psi of pure water is zero.If any solute is added in water, there will be fewer water left, so, water potential decrease.Thats why all the solution will have psi less than zero, i.e, the psi of any solution will be negative. If a solution has psi of -2000kpa ant other will have a psi of -1000kpa, then the solution having psi of -2000kpa will have less water potential than the one having psi of -1000kpa.