Air, as a growth factor, contains the essential nutrient elements carbon and oxygen
in the form of
CO2 (0.03–0.04 vol%) and O2 (approx. 21 vol%). Nitrogen, as the largest component by volume
(approx. 78 vol%), cannot be used directly by higher plants, but it serves as the raw material for
large-scale fertilizer production (Haber–Bosch process, Odda synthesis) and microbial nitrogen
fixation (e.g., rhizobia in legumes).
Oxygen
- Is required in plants for the respiration
of sugars (the reverse of photosynthesis) and serves as
the final electron acceptor. Incomplete sugar breakdown leads to organic acids, which C4-
plants use at night to store CO2.
- Is taken up through the stomata.
- Is absorbed by the roots from the
soil air.
The oxygen concentration in the soil air results from oxygen consumption by plants,
microorganisms, etc., and the diffusion of oxygen from the above-ground atmosphere into the soil
(exceptions: swamp plants like rice, alder, etc., which can form aerenchyma). As the oxygen
content of the soil air decreases, the oxidation states of nutrients and harmful elements change
due to microorganisms reducing existing oxides to gain energy.
- Nitrate is ultimately reduced to
molecular nitrogen and escapes the soil as a gas
(denitrification; intermediate stages like NOx may form as greenhouse gases).
- Manganese is reduced from the +4
to the +2 oxidation state, becoming the Mn²⁺ ion (water-
soluble).
- Iron is reduced from the +3 to the
+2 oxidation state, becoming the Fe²⁺ ion (water-soluble).
- Sulfate is reduced to hydrogen sulfide
(H₂S smells like rotten eggs, e.g., when building a
sandcastle).
- Carbon dioxide is reduced to methane
(a greenhouse gas).
- Protons are reduced to molecular
hydrogen.
With changes in oxidation states in the soil, the availability of these elements to
plants also
changes.
Carbon Dioxide
- Is highly soluble in water as a gas
(1 liter of gas per liter of water, approx. 0.2 wt%). This
explains the importance of the world's oceans for CO₂ storage. Plants consist of about 40%
carbon, and the corresponding amount of CO₂ can only be supplied to the plant through air
circulation and turbulence (greenhouse air exchange rate).
- Uptake occurs through the leaves
(stomata), which open under high light intensity, high
humidity, and low CO2 partial pressure.
- Serves as the raw material for photosynthesis.
Enrichment in the surrounding air can
significantly increase biomass production (CO2 fertilization in greenhouses).
- Is produced by microbial activity
in soils from organic material (plant residues, etc.). The CO₂
subsequently escaping from the soil is sometimes credited with the yield-enhancing effect of
incorporated composts.
