3.1 Soil water content and potential

Soil water content is crucial for plant growth and soil health. It affects nutrient availability, microbial activity, and root respiration. Understanding how water behaves in soil is key to managing agricultural and environmental systems effectively.

Soil water content can be measured gravimetrically or volumetrically. Water potential describes the energy status of soil water, influencing its movement and availability to plants. The relationship between content and potential is complex, affected by soil properties and environmental factors.

Soil Water Content and Potential

Importance of soil water content

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• Soil water content quantifies water present in soil volume or mass as percentage or ratio
• Affects plant water uptake by determining water availability for root absorption
• Influences nutrient availability and transport through soil solution movement
• Impacts soil aeration and root respiration by filling pore spaces (oxygen availability)
• Affects soil microbial activity by providing habitat and influencing metabolism
• Influences soil temperature regulation through heat capacity and thermal conductivity

Gravimetric vs volumetric water content

• Gravimetric water content measures mass of water per unit mass of dry soil
  • Determined by oven-drying method weighing soil before and after drying at 105℃
  • Calculated using formula $θg = (Mw / Ms) × 100$ (Mw: water mass, Ms: dry soil mass)
• Volumetric water content measures volume of water per unit volume of soil
  • Measured using techniques like Time Domain Reflectometry and neutron probe
  • Calculated using formula $θv = (Vw / Vt) × 100$ (Vw: water volume, Vt: total soil volume)

Concept of soil water potential

• Soil water potential quantifies energy status of soil water relative to pure water
• Measured in pressure units (kPa or bar) indicating water's tendency to move
• Components include:
  1. Matric potential: attraction between water and soil particles (texture, structure)
  2. Osmotic potential: effect of dissolved solutes in soil water
  3. Gravitational potential: influence of gravity on water movement
  4. Pressure potential: external pressure applied to soil water (saturated conditions)

Soil water content-potential relationship

• Soil water retention curve (SWRC) graphically represents relationship
• Key points on SWRC:
  1. Saturation point: all pores filled with water
  2. Field capacity: water remaining after free drainage
  3. Permanent wilting point: water unavailable to plants
• Hysteresis effect shows different water content at same potential during wetting and drying
• Factors affecting relationship:
  • Soil texture (sand, silt, clay proportions)
  • Soil structure (aggregation, pore size distribution)
  • Organic matter content (water retention capacity)
• Water movement occurs from high to low potential areas
  • Rate depends on hydraulic conductivity (soil's ability to transmit water)