What does 'field capacity' refer to in irrigation?

Field capacity refers to the maximum amount of water that soil can retain after the excess water has drained away under the force of gravity. This concept is crucial in irrigation as it indicates the level at which the soil holds water without any water being lost to drainage. At field capacity, soil has sufficient moisture levels for optimal plant growth, enabling plants to access water through their root systems.

Understanding field capacity is important for effective irrigation practices because it helps in determining the timing and amount of water to apply. This ensures that the soil remains moist enough to support plant growth while avoiding overwatering, which can lead to issues such as waterlogging or nutrient leaching. In contrast, the other options pertain to different aspects of soil moisture management but do not accurately define field capacity. For example, while the point at which plants begin to wilt relates to the soil moisture being critically low for plant survival, it does not represent the maximum water retention ability of the soil after excess drainage occurs.