Importance of oxygen for soil and plants (Part 1)

Oxygen is not only essential for the respiration of living organisms, but also plays a central role for soil and plant growth. An optimally composed soil consists of around 45 % soil solids, 7 % organic material, 23 % water and 25 % air (approx. 79 % N; 20.5 % O₂; < 1 % CO₂). This composition ensures that the plant roots receive sufficient oxygen for the various metabolic processes (oxidative phosphorylation, energy production, root respiration, ...).

But what happens when this balance is unbalanced? Before we address this question, we first want to clarify what role oxygen plays for soil, roots and plants.

Important ‘player’ for nutrient availability and plant development

Oxygen is responsible for sufficient implementation of aerobic (= oxygen-rich) metabolic processes:

Soil bacteria, actinomycetes and fungi decompose organic material and make some of the bound nutrients available to the subsequent plants. Humus is built up in the process, but CO₂ is also released. As a rough classification, it can be said that approx. 70 % of the CO₂ emitted from the soil is produced by microbial transformation and is therefore one of the end products of aerobic metabolism.

Air conductivity is one of the decisive factors in the aerobic conversion of organic material. It influences the oxygen concentration at depth and varies depending on the soil type. In contrast to sandy soils with a high proportion of coarse pores, soils with an increasing proportion of clay in deeper layers have a lower air conductivity. While in sandy soils aerobic bacteria still work effectively down to 25 cm, in clayey soils organic material is only successfully transformed up to approx. 15 cm depth. The more clayey the soil is, the less deeply organic substances should be worked in, as ‘the air becomes thin’ towards the bottom.

Oxygen also plays an important role in cellular respiration:

While the leaves produce oxygen via photosynthesis and fix CO₂, roots have to gain their energy the other way round because of the lack of sunlight. Oxygen is absorbed and CO₂ is released. By diffusion, air movement and precipitation from the atmosphere, new oxygen is constantly supplied and transported into the soil. In contrast to photosynthesis, cellular respiration also takes place at night.

At the same time, the release of CO₂ acidifies the area around the root tips and forms carbonic acid, which in turn leads to a better release and release of nutrients from the clay and humus complexes.

These examples show that oxygen can be seen as an essential player in many processes. But what happens ‘when the air stays away’?

Oxygen and carbon dioxide uptake of plants

Hypoxia: Causes of oxygen deficiency in the soil

A lack of oxygen is referred to as hypoxia. It occurs when the oxygen content in the root area or in the soil is reduced significantly.  

Possible causes are:

Excess water: The air and water content in the soil are complementary. This means that if there is a high water content, there is little air in the pore volume and vice versa. Flooding or highly water-saturated soils therefore always mean a lack of oxygen. 
Inhibited gas exchange: Soil aeration and thus the supply of fresh air to the soil is made possible by the gas exchange between soil air and the above-ground atmosphere that takes place at the soil surface. This gas exchange results from the different partial pressures of carbon dioxide CO₂ (more concentrated in the soil than in the air, e.g. due to respiration processes of soil organisms or roots) and oxygen O₂ (more concentrated in the atmosphere).

In the case of waterlogged conditions, a very dense and fine-pored soil structure or an encrusted soil surface, however, the gas exchange is inhibited and thus prevents the supply of oxygen to the soil.

Soil compaction: If the soil becomes compacted, it can hardly absorb and drain water. This causes separation of the lower soil layers and water stagnation in the upper part. The water displaces the air in the pores, resulting in a lack of oxygen. The same happens with plough layers or disturbing layers

As varied as the causes are, so too are the consequences.

The effects of oxygen deficiency on microbiological activity, nutrient uptake as well as on plant and root growth are described in the second part of the blog article.