Figure 7.6 Typical soil water profiles for three different types of soils, ranging from very fine grained (clay) to coarse (sand). [After Campbell (1977)]

where P = work rate (joules per second = watts), V = volume flow rate of water out of the plant (m3 s—*), and A^ = the potential energy gradient doing the work (Pa). If we make our calculation simple and assume we know that water is being drawn out of the plant at a rate of 1 ml (10—6 m3) per second, work is being done on the plant at the rate of (10—6 m3 s—1 X 105 Pa) = 0.1 J s—11, or 100 mW. To stay in water balance, the plant must use metabolic energy to transport water in at the same rate it is being lost. This means doing work against the thermodynamic gradient in water potential, essentially doing physiological work on the environment at a rate of 100 mW. If the plant cannot mobilize energy at least at this rate, its inward rate of water transport will not match the water loss, and the plant will wilt. In fact, agronomists use just these sorts of calculations to estimate water requirements of various crop plants in various soils.

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