Why is solute potential always negative? Explain yw = ys + yp

Water potential (Ψw) is the sum of solute potential (Ψs) and pressure potential (Ψp): Ψw= Ψs+ Ψp. So when solute is dissolved in water, the water potential of pure water decreases or starts assuming a negative value.

Solute potential also known as osmotic potential is the potential of solution that allows the water to enter the solution by diffusion or osmosis due to presence of the solute in it. Pressure potential is the hydrostatic pressure that is exerted on water present in a cell. This usually has a positive value.