Which one of the following instruments is specifically used to measure soil water tension (the suction with which water is held in the soil)?

Introduction / Context:

This general knowledge question relates to physics and agricultural science. Soil water tension, also called soil water suction or matric potential, indicates how strongly water is held by soil particles and how available it is to plant roots. Measuring this tension helps farmers and researchers make decisions about irrigation and crop management. Different instruments are used to measure light, temperature, humidity, and soil moisture related quantities.

Given Data / Assumptions:

• We want to measure soil water tension, not just soil moisture content.
• The options include photometer, pyrometer, psychrometer, and tensiometer.
• Each instrument name refers to measurement of a different physical quantity.

Concept / Approach:

A tensiometer is a device used in soil physics to measure the tension or suction with which water is held in the soil. It typically consists of a porous ceramic cup attached to a water filled tube and a vacuum gauge. As water moves between the soil and the cup, the gauge reflects the tension. Photometers measure light intensity, pyrometers measure very high temperatures, and psychrometers measure humidity. Therefore, tensiometer is the only instrument among the options that is designed to measure soil water tension.

Step-by-Step Solution:

Verification / Alternative check:

Agricultural manuals and soil science textbooks describe tensiometers in detail as tools to measure soil matric potential in units like centibars or kilopascals. They are often installed at root zone depths and read periodically to decide when to irrigate. In contrast, photometers appear in optics labs, pyrometers in metallurgical or furnace monitoring, and psychrometers in meteorology and HVAC applications. This clear separation of applications confirms that tensiometer is uniquely associated with soil water tension.

Why Other Options Are Wrong:

• Photometer: Used to measure light or illumination, not soil water properties.
• Pyrometer: Measures very high temperatures by radiation, not soil water tension.
• Psychrometer: Measures relative humidity and dew point using wet and dry bulb thermometers, not soil moisture suction.

Common Pitfalls:

Because the names sound similar, students may confuse psychrometer and tensiometer, both of which are used in environmental measurements. Another trap is thinking that pyrometer might be related to soil due to heat and sunlight, but its specific role is temperature measurement at high values. Carefully linking each instrument name to its main field of use helps avoid such confusion: photometer for light, pyrometer for high temperature, psychrometer for humidity, and tensiometer for soil water tension.

Final Answer:

The instrument used to measure soil water tension is the tensiometer.