The Matric Potential
In discussions of soils, seeds, and cell walls, one often finds reference to yet another component of Ψ: the matric potential (Ψm). The matric potential is used to account for the reduction in free energy of water when it exists as a thin surface layer, one or two molecules thick, adsorbed onto the surface of relatively dry soil particles, cell walls, and other materials. The matric potential does not represent a new force acting on water, because the effect of surface interactions can theoretically be accounted for by an effect on Ψs and Ψp (see Passioura 1980; Nobel 1999). In dry materials, however, this surface interaction effect often cannot easily be separated into Ψp and Ψs components in dry materials, so they are frequently bulked together and designated as the matric potential.
It is generally not valid to add Ψm to independent measurements of Ψs and Ψp to arrive at a total water potential. This is particularly true for water inside hydrated cells and cell walls, where matric effects are either negligible or they are accounted for by a reduction in Ψp. For instance, the negative pressure in water held by cell wall microcapillaries at the evaporative surfaces of leaves, discussed on p. 91 of the textbook, is sometimes described as a wall matric potential. Care is needed to avoid inconsistencies when accounting for this physical effect in definitions of Ψp, Ψs, and Ψm (Passioura 1980).