists of carbon compounds. Without carbon dioxide the plant would cease all activities; it could not continue living. Uncompounded carbon is rare. In crystal form it occurs as diamonds and graphite.

Plants can gather almost all the nutrients they need from the steady stream of passing nutrients, except for carbon, which is most important because the plant must build up almost half of its solid matter out of carbon. In plant nutrition and for humus development the important element in terms of quantity is carbon, followed by nitrogen, and then all other elements occurring in the soil cycle.

It is easy to forget that carbon in the soil in the form of released carbon dioxide (CO2) is the most important raw material for the plant, other than nitrogen. Historically, however, nitrogen has been given a superior position in soil science and carbon was generally considered relatively unimportant.

Carbon dioxide amounts are naturally higher in the air near the soil than above the green shield and in the (upper) free air.

Carbon Dioxide

It is first worth mentioning that carbon dioxide is a gaseous nutrient and that both components of carbon dioxide are important building blocks for plants. Carbon dioxide is a colorless gas with a slightly acidic smell. It is one and a half times heavier than air, has low reactivity, and is noncombustible. Carbon dioxide is dissolved in all natural waters; there can be up to one part of CO2 in each part of water. Although it is true that atmospheric carbon dioxide is generally referred to as “carbonic acid (of the air)”, true carbonic acid (H2CO3) is produced when carbon dioxide is dissolved in water (CO2+H2O). Their salts are the carbonates. But carbon dioxide used for artificial mineral waters is also referred to as carbonic acid. By comparison, carbon monoxide (CO) is a toxic gas, without color or smell. Carbon monoxide can be found in the fumes produced by defective coal ovens, car exhausts, and feed silos and can lead to gas poisoning. Carbon dioxide stems from soil respiration, the burning of coal and oil, volcanic eruptions, and the respiration of superior living beings. Through the consumption of carbon dioxide plants keep up the cycle of carbon dioxide.

Soil-Borne Carbonic Acid

The varied locations of soil-borne carbon dioxide include:

  • in the air around the plant, close to the soil
  • further up where there is no more foliage
  • high above the plant

Carbon dioxide amounts are naturally higher in the air near the soil than above the green shield and in the (upper) free air. Carbonic acid located above the leaf shield cannot be taken up by the plants.

Nature has designed plants so that soil-borne CO2 can be absorbed most effectively through the stomata, located on the underside of the leaf.

Between two and three hundred stomata can exist on one square centimeter of leaf surface “inhaling” carbon dioxide. The width of the stomata is regulated by the stomata guard cells. After passing through these stomata guard cells, CO2 molecules find their way into air pockets and then move farther down into the green leaf tissue where they are dissolved into water and processed. In addition to the stomata, there are very tiny openings on both sides of the leaves, the so-called micropores through which water and the substances dissolved in it can pass. By means of this mechanism it is possible to increase the nutrient supply of the plant by applying nutrients and growth substances in solution to the leaves, a practice called foliar nutrition.

Atmospheric air, besides 21 percent oxygen and around 78 percent nitrogen, contains approximately 0.05 percent carbon dioxide (CO2). Oxygen contents below 20 percent frequently can be found in the air directly above the ground, whereas the carbon dioxide level here is often over 0.2 percent and can reach even higher concentrations (2-3 percent). Thus, carbon dioxide contents above the surface can be ten to a hundred times higher than in the atmospheric air.

The CO2 concentration in the air surrounding the leaves is crucial for the plants’ carbon dioxide supply. The plant cannot make direct use of carbon dioxide that has accumulated overnight, mainly due to the vertical air currents in the early hours of the morning and when there is not a closed shield of leaves. An evergreen field thus is aways an advantage.

Soil Respiration

Two-thirds of the carbon dioxide freed by respiration processes in the soil stems from the activity of microorganisms, less than a third stems from the activity of microorganisms, less than a third stems from root respiration, and the remainder comes from the respiration of the soil-dwelling creatures. All the biological activity of a soil becomes apparent through the soil respiration.

As a result of the assimilation by the plant roots and the respiration of the soil organisms (oxygen consumption, carbon dioxide production), the composition of the air in the soil is different from that in the atmosphere. Most significantly, the content of carbon dioxide in the air just above the soil is greatly increased. If the exchange of gases is inhibited (for example, in a compacted soil without stable crumb structure) the carbon dioxide content can rise above 10 percent, and the oxygen content can sink to below 10 percent, which inhibits root activity.

The Ratio of Carbon Dioxide to Nitrogen

The carbon/nitrogen ratio (C/N ratio) is a useful way to measure organic matter’s ability to decompose as well as to the biotic activity of a soil. The carbon/nitrogen ratio necessary for cellulose decomposition is 30:1. Aerobic and anaerobic bacteria, actinomycetes, and fungi all take part in the decomposition of organic substances. Cellulose, the most important building material in vegetal walls, is the most resistant carbohydrate.

Microbial activity becomes inhibited if there is not enough protein — nitrogen available for their construction. If the carbon/nitrogen ratio is greater than 25:1, as is the case in straw and strawy manure, there is less decomposition of matter and nitrogen is temporarily biotically fixed in the microbes. Organically bound nitrogen can only be released when the organic matter has decomposed and the carbon/nitrogen ration decreases, for example below 20:1. Highly fertile soils should have a low carbon/nitrogen ratio of 10:1 (the actual ratio can be determined in a laboratory).

About the Author:

Erhard Hennig was an agronomist who devoted himself to agriculture from an early age. He worked extensively as a farmer, agricultural consultant, journalist, author, and lecturer and worked and taught at Humboldt University in Berlin. Hennig died in 1998.

Titles of Similar Interest: