Oxygen in Soil and Water: Oxygen is an element belonging to the \({\rm{16th\;}}\) group of the periodic table. We have studied all about its physical and chemical properties and how it is essential for life on Earth for many living organisms. However, one of the most significant facts is that we need oxygen in air, soil and water to maintain the balance and to ensure the organisms get enough of this life-sustaining ingredient to survive.
So, how does oxygen occur in all these three forms?

1. In the air, oxygen is present as molecular oxygen.
2. In water, it is present in the dissolved form to make it ‘dissolved oxygen’.
3. In soil, oxygen is present diffused in the air gaps.

Sufficient oxygen requirement in all these three – air, water bodies and soil, is mandatory to make it healthier for the living organisms to survive. However, due to pollution, excess usage and other reasons, the amount of dissolved oxygen or molecular oxygen may go down in water and soil. To understand what will happen if the diffused oxygen or dissolved oxygen concentration decreases, it is important to know what do we mean by oxygen in soil and water.

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Oxygen in Water

In water, oxygen occurs in the ‘dissolved form’, which means the percentage of oxygen that gets dissolved in water. The gaseous oxygen dissolved in water is used up by the aquatic animals for their respiration. In turn, the DO or dissolved oxygen is enriched in the water further or replenished by the respiration of the aquatic plants. Other than the aquatic animals, organic matter uses this dissolved oxygen in the water for their decomposition. 

The required limit of oxygen in water is a minimum of \(6\,{\rm{ppm}}\) (parts per million) of oxygen (maximum of \(10\,{\rm{ppm}}\)). If it grows below \(6\,{\rm{ppm}}\), the growth of the fish is hindered. DO is a measure of water quality because the lesser the dissolved oxygen present, the more contaminated the water. Hence, the amount of dissolved oxygen gives a fair idea about the quality of water.

The DO content in water varies with temperature. At higher temperatures, the DO of water will be lowered because the solubility of the gases decreases with an increase in temperature.

Oxygen enters the water through two sources:
a. By direct contact of the surface of the water with air: When water flows over rocks and pebbles, it becomes turbulent. This results in the air bubbles getting caught and submerged, thereby increasing the oxygen in the water. While the still waters, such as lakes and ponds, also get oxygen through surface contact with air, it does not get as much as the turbulent waters.
b. From aquatic plants: During the daytime, the water gets supersaturated with oxygen because of the photosynthesis of aquatic plants. However, as night falls and photosynthesis stops, plants respire oxygen and take up some of the dissolved oxygen, thereby levelling it up.

Causes of Low Dissolved Oxygen in Water

One of the major and primary reasons for the lowering of dissolved oxygen in water is pollution. When the level of dissolved oxygen is low, it indicates water pollution. The cause for low dissolved oxygen is the presence of large amounts of organic matter, micro-organisms and chemicals in the water.

The discharges from industries such as tanneries, pulp, and paper industries, etc., require water for decomposition. Such wastes and wastes from human sewage and other natural organic wastes all leach the oxygen from water, thereby reducing the dissolved oxygen content in water.

The micro-organisms which decompose these organic wastes in water require enough oxygen to survive, thereby increasing the demand of oxygen in water, and therefore, decreasing the dissolved oxygen content. The organic waste present in water is estimated by taking into account the oxygen requirement or the oxygen demand of a particular water body. The measurement is done as BOD- biochemical oxygen demand, which is defined as:

‘The total amount of oxygen consumed by the bacteria or the microbes to decompose the organic matter present in a certain volume of water or sample water’.

Dissolved oxygen in water is also measured experimentally by titration or by the colorimetric method. Winkler’s method is the conventional titration method to find the DO in water.

Oxygen in Soil

Air diffuses into the soil from the atmosphere and occupies the gaps in the soil. Oxygen enters the soil by this diffusion process. Diffusion of air and oxygen in turn, depends upon several factors such as the nature or texture of the soil, pore size, and so on. Soil contains \(34{\rm{\% }}\) of air and \(66{\rm{\% }}\) water. Although, the air that occupies the gaps of the soil or the space between the soil particles contain more carbon dioxide and moisture than oxygen. 

Oxygen is an essential ingredient required for the well-being of underground microbes and the parts of the plants which are submerged below the soil.  

Functions of Oxygen in Soil

1. Oxygen is consumed by flora and fauna in the soil.
2. Oxygen is required for plant growth (roots and other parts submerged below the soil).
3. Oxygen is required for the nutrient and water uptake by the plants because the roots form a major part in the nutrient uptake.
4. It is required for the activity of micro-organisms inside the soil. Some of these microbes are very much necessary for plant growth and also for the general well-being of the soil.

Low oxygen content in soil affects the nutrient and water uptake by the plants and the activity of micro-organisms. Oxygen levels decrease with the depth of the soil, as the gaps between the soil lessen due to tightening of the soil and also the distance to the surface of the soil decreases. 

While the oxygen in water is increased to maintain the good health of the soil, the presence of pollutants can leach the soil with the required oxygen, thereby harming both the plants and the microbes present in the soil. For plants especially, the diffused oxygen presents a sign of good health. If oxygen present in the soil is less, roots cannot absorb oxygen, and their growth ceases. As a result, plant growth is affected. Leaves become discoloured.

Oxygen Diffusion Rate

Oxygen diffusion rate in the soil is the rate at which the oxygen is supplied to the soil when it is being continuously used for the respiration of roots and soil micro-organisms. When the oxygen diffusion rate decreases to \(20 \times {10^{ – 8}}{\rm{g}}/{\rm{c}}{{\rm{m}}^2}/{\rm{min}}\), the root growth ceases.

Summary

Oxygen is an essential part of the soil, water and air. While in water, oxygen is dissolved into it, it diffuses through the soil pores to enter into the soil. Oxygen in water is essential for the growth of aquatic animals and for other micro-organisms present inside water. Oxygen enters the water through surface contact with air and also by the photosynthesis of aquatic plants. The presence of unwanted inorganic materials and other pollutants actually leaches the dissolved oxygen in the water, taking away the dissolved oxygen, and therefore, making it polluted. Similarly, in soil, the diffused oxygen is used by the roots and other parts of the plants for their growth and the microbes that grow beneath the soil. The oxygen concentration or percentage of oxygen in soil decreases as we move down the soil because of the distance between the surface increases.

FAQs

Q.1. How much oxygen is in water?
Ans: Water contains a maximum of \(10\,{\rm{ppm}}\) dissolved oxygen. Below \(6\,{\rm{ppm}}\) stops the growth of fish.

Q.2. Why do we need to check the water quality?
Ans: The amount of dissolved oxygen determines the quality of water. The DO of water bodies and drinking water are checked regularly to determine if it is fit for consumption or use.

Q.3. Why is oxygen important in soil?
Ans: Oxygen has important functions in soil. They are used for:
i) The growth of flora and fauna
ii) The growth of the plant roots
iii) The nutrient and water uptake by the plants
iv) The activity of micro-organisms.

Q.4. How does oxygen affect plant growth?
Ans: If oxygen present in the soil is less, roots cannot absorb oxygen, and their growth ceases. As a result, plant growth is affected. Leaves become discoloured.

Q.5. How do you get oxygen into the soil?
Ans: Air diffuses into the soil from the atmosphere and occupies the gaps in the soil. The diffused air usually contains not just oxygen but carbon dioxide and moisture too.

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