Water potential : Free energy per mole of water molecule is called water potential. It is represented by Greek letter ' $\psi$ ' (Psi). Water potential of pure water is zero and addition of solute, decreases its free energy or water potential.
Factors affecting water potential are as following :
(i) Matric potential $\left(\psi_m\right)$ causes by adsorbent or colloidal particles (always negative and almost negligible).
(ii) Solute potential $\left(\psi_{\mathrm{s}}\right)$ caused by presence of solute particles (decreases the water potential).
(iii) Pressure potential $\left(\psi_{\mathrm{p}}\right)$ caused by entry or exit of water (increases the water potential).
$$
\psi_w=\psi_{\mathrm{p}}+\psi_{\mathrm{s}}+\psi_{\mathrm{m}}
$$