Using the data in the question $psiS=-0.243 L MPa$. This more negative solute potential would lower the total soil water potential, making for less movement of water into the roots, explaining the stunting of the grass in this soil.
Work Step by Step
Just plug the numbers from the question into the equation and multiple, using the conversion factor that 1 mol=1000 mmol. Remember that plant growth is driven by water uptake, so less water uptake in the saltier soil will provide less water for growth.