Water potential

Water potential quantifies the tendency of water to move from one area to another due to osmosis, gravity, mechanical pressure and matrix effects such as capillary action (which is caused by surface tension).

A common example is water with dissolved salts, such as seawater or the fluid in a living cell.

Negative pressure potentials occur when water is pulled through an open system such as a plant xylem vessel.

Withstanding negative pressure potentials (frequently called tension) is an important adaptation of the xylem.

This can be the case for marine organisms living in sea water and halophytic plants growing in saline environments.

If no membrane is present, movement of the solute, rather than of the water, largely equalizes concentrations.

On the other hand, osmotic potential has an extreme influence on the rate of water uptake by plants.

In such cases, the soil solution would severely restrict the rate of water uptake by plants.

The magnitude of matrix potential depends on the distances between solid particles—the width of the menisci (also capillary action and differing Pa at ends of the capillary)—and the chemical composition of the solid matrix (meniscus, macroscopic motion due to ionic attraction).

Matrix potential markedly reduces the energy state of water near particle surfaces.

If the matrix potential approaches a value of zero, nearly all soil pores are completely filled with water, i.e. fully saturated and at maximum retentive capacity.

In the case that water drains into less-moist soil zones of similar porosity, the matrix potential is generally in the range of −10 to −30 kPa.

Field capacity is the optimal condition for plant growth and microbial activity.

At a potential of −1500 kPa, the soil is at its permanent wilting point, at which plant roots cannot extract the water through osmotic diffusion.

In contrast, atmospheric water potentials are much more negative—a typical value for dry air is −100 MPa, though this value depends on the temperature and the humidity.

[3][4][5] A tensiometer, electrical resistance gypsum block, neutron probes, or time-domain reflectometry (TDR) can be used to determine soil water potential energy.

A scale can estimate water weight (percentage composition) if special equipment is not on hand.