The amount of rainfall required to saturate the soil is called:

Soil moisture deficiency is the amount of rainfall needed to bring dry soil up to saturation or field capacity.

Soil water holding capacity is the maximum water the soil can retain, not the shortfall to be filled.

Soil moisture content is the water already present in the soil at a given time.

The slender coefficient is unrelated to soil water and is not a moisture term.

The deficiency measures how much water is missing before the soil is fully wetted.

Knowing this deficit helps plan irrigation so crops receive water when the soil runs short.