A tensiometer installed in an agricultural field directly measures soil matric suction. Due to the physical cavitation of water inside the porous ceramic cup, what is the practical upper limit of soil matric tension that a tensiometer can reliably record?
Options:
(A) $0.80 \text{ to } 0.85\text{ bar}$ ($80 \text{ to } 85\text{ kPa}$)
(B) $15.0\text{ bar}$ ($1500\text{ kPa}$)
(C) $0.05\text{ bar}$ ($5\text{ kPa}$)
(D) $100.0\text{ bar}$ ($10,000\text{ kPa}$)
Answer Key:
A
Step-by-Step Solution & Explanation
Tensiometers measure the negative pore water pressure (matric potential). As soil dries and matric suction approaches $1\text{ atmosphere}$ ($1.013\text{ bar}$), dissolved gases come out of solution and water inside the tensiometer tube cavitates (boils at ambient temperature). Hence, their practical operating range is strictly limited to $0 \text{ to } 0.85\text{ bar}$ ($0 \text{ to } 85\text{ kPa}$). At permanent wilting point ($15\text{ bar}$), tensiometers are completely inoperative. Option A is correct.