Why is solute potential always negative? Explain ψw =ψs+ψp.

If some solute is dissolved in pure water, solution has fewer free water molecules and the concentration of water decreases, reducing its water potential. 
Hence, all the solutions have a lower water potential than pure water. The magnitude of this lowering is due to dissolution of solute is called solute potential or ψsψs iS always negative, The morethe solute molecules, the lower (more negative) is the solute potential ψs. water potential of a cell is affected by both solute and pressure potential.
The relationship can be illustrated as where.
ψw =ψsψp
ψw = water potential,
ψs = solute potential
ψp = pressure potential