Question

Which statements are incorrect?

(A) In the field, soil moisture content determined by the gravimetric method is used as the reference in calibrating the other soil moisture instruments.

(B) Resistance blocks are useful for saline soils, since the resistance reading is not affected by salt concentration.

(C) The principle of the neutron probe moisture meter is based on the measurement of the number of carbon nuclei that are present in a unit volume of soil.

(D) Tensiometer satisfactorily measures the entire range of available moisture in all soil types.

(E) In sprinkler irrigation the soil moisture measuring stations should be between the sprinkler heads and 3 to 4 m away from the lateral.

Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (B), (C) and (D) only.
• C. (C), (D) and (E) only.
• D. (A), (B) and (E) only.

Hint:

Check each claim against how gravimetric sampling, resistance blocks, neutron probes and tensiometers actually behave.

Answer 1

The Correct Option is B

Take the five claims one at a time and compare them with what each instrument or method is actually built to do.

Gravimetric sampling (A) is a direct weighing method, so it needs no calibration itself and is used as the yardstick for every indirect instrument. That claim holds.

Resistance blocks (B) sense moisture indirectly through the electrical resistance of a buried porous block. Since dissolved salts carry current on their own, a saline soil pushes the resistance reading down regardless of how much water is actually present, which makes the method unreliable rather than useful in saline conditions. So (B) is wrong.

Neutron probes (C) send out fast neutrons and count how many come back slowed down. Hydrogen atoms, overwhelmingly supplied by soil water, are what slow the neutrons most effectively, so the count reflects hydrogen nuclei, not carbon nuclei. That makes (C) wrong as written.

Tensiometers (D) rely on a continuous water column between the soil and a gauge, and this column breaks once suction passes roughly 0.85 bar. That limits them to the wetter part of the moisture range and rules out the entire range claimed in (D), so (D) is wrong.

Placement guidance for sprinkler systems (E), keeping a sensor between heads and a few metres from the lateral, avoids the extremes of over wetting near the pipe and under wetting far from it, so (E) is a reasonable and correct placement rule.

Only (B), (C) and (D) turn out to be the incorrect statements.

\[\boxed{(B),\ (C),\ (D)\ \text{only}}\]