Q32: The force per unit area that must be exerted in order to extract water from the soil is known as:
📝 Detailed Explanation
This is the exact same question and definition already covered in Q12 earlier in this topic — capillary potential, tested here a second time with reordered answer choices.
✅ Why “capillary potential” Is Correct
Capillary potential is defined as the force per unit area that must be exerted to extract water from the soil — also known as matric potential, reflecting how tightly the soil’s pore structure and surface-tension forces hold onto its water.
🔑 Key Point
Capillary potential = matric potential = force per unit area needed to extract water from soil — see Q12 for this exact same definition, tested with a different set of surrounding distractor options.
💡 Key Concepts for Students
- This exact repetition (Q12 and Q32 testing the identical concept) is a useful reminder that some MCQ banks recycle core definitional questions with reshuffled distractors — recognizing the repeat saves time.
- See Q12’s Going Deeper section for the fuller discussion of capillary rise and capillary/matric potential together.
- Finer-grained soils generally exhibit higher capillary potential (water held more tightly) than coarser soils, consistent with their smaller pore sizes and higher specific surface area.
- This concept bridges into unsaturated soil mechanics and agricultural soil science, beyond the saturated-flow focus of most of this topic’s other questions.
📚 Going Deeper: Capillary Potential in the Context of Unsaturated Soil
Most of this topic (Darcy’s law, permeability testing, well-pumping formulas) assumes fully saturated soil, where every void is water-filled and flow is driven purely by a hydraulic gradient. Capillary/matric potential, by contrast, is most relevant in UNSATURATED conditions, where air and water share the pore space.
In this partially saturated state, water is held in the soil by a combination of capillary forces (surface tension at curved air-water interfaces within the pores) and adsorptive forces (attraction between water molecules and mineral particle surfaces, especially significant in fine-grained soils). The capillary/matric potential quantifies the combined strength of these attractive forces as an equivalent suction pressure — the force per unit area that would need to be applied to pull that water back out. This concept becomes central once analysis moves beyond simple saturated Darcy flow into unsaturated flow, plant-available soil water, and suction-based soil mechanics more broadly.