---
title: "Short note: Soils for agriculture"
subject: "Irrigation and Drainage Engineering"
chapter: 2
key: "semester_6/irrigation_and_drainage_engineering:soils-agricultural-purposes-note"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/soils-agricultural-purposes-note
site: BCT Engineering
---

# Short note: Soils for agriculture

Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements

- Asked in: 2063 Baisakh (old course), 4 marks
- 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/soils-agricultural-purposes-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2

## Question

Write a short note on soils for agricultural purposes.

## Short answer

Soil is the loose upper layer of the earth’s crust providing water, air and nutrients. A good agricultural soil has about 45% mineral matter, 5% organic matter, 25% water and 25% air by volume. Texture (sand, silt, clay) determines water-holding capacity and drainage; loam is best. Important properties include field capacity, wilting point, available moisture, infiltration, pH and salinity.

## Answer

**Soil** is the loose upper layer of the earth's crust, formed by weathering of rocks and decay of organic matter, which supplies water, air and nutrients to plants.

#### Soil constituents
Mineral particles (45%), organic matter (5%), water (25%) and air (25%) by volume in a good agricultural soil.

#### Classification by texture
- **Sand** (0.05-2 mm): coarse, high infiltration, low water-holding capacity.
- **Silt** (0.002-0.05 mm): medium.
- **Clay** (<0.002 mm): fine, low infiltration, high water-holding capacity.
- **Loam** (mix of sand, silt and clay): the best for agriculture, with good drainage and retention.

#### Properties important for irrigation
Texture, structure, porosity, infiltration rate, field capacity, wilting point, available moisture, pH and salinity, and fertility.

#### Suitability
- Sandy loam and loam: most crops; sprinkler/drip for sandy soil.
- Clay and clay loam: rice, but poor drainage; needs careful irrigation.
- Saline-alkaline soils need leaching and gypsum.

Depth of soil, topography and drainage also decide the choice of crop and irrigation method.

## Check yourself

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

1. A good agricultural soil by volume has roughly
   A. 90% mineral, 10% air
   B. 50% water, 50% air
   C. 25% mineral, 75% organic
   D. 45% mineral, 5% organic, 25% water, 25% air
   Answer: D. Pore space shares water and air. (Key point: Soil composition)

2. Which soil is best for agriculture?
   A. Loam
   B. Pure sand
   C. Pure clay
   D. Gravel
   Answer: A. Loam balances drainage and retention. (Key point: Best texture)

3. Clay has
   A. high infiltration, low holding
   B. low infiltration and high water-holding capacity
   C. high infiltration, high holding
   D. no water holding
   Answer: B. Fine particles retain water. (Key point: Clay property)

4. Saline-alkaline soils need
   A. more fertilizer only
   B. less drainage
   C. leaching and gypsum
   D. sand mixing only
   Answer: C. Leaching removes salts. (Key point: Saline soils)

## Related in other subjects

- [Civilization: agricultural, industrial, information society](https://www.bctengineer.com/question.md?key=semester_7%2Ftechnology_environment_and_society%3Aenlighten-civilization-societies) — Technology, Environment and Society, chapter 3