7.1 Soil Water Availability, Infiltration & Matric Potential

Soil Water Availability and Matric Potential

Understanding how water behaves in soil is critical for effective crop management, irrigation scheduling, and environmental protection. Soil hydrology governs the movement, storage, and availability of water to plant roots.

States of Soil Water

Soil water exists in a dynamic equilibrium, constantly influenced by precipitation, evaporation, transpiration, and gravity. We categorize soil water status into several key states based on the energy required to remove water from the soil matrix.

Saturation

When all soil pores—both macro and micro—are completely filled with water, the soil is at saturation. In this state, the matric potential is effectively zero (0 kPa). Saturated conditions typically occur during or immediately after heavy rainfall or intensive irrigation. While saturated soils contain the maximum amount of water possible, they lack oxygen, which is essential for root respiration. Prolonged saturation leads to anoxic conditions, stunting root growth, promoting denitrification, and potentially killing sensitive crops. Gravity rapidly pulls excess water downward from macropores in a process called free drainage.

Field Capacity

Field capacity is defined as the amount of water remaining in the soil after the free drainage of gravitational water has significantly slowed or stopped, typically 1 to 3 days after a saturating rain. At field capacity, macropores are mostly filled with air, providing necessary oxygen, while micropores remain filled with water. The matric potential at field capacity varies by soil texture, generally ranging from -10 kPa in sandy soils to -33 kPa in clay soils. Field capacity represents the optimal balance of water and air for most agricultural crops.

Permanent Wilting Point

As plants extract water and evaporation continues, the remaining water is held increasingly tightly to soil particles. The permanent wilting point (PWP) is reached when plants can no longer extract sufficient water to maintain turgor pressure, and they wilt irreversibly (even if placed in a humid environment overnight). The matric potential at PWP is conventionally defined as -1500 kPa. It is important to note that the soil still contains water at PWP, but it is held so tightly by matric forces that plant roots cannot overcome the tension to extract it.

Plant Available Water (PAW)

Plant Available Water (PAW) is the critical metric for agronomic management. PAW is the total amount of water held in the soil between field capacity and the permanent wilting point. It represents the "bank account" of moisture that crops can draw upon between rainfall or irrigation events.

Mathematically: PAW = Field Capacity - Permanent Wilting Point

Soil texture plays a dominant role in determining PAW.

• Sand: Sandy soils have large pores, leading to rapid drainage. They hold very little water at field capacity and have a low PWP. Consequently, their PAW is very low (e.g., 0.5 - 1.0 inches of water per foot of soil).
• Clay: Clay soils have very high total porosity due to countless micropores. They hold a massive amount of water at field capacity. However, they also hold water extremely tightly, resulting in a very high PWP. Their PAW is moderate (e.g., 1.5 - 2.0 inches/foot).
• Silt Loam: Medium-textured soils like silt loams offer the best balance. They have enough small pores to hold water against gravity (high field capacity) but not so many tiny pores that water is unavailable (moderate PWP). Silt loams typically have the highest PAW (e.g., 2.0 - 2.5 inches/foot).

Matric Potential and Soil Water Energy

Water moves in soil primarily in response to energy gradients, moving from areas of high potential energy to areas of low potential energy. The total water potential is the sum of matric potential, osmotic potential, gravitational potential, and pressure potential.

Matric potential is usually the dominant force in unsaturated soils. It arises from the capillary and adsorptive forces between water molecules and soil particles. Because these forces reduce the free energy of water, matric potential is always a negative value (expressed in kPa, bars, or cm of water). The drier the soil, the more negative the matric potential, and the harder plants must work to extract water.

Infiltration and Percolation

Infiltration is the process by which water enters the soil surface. The infiltration rate determines how much rainfall or irrigation water actually enters the root zone versus running off the surface. It is influenced by:

• Soil Texture: Sandy soils have high infiltration rates; clays have low rates.
• Soil Structure: Well-aggregated soils with continuous macropores have high infiltration rates.
• Surface Crusts: Bare soils exposed to raindrop impact can form surface crusts that drastically reduce infiltration.
• Initial Moisture Content: Dry soils initially absorb water quickly, but the rate declines as the soil wets up.

Percolation refers to the downward movement of water through the soil profile after it has infiltrated. It is driven by gravity and the matric potential gradient.