Complex sentence: At a potential of -1500 kPa, soil is at its permanent wilting point, meaning that soil water is held by solid particles as a "water film" that is retained too tightly to be taken up by plants.
Split into simpler sentences: At a potential of -1500 kPa, soil is at its permanent wilting point. At the permanent wilting point, soil water is held by solid particles as a "water film" that is retained too tightly to be taken up by plants.