What is the Field Soil Water Holding Capacity?

The field soil water holding capacity, precisely defined as Field Capacity, refers to the maximum amount of water a soil can retain against the force of gravity after excess water has drained away. It is a critical metric for understanding how much water is available for plant uptake.

Understanding Field Capacity

Field Capacity represents a key benchmark in soil moisture content. As per the provided reference: "Field capacity is the water remaining in a soil after it has been thoroughly saturated and allowed to drain freely, usually for one to two days." This state signifies that gravitational water (water that drains due to gravity) has moved out of the root zone, leaving behind water held by capillary forces within the soil pores. This water is generally considered readily available for plant absorption.

The Role of Permanent Wilting Point

While Field Capacity indicates the upper limit of water available after drainage, it's equally important to understand the lower limit. The reference also defines this critical threshold: "Permanent wilting point is the moisture content of a soil at which plants wilt and fail to recover when supplied with sufficient moisture." At this point, the remaining soil water is held so tightly by soil particles that plant roots cannot extract it, leading to irreversible wilting and plant death.

Available Water Holding Capacity: The Usable Range

The true "usable" water holding capacity for plants is the difference between Field Capacity and the Permanent Wilting Point. This range is known as the Available Water Holding Capacity (AWHC). It represents the volume of water stored in the soil that is accessible and beneficial for plant growth. Understanding AWHC is vital for efficient water management in agriculture and horticulture.

Factors Influencing Soil Water Holding Capacity

Several factors dictate a soil's ability to hold water:

• Soil Texture: This refers to the proportion of sand, silt, and clay particles.
  • Clays have very small pores and large surface areas, allowing them to hold a significant amount of water, often resulting in high Field Capacity.
  • Silts have medium-sized particles and good water retention.
  • Sands have large particles and large pore spaces, leading to rapid drainage and lower Field Capacity.
• Organic Matter Content: Organic matter acts like a sponge, significantly increasing a soil's water holding capacity across all soil textures. It improves soil structure, creating more stable pores that can hold water.
• Soil Structure: The arrangement of soil particles into aggregates influences pore size distribution. Well-structured soils with stable aggregates typically have a good balance of macro- and micropores, optimizing both drainage and water retention.
• Compaction: Compacted soils have reduced pore space, which restricts both water infiltration and storage, leading to lower Field Capacity.

Practical Applications and Importance

Understanding field soil water holding capacity is fundamental for sustainable land and water management:

• Efficient Irrigation Scheduling: Knowing a soil's Field Capacity and Available Water Holding Capacity allows farmers and gardeners to apply the right amount of water at the right time, preventing both waterlogging and drought stress. This optimizes water use efficiency and reduces waste.
• Crop Selection: Different crops have varying water requirements and tolerances to soil moisture fluctuations. Selecting crops suited to the soil's inherent water holding capacity can enhance yield and reduce the need for supplemental irrigation.
• Soil Health Management: Practices that improve soil health, such as adding organic matter, cover cropping, and minimizing tillage, directly enhance the soil's water holding capacity, making systems more resilient to climatic variability.
• Environmental Benefits: Improved water retention reduces surface runoff and erosion, helping to protect water quality and minimize nutrient leaching.

Example: Typical Available Water Holding Capacity by Soil Type

Note: These values are approximations and can vary significantly based on organic matter content, specific mineralogy, and soil structure.