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

Step 1: Go through each statement and check it against how these soil moisture methods actually work.

Step 2: Statement (A) is correct. The gravimetric method (oven drying a soil sample and weighing before and after) gives the true moisture content directly, so it is the standard reference used to calibrate every other indirect method, including neutron probes and resistance blocks.

Step 3: Statement (B) is false. Electrical resistance blocks work by measuring the resistance of a porous block in equilibrium with the soil water. Dissolved salts in the soil solution conduct electricity and change this resistance reading independent of the actual moisture present, so resistance blocks give unreliable readings in saline soils rather than being useful for them.

Step 4: Statement (C) is false. A neutron probe works by counting fast neutrons that get slowed down mainly by collisions with hydrogen nuclei, and water is by far the largest source of hydrogen in soil. The count of slow neutrons is related to hydrogen nuclei, not carbon nuclei.

Step 5: Statement (D) is false. A tensiometer only reads suctions up to about 0.8 to 0.85 bar before air enters the porous cup and breaks the water column, so it works well in the wet range near field capacity but cannot cover the drier part of the available moisture range, especially in fine textured soils.

Step 6: Statement (E) is correct. Placing soil moisture stations between sprinkler heads, a few metres away from the lateral line, gives a location representative of the wetted area rather than right under a head or right on the pipe.

Step 7: The false statements are (B), (C) and (D), which matches option 2.