Question

In general, what must be true of the water potential, solute potential, and pressure potential of soil and roots to allow water to diffuse from the soil into the roots?

Answer 1

Water potential: Water potential is a measure of the free energy of water per unit volume (J m–3). Water potential of a pure water is zero. Water potential is reduced with the addition of solute.

Solute potential (Osmotic potential): It is the effect of dissolved solutes on water potential. Solutes reduce the free energy of water by diluting the water. Solute potential decreases with increasing solute concentration.

Pressure potential: The hydrostatic pressure of the solution represents pressure potential. Positive pressures increase  the water potential whereas negative pressures decrease it.

water potential = solute potential + Pressure potential

Water moves from high water potential to low water potential. High solute concentration decreases the water potential whereas high positive pressure increases the water potential. Root cells gain or lose water until their water potential matches that of its surroundings.

For water to diffuse from the soil into the roots water potential in the soil should be high, solute potential should be high and pressure potential should be high as compare to the root cells.