Under saturated condition, the soil water potential is

Correct option is D

The correct answer is (D) May be zero or negative

Explanation:
• Soil water potential ($ψ_s$) is a measure of the energy state of water in the soil compared to pure free water at a reference elevation.
• Under saturated conditions, all soil pores are filled with water. At the water table (the upper boundary of the saturated zone), the matric potential is zero.
• Below the water table, the water is under hydrostatic pressure, which contributes a positive pressure potential. However, in most soil physics contexts focusing on the root zone, we consider the total potential which can be zero at the surface of saturation.
• It is important to note that osmotic potential (due to dissolved salts) is almost always negative. Therefore, even if the matric potential is zero (saturation), the presence of solutes makes the total potential zero or negative depending on the reference point and salt concentration.

Information Booster:
• Total Soil Water Potential ($ψ_t$) = Matric Potential ($ψ_m$) + Osmotic Potential ($ψ_o$) + Gravitational Potential ($ψ_g$) + Pressure Potential ($ψ_p$).
• In unsaturated soils, matric potential is always negative due to adsorption and capillarity.
• Water always moves from a region of higher potential (less negative) to lower potential (more negative).

Additional Knowledge:
• Option A: While true for unsaturated soils, it doesn't account for the zero point at the saturation boundary.
• Option C: Soil water potential is rarely "always positive" unless discussing hydrostatic pressure deep within a saturated aquifer or a pressurized pipe.