After reading this article you will learn about:- 1. Definition of Soil Aeration 2. Causes of Poor Aeration 3. Mechanism of Gaseous Exchange 4. Aeration Status of Soils 5. Factors Affecting 6. Importance.
Definition of Soil Aeration:
Soil aeration is phenomenon of rapid exchange of oxygen and carbon dioxide between the soil pore space and the atmosphere, in order to prevent the deficiency of oxygen and/or toxicity of carbon dioxide in the soil air. The well aerated soil contains enough oxygen for respiration of roots and aerobic microbes and for oxidation reaction to proceed at optimum rate.
Causes of Poor Aeration:
Compact soils of finer textures suffer from poor aeration. Cultivation (working-the soil when crops are growing) of soil prevents it. Water logging is another important cause of poor aeration especially in the case of soils of finer texture.
The gaseous exchange between the soil air and the atmosphere may not be rapid enough to remove carbon dioxide from the soil air and to supply oxygen to the growing roots. This may happen if an excessive amount of readily decomposable organic matter has been added to the soil.
Mechanism of Gaseous Exchange:
The exchange of gases between the soil air and the atmosphere takes place mainly by the following two mechanisms:
(i) Mass flow:
Gases may move in a mass in the soil or out of it. The soil temperature is higher than the atmospheric temperature at midday when the soil gases expand and move out of the soil pore space into the atmosphere. The soil is cooler than the atmosphere during right when the atmosphere. The soil is cooler than the atmosphere during night when the atmospheric gases enter the soil.
When the atmospheric pressure is increased, volumes of gases present in the soil are decreased and therefore atmospheric gases enter the soil. Rain water displaces soil gases in the pore space and also carries gases dissolved in it to the soil. Rain fall usually account for 1/12 to 1/16th of the normal soil aeration. Variations in temperature and pressure between the soil and at the atmosphere play an insignificant role in soil aeration.
Most of the gaseous interchange between the soil and the atmosphere takes place by diffusion. Diffusion is the process by which each gas tends to move in the space occupied by another as determined by the partial pressure of each gas.
The partial pressure of a gas is the pressure which the gas would exert if it were present alone in the volume which has been occupied by the mixture of gases. The atmosphere contains higher amounts of oxygen than the pore spaces of soils which contain more carbon dioxide than the atmosphere.
So the partial pressure of oxygen is higher in the atmosphere than in the soil pore space and the partial pressure of carbon dioxide is higher in soil pore spaces than in the atmosphere even though the total pressure in the atmosphere and the soil pore spaces may be the same. So oxygen moves in the soil and carbon dioxide moves out of the soil.
Aeration Status of Soils:
It can be determined in the three ways i. e:
(i) Percentage oxygen and carbon dioxide content of the soil,
(ii) Oxygen diffusion rate, and
(iii) The oxidation reduction potential (Redox potential).
(i) Percentage composition of soil air:
The average inorganic soil contains about eight times more carbon dioxide and a little less oxygen than the atmosphere as shown below:
Well aggregated soils contain enough macrospores to keep the soil aerated for proper growth and functioning of roots and micro-organisms. After a heavy rain, the macrospores are filled up with water but the soil may still contain some quantity of air dissolved in water. So micro-organisms can grow for a short time only, after which the soil must be drained so that the macrospores are re-filled with air.
(ii) Oxygen Diffusion Rate (ODR):
It determines the rate at which the oxygen should be supplied to the soil when it is being continuously used for the respiration of roots and soil micro-organisms. The growth of roots of most crops ceases when the oxygen diffusion rate decreases to about 20 x 10-8 gm./sq. cm/min. It should be above 40 x 10-8 gm./sq.cm/min. for good growth of most crops.
(iii) Oxidation-reduction potential of soil.
The oxidation potential of chemical systems including soil is a measure of the tendency of the oxidation reaction to occur in that system, including soils.
This means that the system is in a reduced condition. Highly reduced soils have a high oxidation potential of +0.50 volts.
