Fundamentals of Soil Science (1+1)

Introduction

• Soil atmosphere is the gaseous phase of the soil. Soil air occupies the pores which are not occupied by the liquid. Soil air is a continuation of the atmospheric air. It is in constant motion from the soil pores into the atmosphere and from the atmosphere into the pore space.
• The exchange of gases between the soil pore spaces and the atmospheric air is known as soil aeration. Soil aeration is essential for the respiration and survival of soil organisms and plant roots. This process controls the deficiency of oxygen consumed during respiration of plant roots and soil micro-organisms and prevents toxicity of carbon dioxide evolved during respiration in the soil air.

Composition of Soil Air

• Soil air contains gases like nitrogen, oxygen, carbon dioxide, water vapour and others. The composition of soil air is different from atmospheric air. Soil air contains more carbon dioxide and less oxygen than atmospheric air. It also contains more water vapour than atmospheric air. The nitrogen content of soil air is almost equal to atmospheric air.

Component	Per cent by volume
	Soil air	Atmospheric air
Nitrogen	79.10	79.00
Oxygen	20.60	20.79
Carbon dioxide	0.25	0.03

Air Capacity:

• This term is used to describe aeration status of soil.The air capacity refers to the volume of pore space filled with air when the soil under a tension of 50 milli bar. The aeration capacity can be characterized in three ways as given below.

1. Content of oxygen and other gases (as discussed above).
2. Oxygen Diffusion Rate (ODR):
• It is the best and most reliable measurement of aeration capacity. It determines the rate at which O₂ in soil air is replenished. ODR decreases with soil depth. ODR should be above 40 x 10^-8 g/cm²/minute for good growth of most of the crops. However, the root growth is drastically reduced when the ODR decreases to about 20 x 10^-8 g/cm²/minute
3. Oxidation - Reduction potential (Eh) of soil:

• It is an important chemical characteristic of soil related to soil aeration. It indicates the oxidation and reduction states of soil system.
• In oxidized soil, ferric (Fe³⁺),manganic (Mn⁴⁺), nitrate (NO₃^-) and sulphate (SO₄^2-) ions dominate.
• In reduced soil, ferrous (Fe²⁺), manganous (Mn²⁺), ammonium (NH⁴⁺) and sulphides (S^2-) are present.
• The redox potential is measured using platinum electrodes and expressed in millivolts. A positive Eh value indicate oxidized state and a negative Eh value indicate reduced state.