Salt significantly lowers the water potential of a solution.
Water potential is a measure of the potential energy of water per unit volume relative to pure water under standard conditions. It determines the direction of water movement – water moves from areas of higher water potential to areas of lower water potential.
Several factors contribute to the total water potential ($\Psi_w$), including pressure potential ($\Psi_p$), gravitational potential ($\Psi_g$), and osmotic potential ($\Psi_s$, also known as solute potential).
$\Psi_w = \Psi_p + \Psi_g + \Psi_s$
The Role of Osmotic Potential
Osmotic potential ($\Psi_s$) is influenced by the concentration of solutes dissolved in water. Pure water has an osmotic potential of zero. Adding solutes lowers the osmotic potential, making it a negative value. The more solutes present, the lower (more negative) the osmotic potential becomes.
How Salt Impacts Osmotic Potential
Salt, when dissolved in water, dissociates into ions (like Na⁺ and Cl⁻). These ions are solutes.
According to the reference, "Accumulated salt decreases the osmotic potential of the soil water". This means that as the concentration of salt in water increases, the osmotic potential becomes more negative.
Lowering the Overall Water Potential
Since osmotic potential ($\Psi_s$) is a component of the total water potential ($\Psi_w$), a decrease (making it more negative) in osmotic potential directly leads to a decrease in the overall water potential of the water.
- Low Salt Concentration: Higher (less negative) osmotic potential -> Higher water potential.
- High Salt Concentration: Lower (more negative) osmotic potential -> Lower water potential.
Consequences for Plants and Water Uptake
This reduction in water potential due to dissolved salt has critical implications, particularly for plants. Plants absorb water from the soil, and this process relies on the water potential gradient between the soil water and the plant's roots. Water moves into the roots because the water potential inside the root cells is typically lower than in the surrounding soil water.
However, when salt accumulates in the soil water, its water potential drops significantly (becomes more negative). The reference notes this reduction "reduces the root water uptake rate". If the soil water potential becomes too low (highly negative) due to high salt concentration, it can become difficult or impossible for plants to absorb water. In extreme cases, water may even move out of the plant roots into the drier, saltier soil, leading to dehydration and wilting, even if the soil appears physically wet.
The reference also highlights that "the patterns of water uptake and salt accumulation temporally change as the soil water's salt concentration increases as a result of water uptake by roots," indicating a dynamic process where water removal leaves salt behind, further concentrating it and lowering the potential over time.
Summary of Salt's Effect
| Factor | Effect of Salt Accumulation | Impact on Water Potential Component | Result on Total Water Potential |
|---|---|---|---|
| Dissolved Solutes | Increases solute concentration | Decreases Osmotic Potential ($\Psi_s$) (becomes more negative) | Decreases Total Water Potential ($\Psi_w$) (becomes more negative) |
In essence, salt acts as a solute that reduces the free energy of water molecules, making them less likely to move away from the solution. This reduction in free energy is quantified as a lower (more negative) water potential.
