Soil –Plant –Water relation relates to the physical properties of soil and plants that effect the movement, retention and use of water. These relations must be considered in designing and operation systems. Soil is a store house of plant nutrients, a habitat for bacteria , an anchorage for plant and a reservoir that hold the water needed for plant growth. The amount of water a soil can hold in available from for plant use is determined by its physical properties. This amount determines the length of time a plant can survive without water being added. It determines both the frequency of irrigation and the capacity of irrigation system needed to ensure continuous crop growth.

Soil is a three phase system comprising of the soil phase made of mineral and organic matter and various chemical compounds, the liquid phase called soil moisture and the gaseous phase called the soil air. They also contain variety of living an organism. Two important physical properties of soil, which differ the supply of water and air in soil, are texture and structure.