In unsaturated soil-water mechanics, the total soil water potential ($\psi_t$) is the sum of matric ($\psi_m$), osmotic ($\psi_s$), pressure ($\psi_p$), and gravitational ($\psi_g$) potentials. In an unsaturated root zone above the water table under atmospheric pressure, the matric potential $\psi_m$ is always:
Options:
(A) Negative (representing capillary and adsorptive suction forces pulling water into soil pores)
(B) Strictly positive and equal to hydrostatic head
(C) Zero at all soil moisture contents
(D) Equal to the atmospheric pressure head in absolute units
Answer Key:
A
Step-by-Step Solution & Explanation
Matric potential ($\psi_m$) results from capillary menisci and surface adsorption forces binding water to soil solid matrices. Because energy must be expended to extract this water relative to a free water pool at the same elevation, matric potential in unsaturated soil is strictly negative (or matric suction is positive). Option A is correct.