Water retention in soil exhibits diverse phenomena, including
suction-saturation hysteresis, non-unique air entrapment at zero suction
and negative suction under partial saturations. The constancy of suction
after a long rest can be broken by relatively minor mechanical or
hydraulic agitations such as low-amplitude wetting cycles -- this fact
is here being related to metastable states that differ from the true
equilibrium. The complete suction-saturation relationships are thus
being recovered using non-equilibrium Landau's hydrodynamic theory and
Onsager's reciprocity principles. Equilibrium suction does not pertain
to hysteresis, yet can be approached through small amplitude agitations
over long duration. Conditions for rate independence are being
described, while rate-dependency are also accommodated and illustrated.
Finally, it is shown that the new non-equilibrium theory retains the
rigorously derived equilibrium result of the effective stress of
partially saturated soils.