42.2. Field Capacity and Wilting Point

Field Capacity (FC): The amount of soil moisture remaining after excess water has drained away, typically 1–3 days after irrigation or rainfall.

• Units: % by volume or weight.
• Significance: Marks the upper limit of available water.

Field Capacity refers to the amount of moisture that soil can hold after excess water has drained from it. This occurs a few days after irrigation or rainfall, usually around 1 to 3 days. At field capacity, the soil is neither too dry nor waterlogged. The units of measurement for field capacity can be expressed either as a percentage of volume or weight. Understanding field capacity is crucial because it indicates the maximum amount of water that plants can access efficiently, right before the soil begins to dry out.

Imagine a sponge that has soaked up water. After removing it from the water, the sponge retains some moisture for a while. Field capacity is like that 'sogginess' of the sponge after the excess water has dripped away, representing the ideal amount of moisture available for plants to absorb.

Permanent Wilting Point (PWP): The soil moisture level at which plants can no longer extract water, leading to permanent wilting.

• At this stage, only hygroscopic water remains.
• Varies by plant species and soil texture.

The Permanent Wilting Point is the moisture level in the soil where plants cannot extract any more water. This situation arises when the soil moisture has declined to a point where only hygroscopic water remains. Hygroscopic water is tightly bound to the soil particles and is unavailable to plants. The PWP can vary depending on the species of plant and the texture of the soil; some plants can survive longer without water, while others will wilt more quickly.

Think of a plant that has been neglected and the soil has dried out considerably. The moment the plant starts to droop and cannot recover even if watered, it has reached the Permanent Wilting Point. It’s akin to a car running out of gas entirely; no matter how much you try to push it, it won't go anywhere without fuel.

Available Water (AW): AW = FC − PWP. It is the water stored in the soil profile between field capacity and wilting point, usable by plants.

Available Water refers to the amount of water that is accessible to plants, calculated by subtracting the Permanent Wilting Point from the Field Capacity (AW = FC - PWP). This value tells us how much water is actually available for plant uptake between the two critical points of moisture content. This concept is essential, as it guides farmers and agriculturalists in understanding how much water they need to apply for optimal plant growth.

Consider a bathtub filled with water. The water level where the overflow starts is similar to the Field Capacity, while the level at which water almost runs out is like the Permanent Wilting Point. The water that remains between these two levels is what you can actually use for washing. In agricultural terms, that 'usable' water is what plants rely on to thrive.

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