This also approximates what happens in the plant-soil atmosphere continuum. In Figure 4, the soil is at -0.3 MPa and the roots are slightly more negative at -0.5 MPa. This means the roots will pull water up from the soil. Then the water will move up through the xylem, out through the leaves across this potential gradient. The atmosphere, at -100 MPa, is what drives this gradient. So the water potential defines which direction water will move in the system.
A soil moisture release curve ties together the extensive variable of volumetric water content with the intensive variable of water potential. Graphing the extensive and intensive variables together allows researchers and irrigators to answer critical questions, such as where soil water will move. For example, in Figure 5 below, if the three soils below were different soil horizon layers at 15% water content, the water in the loamy fine sand would begin to move toward the fine sandy loam layer because it has a more negative water potential.
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