Pressure Sucks Moisture From Soil

Biological Strategy

AskNature Team

Image: Sanjay Acharya / wikimedia commons /

Capture, Absorb, or Filter Liquids

The most common liquid used by living systems is water, which they require to survive. But there are many other liquids that provide nourishment, play a role in defense mechanisms, or serve other purposes. Water varies in its availability; it is sometimes plentiful and sometimes very scarce or only available as fog or mist. To minimize the energy required to capture, absorb, or filter liquids, living systems have strategies that take advantage of the unique properties of the given liquid. For example, water moves from a gaseous to liquid state when it encounters a surface colder than the air. Plants in forests that experience fog and clouds more than rain have strategies that condense liquid water from moist air.

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The roots of desert plants extract hard to remove water from soil using negative pressure.

“Plants again. Even in a desert the soil a little ways below the surface contains liquid water. It’s called ‘capillary water’ and is often thought of as firmly stuck to soil particles. The binding, though, is as much physical as chemical – the water in the soil interstices lie in tiny recesses between soil crumbs where it has minimized its exposed interface with air (Rose 1966). For the roots of a plant to extract the water requires making more surface, and thus it takes a very great pull, one that appears as an additional (negative) component of the pressure in the vessels running up a stem or trunk. The lowest (most negative) pressures known in plants occur in desert shrubs, which must suck really hard on the ground to get any water out. The most extreme value on record is, I think, minus 120 atmospheres (Schlessinger et al. 1982) – that would hold up a column water over 1,200 meters (4,000 feet) high. So the pull needed to get water free of soil can exceed both the pull that keeps water moving in the vessels and the pull that counteracts gravity.” (Vogel 2003:113)