Soil Air and Soil Water for B.Sc. Agriculture

B. Sc. Agriculture (Hons.) Ist. Semester (Six Deam Commitee of ICAR)

Soil Air and Soil Water

Soil Air

1.1 Definition

Soil air is the mixture of gases that occupies the pore spaces not filled with water in the soil.
It is essential for root respiration, microbial activity, and nutrient transformation.

1.2 Composition of Soil Air

The composition of gases in soil air differs from that of the atmosphere because of biological activities such as root respiration and microbial decomposition.

Gas	Atmospheric Air (%)	Soil Air (%)	Remarks
Oxygen (O₂)	20.9	10–15	Used by roots and microbes
Carbon dioxide (CO₂)	0.03	0.1–10	Produced by respiration
Nitrogen (N₂)	78.0	78.0	Nearly same as atmosphere
Water vapour	Variable	Higher	Depends on soil moisture

1.3 Soil Aeration

The exchange of gases between the soil and the atmosphere is called soil aeration.

Types of aeration:

Mass flow: Movement of air due to pressure differences (wind or temperature).
Diffusion: Movement of gases from high to low concentration — main mechanism.

1.4 Factors Affecting Soil Air

Factor	Effect on Aeration
Soil texture	Sandy soils → good aeration; Clayey soils → poor aeration
Soil structure	Granular → improves porosity and gas exchange
Soil moisture	Excess water replaces air → causes poor aeration
Organic matter	Enhances aggregation and pore continuity
Tillage and compaction	Proper tillage improves, compaction reduces aeration
Biological activity	Roots and earthworms create channels for air movement

1.5 Types of Soils Based on Aeration

Type	Condition	Example / Impact
Aerobic soils	Adequate oxygen; good drainage	Favorable for most crops
Anaerobic soils	Lack of oxygen; waterlogged	Rice fields, marsh soils

1.6 Importance of Soil Air

Function	Importance
Root respiration	Oxygen is required for root energy and growth.
Microbial activity	Aerobic microbes decompose organic matter efficiently.
Nutrient transformations	Nitrification and oxidation require oxygen.
Redox reactions	Influence Fe and Mn oxidation states.
Plant health	Poor aeration → root diseases, reduced yield.

1.7 Management Practices for Good Aeration

Maintain proper drainage to remove excess water.
Avoid over-irrigation and soil compaction.
Apply organic matter to improve structure.
Tillage and subsoiling to break hardpans.
Grow deep-rooted crops to increase pore continuity.

Soil Water

2.1 Definition: Soil water is the water present in the soil pores, which is available to plants and influences all physical, chemical, and biological processes in the soil.

2.2 Sources of Soil Water

Precipitation (rain, snow, dew)
Irrigation
Groundwater rise (capillary action)

2.3 Forms (or Types) of Soil Water

Type	Location in Soil	Availability to Plants	Remarks
1. Gravitational water	Occupies large pores (macropores) after rainfall or irrigation	Not available	Drains quickly due to gravity
2. Capillary water	Held in micropores by surface tension	Available	Main source of plant water
3. Hygroscopic water	Thin film adsorbed around soil particles	Unavailable	Held tightly by adsorption
4. Combined water	Chemically bound in minerals	Unavailable	Released only at very high temperature
5. Vapor phase	Water vapour in soil air	Negligible	Found in dry upper layers

2.4 Important Soil Moisture Constants

Term	Definition	Typical Tension (bars)	Remarks
Saturation capacity	All pores filled with water	0	Gravitational water present
Field capacity (FC)	Water retained after drainage of gravitational water	1/10 to 1/3 bar	Optimum for plant growth
Permanent wilting point (PWP)	Water content when plants permanently wilt	15 bar	Below this, water unavailable
Available water	FC – PWP	—	Usable by plants

2.5 Soil Moisture Tension

It is the force with which water is held in the soil and must be overcome by plant roots to absorb water.

Expressed in bars or kilopascals (kPa).
High tension → water held tightly → less available.

2.6 Measurement of Soil Water

Method	Instrument	Principle / Use
Gravimetric method	Oven drying	Direct measurement of moisture (%)
Tensiometer	Porous ceramic tube	Measures soil water tension (0–1 bar)
Electrical resistance blocks	Gypsum block	Indirect measure via electrical conductivity
Neutron moisture meter	Radioactive source	Measures hydrogen content (water proxy)
Time Domain Reflectometry (TDR)	Electrical pulses	Modern, accurate method for soil moisture

2.7 Importance of Soil Water

Function	Importance
Plant growth	Essential for germination and nutrient transport
Nutrient availability	Nutrients move in soil solution
Microbial activity	Influenced by soil moisture content
Soil temperature regulation	Water absorbs and retains heat
Soil structure	Affects aggregation and compaction
Chemical reactions	Medium for most soil reactions

2.8 Classification Based on Availability

Class	Soil Water Range	Plant Availability
Available water	Between FC and PWP	Usable by plants
Unavailable water	Below PWP	Tightly held, inaccessible
Superfluous water	Above FC	Drains quickly, causes poor aeration

2.9 Movement of Soil Water

Type of Movement	Direction	Occurs In
Saturated flow	Downward (gravity)	Saturated soils
Unsaturated flow	From wet → dry area	Moist soils
Vapour movement	In gaseous form	Dry upper layers

2.10 Management of Soil Water

Adopt proper irrigation scheduling (avoid overwatering).
Maintain optimum field capacity.
Use mulching to reduce evaporation.
Improve drainage in heavy soils.
Apply organic matter to improve water-holding capacity.
Use soil moisture sensors for precise monitoring.

Summary Table

Parameter	Soil Air	Soil Water
Definition	Mixture of gases in soil pores	Water held in soil pores
Main components	O₂, CO₂, N₂	Gravitational, Capillary, Hygroscopic
Source / Exchange	Diffusion and respiration	Rainfall, irrigation, groundwater
Role in soil	Aeration and respiration	Medium for plant nutrients
Limiting factor	Excess water → poor aeration	Water deficit → drought stress
Optimum condition	50% pores filled with air	50% with water (field capacity)

Soil Air and Soil Water

Soil Water

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