The Soil Moisture Characteristic Curve (SMCC), also called the Soil-Water Characteristic Curve (SWC) or Moisture Retention Curve, is a critical tool in understanding how soils retain and release water. It represents the relationship between the soil’s water content (either mass or volume basis, θm or θv) and its matric potential (or suction).
In simple terms, SMCC shows how tightly water is held by the soil at various moisture levels. This curve is essential in agricultural and environmental sciences for modeling water flow, irrigation planning, and predicting solute transport, especially in partially saturated soils.
How is Soil moisture characteristics curve (SMCC) Measured?
Soil samples are brought to different known tension (suction) levels in laboratory conditions. At each level, the corresponding water content is measured. Plotting these values gives us the soil moisture characteristic curve, which tells us how much water remains in soil at each suction level.
Why SMCC Matters in Agriculture and Veterinary Sciences
- Helps estimate field capacity, plant-available water, and permanent wilting point.
- Crucial for designing irrigation systems and water conservation strategies.
- Predicts soil aggregate stability under varying moisture regimes.
- Supports livestock forage and pasture health by understanding plant-soil water relations.
- Aids in environmental assessments involving contamination and solute transport.
Factors Affecting the Soil Moisture Characteristic Curve
1. Soil Texture
- Clayey soils retain more water due to:
- High surface area
- Negative charge attracting water
- Abundant micro-pores that hold water tightly
- Sandy soils have large pores (macro-pores), resulting in:
- Lower water retention at all suctions
- Quick drainage
- At high suction, clay holds water too tightly for plant uptake.
2. Soil Structure
- At low suction (0 to -1 bar), pore arrangement becomes key:
- Compacted soil has:
- Fewer total pores
- High water retention at high suction
- Aggregated soil has:
- More macro-pores → better low-suction retention
- Less water retention at high suction
- Compacted soil has:
3. Swelling and Shrinkage in Clays
- Soils with swelling clays (e.g., Na⁺, Ca²⁺ clays) hold more water.
- Cation swelling order: Li⁺ > Na⁺ > Ca²⁺ > Ba²⁺ > H⁺ > K⁺
- However, swelling effect on the moisture curve is still being explored.
4. Entrapped Air Bubbles
- Air trapped in soil pores reduces available water storage capacity.
- Alters saturation and drainage characteristics.
5. Sudden Drying and Wetting
- Rapid changes in moisture:
- Disturb pore distribution
- Cause cracking (especially in clay) or crusting (in silt/sand)
- Disrupt water retention patterns
6. Prolonged Saturation
- Long-term waterlogging:
- Damages soil structure
- Increases anaerobic conditions
- Alters water holding capacity
What is Hysteresis in SMCC?
Hysteresis refers to the difference in soil water retention behavior during drying vs. rewetting of the same soil.
Even when the matric potential is the same, the water content can differ based on the soil’s history of wetting and drying.
Causes of Hysteresis:
- Non-uniform pore sizes
- Entrapped air that prevents complete wetting
- Bottleneck effect: Larger pores surrounded by smaller ones
- Clay swelling/shrinking during moisture changes
This phenomenon is critical in soil physics as it affects irrigation strategies and hydrological modeling.
Key Takeaway
- SMCC helps predict how much water soil can store and release to plants.
- It is influenced by texture, structure, clay mineralogy, and moisture history.
- Understanding SMCC is essential for water management, sustainable farming, and veterinary pasture health.
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