---
title: "Soil moisture tension and related terms"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:soil-moisture-tension-terms"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/soil-moisture-tension-terms
site: BCT Engineering
---

# Soil moisture tension and related terms

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2068 Baisakh (old course), 3 marks
- Video: [Irrigation Terminology](https://www.youtube.com/watch?v=MZ95qFqhVQU) — ABHISHEK Singh
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/soil-moisture-tension-terms
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

Explain soil moisture tension, osmotic pressure, field capacity, wilting point and available moisture.

## Short answer

Soil moisture tension is the suction force (≈1/3 atm at FC, 15 atm at PWP). Osmotic pressure from salts adds to total stress. FC is the upper limit, PWP is the lower limit; available moisture = FC – PWP.

## Answer

- **Soil moisture tension (suction):** the force (negative pressure) with which water is held by the soil; the energy per unit mass that a plant has to apply to take water from soil. It is measured in atm, kPa or pF and increases as the soil dries (about 1/3 atm at FC and 15 atm at PWP).
- **Osmotic pressure:** pressure caused by dissolved salts in soil water, which opposes the entry of water into root cells. A saline soil has high osmotic pressure, so total water stress for the plant is the sum of soil moisture tension and osmotic pressure.
- **Field capacity:** moisture content retained after free gravity drainage; tension about 1/3 atm; upper limit of available moisture.
- **Permanent wilting point:** moisture content at which plants wilt permanently; tension about 15 atm; lower limit of available moisture.
- **Available moisture:** the water between FC and PWP that is usable by plants: $AM = FC - PWP$.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does soil moisture tension change as soil dries?
   A. It decreases from 15 atm to 1/3 atm
   B. It stays constant
   C. It increases, about 1/3 atm at FC to 15 atm at PWP
   D. It is zero at PWP
   Answer: C. Drier soil holds water more tightly. (Key point: Moisture tension)

2. Why does osmotic pressure matter in saline soils?
   A. Salts speed up root absorption
   B. It replaces soil moisture tension
   C. It only affects drainage
   D. Dissolved salts oppose water entry, adding to total water stress
   Answer: D. Total stress = soil moisture tension + osmotic pressure. (Key point: Osmotic pressure)

3. Which pair correctly gives the limits of available moisture?
   A. Upper FC (1/3 atm), lower PWP (15 atm)
   B. Upper PWP, lower FC
   C. Upper saturation, lower FC
   D. Upper FC (15 atm), lower PWP (1/3 atm)
   Answer: A. Available moisture lies between FC and PWP. (Key point: Limits of available moisture)

## Related in Irrigation and Drainage Engineering

- [Field capacity, available moisture definitions](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adefine-fc-am-ram-deficiency) — Irrigation and Drainage Engineering, chapter 2
- [Readily available moisture, PWP, interval](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Armc-pwp-irrigation-interval-terms) — Irrigation and Drainage Engineering, chapter 2
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