The reduction potential of a chemical system including soil is a measure of the tendency of the reduction reaction to occur in that system. This means that the system is an oxidized condition. Reduction Potential or Redox Potential (Eh) is the opposite in sign to the oxidation potential.
So a highly reduced soil which has a tendency to be oxidized has a Reduction Potential or Redox Potential Eh of- 0.50 volts. Well drained and aerated soils which are highly oxidized usually have a redox potential of +0.50 volts. The value of the Redox Potential increases when the oxygen content of soils decreases.
Factors Affecting Soil Aeration:
(i) Soil organic matter:
When organic matter is added to the soil, it is readily decomposed by the soil micro-organisms to liberate the carbon dioxide content of the soil air.
(ii) Since the top soil contains much more macrospore space than the subsoil, the opportunity for gaseous exchange is more in top soil than in the sub soil. Hence the oxygen content of the top soil is greater than that of the sub soil.
(iii) Soil moisture:
The macrospores are filled up with water immediately after heavy rain when the oxygen content falls to near zero. When the soil is artificially drained again, the macrospores are filled up with air and the oxygen content of the soil increases.
Importance of Soil Aeration:
Soil aeration affects the availability of some nutrients elements to plant roots. Manganese and iron occurs in the well aerated soil in their higher valent forms (Mn++++, Mn+++, Fe+++) and in poorly aerated soils in their lower valent forms (Mn++, Fe++). They are available to plants only in their lower valent forms.
Crops suffer from manganese toxicity if an excessive amount of manganese occurs in the soil in the soluble form. Manganese toxicity to plant roots, under this circumstance, may be corrected either by making the soil more aerated by tilling the soil and improving drainage of the soil or by increasing the soil pH by applying lime to the soil.
When iron and manganese are in short supply to the soil, then the soil may be subjected to anaerobic condition by applying readily decomposable organic matter to it.
Carbon dioxide would be produced from the decomposition of organic matter to make the soil relatively more anaerobic when manganese and iron will be reduced from their respective higher Volant (Mn++++, Mn+++, Fe+++) forms to their respective lower valent form (Mn++, Fe++) i.e. divalent forms and would be available to plant roots.
Ferric phosphate would be reduced to ferrous phosphate. Carbon dioxide produced from the decomposition of organic matter reacts with water to form carbonic acid which slowly dissolves insoluble phosphate. So the availability of phosphates (would be increased to the plant roots.
Sulphur occurs as sulphate in well aerated soil. Plant roots assimilate sulphate. Sulphate is reduced to sulphide in poorly aerated (water logged) soils. Hydrogen sulphide is toxic to plant a root which suffers from it in water logged soil.
Organic matter is decomposed by aerobic bacteria in well aerated soil when complex organic nitrogen and phosphorus compounds are decomposed to their respective simple inorganic compounds which plant roots readily assimilate symbiotic and non-symbiotic nitrogen fixation takes place only in well aerated soils.
Nitrates are reduced to oxides of nitrogen and nitrogen gases in poorly aerated soils. These gases escape to the atmosphere, long light coloured roots develop in well aerated soils. Root hairs develop best under well aerated condition.
Roots get thicker, shorter and darker in anaerobic soils that also retard the development of root hairs. Poor aeration causes abnormal development of roots, e.g. abnormal shaped sugar beet and carrot roots have been found in poorly aerated soils.
Nutrient absorption is an energy consuming process. Energy is available from respiration is expended in absorbing nutrient ions from the soil. Hence nutrient absorption is retarded in poorly aerated soils.
If an excessive amount of readily decomposable organic matter has been added to the soil, then it would decompose to evolve high amounts of carbon dioxide to the soil. Consequently root growth and germination of seeds would be adversely affected.
Some crops become infested with pathogens in poorly aerated soils. The incidence of will disease caused by the fungus (Fusarium sp) has been attributed to poor aeration. Citrus and suffers from die-back in poorly aerated soils have also reviewed the works of some investigators who have observed that poorly aerated soils (waterlogged soils) has an effect on the pathogenicity of root infesting fungi.