Soil Moisture and Soil Water Movement

Soil Moisture refers to the water held in the spaces (pores) between soil particles. It acts as a critical link between the surface and groundwater and is the primary reservoir for terrestrial plant life.

Classification of Soil Water by Physical Force

Water in the soil is not uniform; it is held by different forces which determine its mobility and availability to plants.

Soil Moisture Constants and Terminology

These constants define the limits of water storage and utility within a specific soil profile.

Saturation Capacity

When all the pore spaces (both macro and micro) are filled with water. In this state, the soil is anaerobic (lacks oxygen), which can lead to root rot in most crops.

Field Capacity (FC)

The amount of water remaining in the soil after the excess gravitational water has drained away (usually 2-3 days after heavy rain). This is the optimal state for plant growth as it provides both water and air.

Permanent Wilting Point (PWP)

The stage where the soil moisture is so low that the tension with which it is held exceeds the suction power of plant roots. The plant wilts and cannot recover.

Available Water Capacity (AWC)

The numerical difference between Field Capacity and Permanent Wilting Point (AWC=FC−PWP). It represents the “storage tank” of water that plants can actually use.

Mechanics of Soil Water Movement

Water moves through the soil in three distinct states: saturated, unsaturated, and vapor.

Saturated Flow

Occurs when the soil pores are completely filled with water. The movement is downward, driven primarily by gravity. The rate is determined by Hydraulic Conductivity, which is much higher in sandy soils than in clay.

Unsaturated Flow

Occurs when pores contain both air and water. Water moves from thick films to thin films (from wet areas to dry areas) driven by Matric Potential (capillary suction). This flow can happen in any direction—sideways, downward, or even upward.

Capillary Rise

A specific type of unsaturated flow where water moves upward from the water table through narrow soil pores. This is significant in arid regions where it can bring salts to the surface, leading to Salinization.

Factors Influencing Movement

• Soil Texture: Sand has large pores (high saturated flow), while clay has many small pores (high capillary/unsaturated flow).
• Soil Structure: Well-aggregated soils (granular structure) facilitate better water movement than compacted or “massive” soil structures.
• Organic Matter: Increases the water-holding capacity and improves the structure, facilitating both infiltration and retention.
• Temperature: Viscosity of water decreases as temperature rises, slightly increasing the rate of flow.

Significance for UPSC Aspirants

• Irrigation Scheduling: Farmers must aim to keep soil moisture between Field Capacity and the Wilting Point to maximize “Water Use Efficiency.”
• Dryland Farming: Techniques like deep ploughing and mulching are used to break capillary tubes to prevent “Capillary Rise” and subsequent evaporation loss.
• Salinity Management: In states like Punjab and Haryana, over-irrigation leads to an elevated water table. This triggers upward capillary movement, depositing salts on the surface (Reh/Kallar) and ruining soil fertility.
• Watershed Management: Understanding soil water movement is essential for designing “In-situ moisture conservation” structures like bunding and trenching.

Trivia for Prelims

• Tensiometer: An instrument used to measure the “Soil Moisture Tension” or how hard the soil is “holding” onto its water.
• Pfm Scale: A logarithmic scale used to express the energy with which water is held in the soil.
• Hydraulic Gradient: The “slope” or pressure difference that drives the speed of water movement through the soil.