Water Potential Calculator: Solute & Pressure Potential

What is water potential?

A water potential calculator finds the total water potential (Ψw) of a plant cell or solution by adding its solute potential (Ψs) and its pressure potential (Ψp). Water potential measures the tendency of water to move from one place to another. It is reported in megapascals (MPa), a unit of pressure.

Water always moves from a region of higher water potential to a region of lower water potential. This single rule explains how water enters plant roots, rises through stems, and leaves through leaves.

The water potential formula

The formula used by the calculator is:

Ψw = Ψs + Ψp

• Ψw: water potential, in MPa
• Ψs: solute potential (also called osmotic potential), in MPa
• Ψp: pressure potential, in MPa

Pure water at atmospheric pressure has a water potential of 0 MPa. That value is the reference point. Dissolved substances and physical pressure push the number up or down from there.

Components of water potential

Solute potential (Ψs)

Solute potential measures the effect of dissolved particles, such as sugars or salts, on water's energy. Dissolved particles attract and hold water molecules, which lowers the water's free energy. For this reason, solute potential is always zero or negative. A dilute solution has a value close to 0 MPa. A concentrated solution reaches much lower values. Seawater has an osmotic pressure of about 2.5 MPa, which means a solute potential of about -2.5 MPa.

Pressure potential (Ψp)

Pressure potential measures physical pressure on water. Inside a healthy, hydrated plant cell, the cell's contents push outward against the cell wall. This is called turgor pressure, and it produces a positive pressure potential, typically around +0.6 to +0.8 MPa and up to about +1.5 MPa in a well-watered plant. In the water-conducting tubes of a plant (the xylem), water is often pulled rather than pushed. Evaporation from leaves can create a tension of about -2 MPa at the leaf surface, a value that varies with how dry the air is.

How to calculate water potential

1. Measure or obtain the solute potential of the sample, in MPa. This value is usually negative or zero.
2. Measure or obtain the pressure potential of the sample, in MPa. This value can be positive, negative, or zero.
3. Add the two numbers together. The sum is the water potential, Ψw.

The calculator performs this addition automatically once both values are entered.

Example calculation

A leaf sample has a solute potential of -0.7 MPa and a pressure potential of +0.4 MPa.

Ψw = Ψs + Ψp Ψw = -0.7 + 0.4 Ψw = -0.3 MPa

The leaf's water potential is -0.3 MPa. Placed in pure water (0 MPa), the leaf cells would gain water, because water moves toward the higher potential. Placed against soil water at -0.5 MPa, the leaf cells would lose water instead, because -0.5 MPa is lower than -0.3 MPa.

A second example: a cell has Ψs = -1.8 MPa and Ψp = +0.5 MPa.

Ψw = -1.8 + 0.5 = -1.3 MPa

Reading the result

A more negative water potential means the water is under more stress and has a stronger pull on surrounding water. A less negative or positive value means water is more freely available. When Ψs and Ψp are equal and opposite, such as -0.5 MPa and +0.5 MPa, the total is 0 MPa, the same as pure water. Such a cell is fully firm and gains no water from pure water. At the other extreme, when pressure potential falls to 0 MPa, the water potential equals the solute potential. Leaves wilt as their pressure potential approaches 0 MPa. If water loss continues past that point, the cell membrane detaches from the cell wall, a condition called plasmolysis.

Units of water potential

Megapascals (MPa) are the standard unit in modern plant physiology. Two other units appear in older or related literature:

• Bars: 1 bar = 0.1 MPa
• Kilopascals (kPa), common in soil science: 1 MPa = 1,000 kPa

Why water potential matters

Water potential explains several everyday plant processes. Roots absorb water when the root's water potential is lower than the surrounding soil's. Leaves wilt when they cannot maintain a low enough water potential to keep their cells firm. Researchers use water potential to compare how well different crop varieties tolerate drought, and irrigation managers use it to decide when a field needs water. The lower a plant's water potential falls, the more drought stress it is under. Healthy ranges differ by species and time of day.

Because the formula only combines solute and pressure potential, it covers the components most relevant to plant cells and simple solutions. Soil water potential also includes matric potential, which describes water bound to soil particles, and is left out of this simplified two-term calculation.

Frequently asked questions

What is water potential in simple terms? It is a measure of how strongly water in a system tends to move. Water flows from high water potential to low water potential, similar to how heat flows from hot to cold.

What is the water potential formula? Ψw = Ψs + Ψp, where Ψw is water potential, Ψs is solute potential, and Ψp is pressure potential. All three are measured in megapascals.

Why is solute potential usually negative? Dissolved particles bind water molecules and reduce the amount of free water available to move, which lowers the water's energy. Zero or negative values are typical; solute potential is never positive.

Can water potential be positive? Yes, when the pressure potential is positive and larger than the size of the negative solute potential. For example, Ψs = -0.3 MPa and Ψp = +0.5 MPa give Ψw = +0.2 MPa. The calculator accepts such inputs and returns the positive sum.

What is the difference between water potential and water content? Water content is the amount of water present. Water potential is how available that water is for movement. Two soils can hold the same amount of water yet have very different water potentials, because soil texture affects how tightly the water is held.

Does this calculator account for soil matric potential? No. It adds only solute potential and pressure potential, which is accurate for plant cells and simple solutions. Soil water potential in unsaturated soil also depends on matric potential, which requires separate measurement.

References

1. OpenStax. Biology 2e, 30.5 Transport of Water and Solutes in Plants. Transport of Water and Solutes in Plants — water moves from higher to lower water potential; pure water is 0 MPa; solute potential is negative in a cell and zero in pure water; pressure potentials of about 0.6 to 0.8 MPa, up to 1.5 MPa; about -2 MPa of tension at the leaf surface; pressure potential approaches 0 MPa at the wilting point.
2. Harvard Medical School, BioNumbers. Osmotic pressure of sea water (BNID 108541). Osmotic pressure of sea water — seawater has an osmotic pressure of about 2.5 MPa, citing Nobel, Physicochemical and Environmental Plant Physiology.
3. National Institute of Standards and Technology. NIST Guide to the SI (Special Publication 811), Appendix B.8. Factors for units listed alphabetically — 1 bar = 100 kPa = 0.1 MPa.
4. OpenStax. Biology 2e, 5.2 Passive Transport. Passive Transport — in a plant cell that keeps losing water, the cell membrane detaches from the cell wall; this is plasmolysis, and the plant loses turgor pressure and wilts.