H ho Wmpg

Remember that =—100 kPa =—100 X 103 Pa, p = 103 kg m—2, and g = 9.8 m s—2. Plugging in these values, we see that the soil can draw water up 10.2 m above the water table. Soils with stronger matric potentials will draw the water up further.

The water potential also allows one to estimate the energy costs of transporting water, which is very important for understanding how organisms interact physiologically with the soil. Suppose, for example, that a plant is living in a soil with a matric potential of — 101 kPa and that the water inside the plant has a water potential of only — 1 kPa. There will therefore be a water potential gradient of —100 kPa drawing water out of the plant. Unless the plant actively moves water into its "body," it will dry up. It would be useful to know how much energy the plant will have to expend transporting water in at a rate sufficient to keep its water content steady. We know that the soil is doing work on the plant when it draws water out, and the rate this work is being done is the product of the water flux rate and the potential energy gradient doing the work:

water content (kg water / kg soil)

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