Soil Moisture and Soil Moisture Constants

• Introduction

Water present in the soil plays a vital role in plant growth, nutrient availability, and microbial activity. The amount and form of water retained in soil depend upon:

• Soil texture and structure
• Organic matter content
• Pore size distribution

The study of soil water and its availability to plants is essential for effective irrigation scheduling and water management.

• Forms of Soil Water

After rainfall or irrigation, water infiltrates the soil and occupies the pore spaces between soil particles. Depending on how tightly the water is held by the soil, it exists in three main forms:

i) Gravitational Water

• Definition: The water that moves downward through the soil due to gravity after saturation is called gravitational water.
• Characteristics:
  • Occupies macro-pores (large pores) of the soil.
  • Moves freely downward and drains out quickly.
  • Not available to plants because it moves beyond the root zone before plants can absorb it.
  • Responsible for leaching of nutrients.
• Importance:
  • Helps in recharging groundwater and maintaining deep percolation.
  • Excess gravitational water can cause waterlogging and poor aeration.
• Example: Water that drains out after heavy rainfall or irrigation.

ii) Capillary Water

• Definition: The water held in micropores (capillary pores) of the soil by surface tension and adhesion-cohesion forces is called capillary water.
• Characteristics:
  • Held with moderate tension (between 1/3 and 15 bars).
  • Available form of water for plants.
  • Moves in all directions — upward, downward, and sideways.
  • Found in the field capacity range of soil.
• Importance:
  • Main source of water for plant roots.
  • Readily available and replenished through irrigation or rainfall.

iii) Hygroscopic Water

• Definition: The thin film of water adsorbed tightly around soil particles due to adhesion forces is called hygroscopic water.
• Characteristics:
  • Held very tightly (at tension >31 bars or 10,000 kPa).
  • Unavailable to plants because roots cannot exert enough force to absorb it.
  • Exists as a very thin layer around colloidal particles even in air-dry soil.
• Importance:
  • Contributes to soil humidity and microbial survival in dry conditions.
  • Not useful for crop growth.

Comparison of Forms of Soil Water

• Soil Moisture Constants

Soil moisture constants represent specific points of soil water content that define how much water is available or unavailable to plants. These are key indicators for irrigation and crop management.

i) Saturation Capacity (Saturation Point)

• Definition: The condition when all soil pores (macro and micro) are completely filled with water.
• Soil Water Potential: 0 bar
• Condition: No air in the soil; anaerobic for plant roots.
• Importance: Temporary condition after heavy rain or irrigation; quickly followed by drainage.

ii) Field Capacity (FC)

• Definition: The amount of water retained in the soil after gravitational water has drained away, usually 2–3 days after irrigation or rainfall.
• Soil Water Potential: ≈ -0.1 to -0.3 bar
• Water Held: In micro-pores (capillary water).
• Importance:
  • Ideal condition for plant growth (balance of water and air).
  • Upper limit of available water for plants.

iii) Permanent Wilting Point (PWP)

• Definition: The soil moisture level at which plants can no longer extract water and wilt permanently, even if placed in a humid environment.
• Soil Water Potential: ≈ -15 bars
• Water Held: Mostly hygroscopic water and some tightly held capillary water.
• Importance:
  • Lower limit of available water for plants.
  • Below this point, water is unavailable to crops.

iv) Available Water Capacity (AWC)

• Definition: The difference between field capacity and permanent wilting point, representing the water that can be absorbed and utilized by plants.
• Available Water Capacity = FC − PWP 
• Importance: Indicates the soil’s ability to store plant-available water.

v) Hygroscopic Coefficient

• Definition: The amount of water retained by the soil when it is air-dried under room conditions (mainly hygroscopic water).
• Soil Water Potential: > -31 bars
• Unavailable to plants.

vi) Maximum Water Holding Capacity

• Definition: The maximum amount of water a soil can hold against gravity when all pores are filled.
• This depends on texture, structure, and organic matter content.
• Clay soils have higher capacity than sandy soils.

• Soil Water Availability Range

Importance of Soil Moisture Constants

1. Guide for irrigation scheduling — determines when and how much to irrigate.
2. Helps in soil classification and understanding water-holding behavior.
3. Determines plant water stress levels.
4. Used in designing irrigation systems and calculating water balance.
5. Improves water-use efficiency and prevents waterlogging or drought stress.

Summary