Water is essential to all living things, and plants are no exception. While they can’t move around as animals do, they can still find water to use in their metabolism. One way that plants get water is through the process of capillary movement—where water moves up a narrow channel, called a capillary tube (or capillary), into the leaves of the plant.
Capillary and pore water
Capillary water is the water held in the spaces between soil particles. This capillary water comes from precipitation and snowmelt, or it may be added to the soil by irrigation water. It can be drawn into plant roots through their natural pores (or “pores”) as well as through artificial openings made by tractors and other farm equipment.
Pore water, on the other hand, refers to all of the liquid contained within the pore space of a material that has been saturated with air until its free air has been displaced by liquid at atmospheric pressure. In soils, this would include both capillary water and percolating rainwater that may have drained away from above-ground sources like lakes or rivers but then infiltrated into underground aquifers before being re-discharged somewhere else as groundwater discharge (GWD).
Water potential is a measure of the energy available to water molecules at a given location. It can be thought of as being due to an energy gradient between two points in space, with water moving from high to low. Water pressure is caused by these gradients and will cause water to flow downhill, like when you step off a curb and your foot sinks into the sidewalk because of differences in elevation.
The osmotic component of water potential is the proportion of the total water potential that is due to the osmotic force. It can be calculated based on the number of solute particles in a solution and the temperature of the system.
The matrix component of water potential varies greatly depending on where you are in the soil. At any given point, there is more pressure pushing down on water molecules from above than pushing up from below. Soil particles also affect the matrix component because they are solid. Water in between soil particles doesn’t have much room to move around, so at any given time most of it is being pushed back into its neighbors by gravity.
Solute molecules and dissolved minerals can also affect the matrix component in a few ways:
- They decrease the ability for water to move through the soil, reducing its flow rate (which increases pf).
- They increase pf by increasing surface tension (which decreases qs).
The air in your soil is under a pressure of about 14.7 psi, which is the same as the air pressure at sea level (1 bar). This means that when you pump water out of your well, it has a higher pressure than the surrounding soil.
The opposite happens when water flows into your well: It has a lower pressure than that of the surrounding soil. The difference between these two pressures can be used for energy generation or other purposes such as digging deeper wells or creating oil reservoirs.
If the solute concentration of a solution increases, the potential for the water in that solution to undergo osmosis decreases.
A low water potential means that water has a low force driving it to move from one area to another.
The pressure required to force water out of the stem of a severed leaf equals the water potential and is measured by a pressure gauge.